LC Applications

LC Applications

Behzad PourabbasPolymer Eng. DepartmentSahand University of [email protected]

2

Overview:

• Order Parameter• Anisotropic Properties• Light, polarization and materials

ORDER PARAMETER “S”

qn

The Order Parameter

n

22

1(cos ) (3 cos 1)2

q q S P

2

2

(cos ) 1

(cos ) 0

q

q

S P

S P

perfect crystal

isotropic fluid

Maier-Saupe Theory - Mean Field Approach

Temperature

Nematic LiquidCrystal

Isotropic Fluid

-0.6

0.0

1.0

Ord

er P

aram

eter

, S

n

n

The Order Parameter: How does it affects display performance ?

The order parameter, S, is proportional to a number of importantparameters which dictate display performance.

Parameter Nomenclature Elastic Constant Kii S2

Birefringence Dn SDielectric Anisotropy De SMagnetic Anisotropy Dc SViscosity Anisotropy Dh S

Example: Does the threshold switching voltage for a TN increase or decrease as the operating temperature increases.

Scales as the square root of S therefore lowers with increasing temperature

2

THK SV S

Se

D

proportional to

Response to Electric and Magnetic Fields

External Electric Field and Dielectric Properties of LC molecules

Anisotropy: Dielectric Constant++

+++

- -- --

E

e

e

De e e > 0

E

De e e < 0

++++

----

positive

negative

all angles inthe plane to E arepossible for the-De materials

E

Anisotropy: Duel Frequency

MLC-2048 (EM Industries), Duel Frequency Material Frequency (kHz) 0.1 1.0 10 50 100Dielectric Anisotropy (De) 3.28 3.22 0.72 -3.0 -3.4

low frequency, De>0 high frequency, De<0

Dielectric Constant

ke0L = C = q/V

Dielectric Constant

Dielectric Material?

E

• Dielectric materials consist of polar molecules which are normally randomly oriented in the solid.

•They are not conductors.

•When a dielectric material is placed in an external electric field, the polar molecules rotate so they align with the field. This creates an excess of positive charges on one face of the dielectric and a corresponding excess of negative charges on the other face.

Dielectric Material is smaller in many materials than it would be in a vacuum for the same arrangement of charges.

Eg. Parallel plates:

E

Eo

ke

k oEE

+ + ++

Dielectricmaterial

This makes the potential difference smaller (V=Ed) between the parallel plates of the capacitor for the same charges on the plates and thus capacitance is larger, since Q=C/V.

Ei

Net field: E=Eo-Ei

Dielectric Constant

k

(“kappa”) = “dielectric constant”

= (a pure number ≥ 1)k

So,

dAC ke (for parallel plates)

Or 0C Ck

Where C0 is the capacitance without the dielectric.

Hence, the capacitance of a filled capacitor is greaterthan an empty one by a factor

Dielectric Constants (@20oC, 1kHz)

*Mixture Application De e e

BL038 PDLCs 16.7 21.7 5.3MLC-6292 TN AMLCDs 7.4 11.1 3.7ZLI-4792 TN AMLCDs 5.2 8.3 3.1TL205 AM PDLCs 5 9.1 4.118523 Fiber-Optics 2.7 7 4.395-465 -De material -4.2 3.6 7.8

Materials Dielectric ConstantVacuum 1.0000Air 1.0005Polystyrene 2.56Polyethylene 2.30Nylon 3.5Water 78.54

*EM MaterialsPD: Polymer DispersedAM: Active MatrixTN: Twisted Nematic

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

mm

Dielectric Anisotropy and Permanent Dipole Moment

Dielectric Constants: Temperature Dependence

1 6

1 4

1 2

1 0

8

62 5 3 0 3 5

T - T N I ( ° C )

/ /1 23

e e eD

( )S TeD

Die

le

ctric C

on

sta

nt

e i s

e

E x t r a p o l a t e d f r o m i s o t r o p i c p h a s e

e

4’-pentyl-4-cyanobiphenyl

CH3-(CH2)4 C N

( )S TeD

//1 23

e e eD

Temperature Dependence

Average Dielectric Anistropy

Dielectric Anisotropy and Induced Dipole Moment

easily polarized

Molecular axis

minduced is large e is large

minduced is small e is small

+ -r//

+

-

r

e dielectric constant along the direction perpendicular to the molecular axis

e dielectric constant along the direction parallel to the molecular axis

Magnetic Anisotropy: DiamagnetismDiamagnetism: induction of a magnetic moment in opposition to an applied magnetic field. LCs are diamagnetic due to thedispersed electron distribution associated with the electron structure.

Delocalized charge makesthe major contribution to diamagnetism.

Ring currents associated witharomatic units give a largenegative component to c for directions to aromatic ringplane. Dc is usually positive since:

0ll llc c c c c D > >

Magnetic Anisotropy: Diamagnetism

C 5 H 1 1

C 7 H 1 5

C N

C N

C N

C 5 H 1 1

C N

C 7 H 1 5

C 7 H 1 5

C N

9 3 1/ 1 0 m k gc D

1 . 5 1

1 . 3 7

0 . 4 6

0 . 4 2

- 0 . 3 8

Compound

Light is a high frequency electromagnetic wave and will only polarize electron cloud.In general, De = e e > 0 or e > e

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

For high electrical resistance materials, n is proportional to e1/2

Dn = n n > 0 in generalDn is a very important parameter for a LC device. Larger the Dn value, thinner the LC device and faster the response time

O

S C NC5H11

De = +33

C - N - I76 98

O

O C7H15

CN

C5H11 De = - 4.0

C - N - I45 101

Examples

Magnetic Susceptibility and Anisotropy

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LIGHT, POLARIZATION AND MATERIALS

28

Optical polarization

• for any wavevector, there are two field components

• light is a transverse wave: perpendicular to E k

• any wave may be written as a superposition of the two polarizations

Light as Electromagnetic Wave

Plane Polarized light can be resolved into Ex and Ey

32

BIREFRENGENCE

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

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//

e//

E field

Induced dipole by electromagnetic wave

Propagation of the light parallel to the molecular axis

Change of the speed is relatively small

// = C// / e

e//

Optical Anisotropy: Birefringenceordinary ray (no, ordinary index of refraction)

extraordinary ray (ne, extraordinary index of refraction)

Optical Anisotropy: Birefringenceordinary wave

q

extraordinary wave

on n2 2

2 2 2

1 cos sin

o en n nq q

For propagation along the opticaxis, both modes are no

optic axis

Birefringence (20oC @ 589 nm)

EM Industry Dn ne no Application Mixture BL038 0.2720 1.7990 1.5270 PDLCTL213 0.2390 1.7660 1.5270 PDLCTL205 0.2175 1.7455 1.5270 AM PDLCZLI 5400 0.1063 1.5918 1.4855 STNZLI 3771 0.1045 1.5965 1.4920 TNZLI 4792 0.0969 1.5763 1.4794 AM TN LCDsMLC-6292 0.0903 1.5608 1.4705 AM TN LCDsZLI 6009 0.0859 1.5555 1.4696 AN TN LCDsMLC-6608 0.0830 1.5578 1.4748 ECB95-465 0.0827 1.5584 1.4752 -De devicesMLC-6614 0.0770 --------- --------- IPSMLC-6601 0.0763 --------- --------- IPS18523 0.0490 1.5089 1.4599 Fiber OpticsZLI 2806 0.0437 1.5183 1.4746 -De device

Birefringence: Temperature Dependence

1 . 8

1 . 7

1 . 6

1 . 5

1 . 45 0 4 0 3 0 0

T - T N I ( ° C )

In

dex of R

efractio

n

2 0 1 0

n e

n o

n i s o

2 220

1 23 en n n

E x t r a p o l a t e d f r o m i s o t r o p i c p h a s e

2 220

1 23 en n n

Average Index

TemperatureDependence

( )n S TD

CIRCULAR POLARIZATION OF LIGHT

Circular Birefringence

44

Categories of optical polarization• linear (plane) polarization• coefficients differ only by real

factor• circular polarization• coefficients differ only by factori

• elliptical polarization• all other cases

45

Characterizing the optical polarization• wavevector insufficient to

define electromagnetic wave• we must additionally define the polarization vector

k

x

yz

• e.g. linear polarization at angle

Reflection of Circular Polarized Light

LCP RCP

Dynamic Scattering Mode LCD Device

Twisted Nematic (TN) Device 1971 by Schadt

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

Electrically Controlled Birefringence (ECB) Device (DAP type)

Polymer Dispersed Liquid Crystal (PDLC) Device

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GENERAL STRUCTURE

AX Y

Z Z’

• Aromatic or saturated ring core• X & Y are terminal groups• A is linkage between ring systems• Z and Z’ are lateral substituents

CH3 - (CH2)4C N

4-pentyl-4’-cyanobiphenyl (5CB)

General Structure

Mesogenic Core Linking Groups Ring Groups

N

N

phenyl

pyrimidine

cyclohexane

biphenylterphenyldiphenylethanestilbenetolaneschiffs baseazobenzeneazoxyben-zenephenylbenzoate(ester)phenylthio-benzoate

CH CH2 2 CH CH CH CH CH N N N

N NO

C O

C S

O

O

Common Groups

NomenclatureMesogenic Core

phenylbenzylbenzene

biphenyl terphenyl

phenylcyclohexane (PCH)cyclohexane cyclohexyl

Ring Numbering Scheme

3’ 2’

1’

6’5’

4’

32

1

6 5

4

Terminal Groups (one terminal group is typically an alkyl chain)

CH3

CH2

CH2

CH2

CH3

CH2

C*HCH2

CH3

straight chain

branched chain (chiral)

Attachment to mesogenic ring structureDirect - alkyl (butyl)Ether -O- alkoxy (butoxy)

CH3-

CH3-CH2-

CH3-(CH2)2-

CH3-(CH2)3-

CH3-(CH2)4-

CH3-(CH2)5-

CH3-(CH2)6-

CH3-(CH2)7-

methyl

ethyl

propyl

butyl

pentyl

hexyl

heptyl

octyl

CH3-O-

CH3-CH2-O-

CH3-(CH2)2-O-

CH3-(CH2)3-O-

CH3-(CH2)4-O-

CH3-(CH2)5-O-

CH3-(CH2)6-O-

CH3-(CH2)7-O-

methoxy

ethoxy

propoxy

butoxy

pentoxy

hexoxy

heptoxy

octoxy

Terminal Groups

Second Terminal Group andLateral Substituents (Y & Z)

H -F flouroCl chloroBr bromoI iodoCH3 methylCH3(CH2)n alkylCN cyanoNH2 aminoN(CH3) dimethylaminoNO2 nitro

phenyl

cyclohexyl

Odd-Even EffectClearing point versus alkyl chain length

0 1 2 3 4 5 6 7 8 9 10 11 carbons in alkyl chain (n)

cle

arin

g po

int

18

16

14

12

10

CH3-(CH2)n-O O-(CH2)n-CH3C-O

O

CH3-(CH2)4 C N

CH3-(CH2)4-O C N

4’-pentyl-4-cyanobiphenyl

4’-pentoxy-4-cyanobiphenyl

NomenclatureCommon molecules which exhibit a LC phase

Structure - Property

N

N

CH3-(CH2)4 C N

vary mesogenic core

A

A C-N (oC) N-I(oC) Dn De

22.5 35 0.18 11.5

71 52 0.18 19.7

31 55 0.10 9.7

Structure - Property

CH3-(CH2)4 COO

vary end group

X

X C-N (oC) N-I (oC)

HFBrCNCH3

C6H5

87.592.0115.5111.0106.0155.0

114.0156.0193.0226.0176.0266.0

Lateral Substituents (Z & Z’)

AX Y

Z Z’

• Z and Z’ are lateral substituents • Broadens the molecules• Lowers nematic stability • May introduce negative dielectric anisotropy

E

Solid

Liquid Crystal

Isotropic Liquid

Concentration (c2), %

0 50 100

Why Liquid Crystal MixturesMelt Temperature:Liquid Crystal-Solid

ln ci = DHi(Teu-1 - Tmi

-1)/R

DH: enthalpiesTeu: eutectic temperature

Tmi: melt temperatureR: constant

Nematic-IsotropicTemperature: TNI

TNI = S ciTNIi

Tem

pera

ture

eutecticpoint