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LC Applications
Behzad Pourabbas
Polymer Eng. Department
Sahand University of Technology
Tabriz-Iran
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 S P
2
2
(cos ) 1
(cos ) 0
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
TH
K SV S
S
proportional to
Response to Electric and Magnetic Fields
External Electric Field and Dielectric Properties of LC molecules
Anisotropy: Dielectric Constant++
+++
- -- --
E
e
e
= - > 0De e e
E
= - < 0De e e
++++
----
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
oE
E
+ + ++
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
(“kappa”) = “dielectric constant”
= (a pure number ≥ 1)
So,
dA
C (for parallel plates)
Or 0
C CWhere 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
( )S T
Die
lect
ric C
onst
ant
i s
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
4’-pentyl-4-cyanobiphenyl
CH3-(CH2)4 C N
( )S T
//1
23
Temperature Dependence
Average Dielectric Anistropy
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
Magnetic Anisotropy: Diamagnetism
Diamagnetism: 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 ll
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 g
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, = > 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
27
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 field
Induced dipole by electromagnetic wave
Propagation of the light parallel to the molecular axis
Change of the speed is relatively small
// = C// /
//
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 n
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 . 4
5 0 4 0 3 0 0
T - T N I ( ° C )
Inde
x of
Ref
ract
ion
2 0 1 0
n e
n o
n i s o
2 220
12
3 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
12
3 en n n
Average Index
TemperatureDependence
( )n S T
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
y
z• 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
55
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 N
O
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*H
CH2
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)4C N
CH3-(CH2)4-O C N
4’-pentyl-4-cyanobiphenyl
4’-pentoxy-4-cyanobiphenyl
Nomenclature
Common molecules which exhibit a LC phase
Structure - Property
N
N
CH3-(CH2)4C 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)4COO
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
