chemistry of LC

1
Liquid Crystal Materials
2
Broad Classification
Lyotropics Thermotropics
molecules consisting of a rigid core and
flexible tail(s) form liquid crystal phases over
certain temperature ranges.
amphiphilic molecules, polar and non-polar parts
form liquid crystal phases over
certain concentration ranges when mixed with a
solvent
hydrophobic non-polar tail
flexible tail
-
hydrophilic polar head
rigid core
3
The Lyotropic Phases
micelle
cross section
reverse micelle
cross section
4
The Thermotropic Liquid Crystal Molecule
Physicists Engineers View
Chemists View

• Shape Anisotropy

• Length gt Width

The molecule above (5CB) is 2 nm 0.5 nm
5
Geometrical Structures of Mesogenic Molecules
Low Molecular Weight High Molecular
Weight
(polymers)
( )
disk-like rod-like
n
( )
n
most practical applications
6
The Liquid Crystal Phase
n
Crystal Nematic LC Isotropic
Temperature
7
The Nematic Director n
n
director
The local average axis of the long molecular axis
8
Other Liquid Crystal Phases
n
z
n
n
q
Smectic C Smectic A
Nematic
Temperature
9
Chirality
The methyl group on the 2nd carbon atom on the
alkyl chain of the molecules extends out of the
plane of the paper and the hydro- gen atom
extends into the plane of the paper. Therefore
the 2nd carbon can be thought of as a right or
left handed coordinate system
left-handed right-handed
H
H
H
H
H
mirror images
C N
H-C-C-C-C-C
H
H
H
H
H
non-chiral
H
H
H
H
H
C N
H-C-C-C-C-C
CH3
non-superimposable
chiral (RH)
H
H
H
H
10
The Chiral Nematic
Ordinary Nematic
Chiral Nematic
CN
director
pitch
n
P
11
The Chiral Doped Nematic
You can create a cholesteric material by doping a
conventional nematic with a chiral dopant.
For dilute solutions
For a 10 doping of S-811
Chiral Dopant HTP (mm)-1 S-811
-14 IS-4651 -13.6 - indicates
left twist sense
12
The Chiral Smectic C Ferroelectrics
q
m
Eye- dipole moment m fin -
chiral ferroelectric LC has a dipole moment
perp- endicular to its long axis, and is chiral.
13
The Chiral Smectic TGB
Twisted Grain Boundary (TGB)
A twisted grain boundary smectic A phase
(frustrated) - TGBA
14
R
Discotic Liquid Crystal
C
C
R
O
O
R
C
O
C
O
O
O
C
R
O
O
O
O
C
example ROCOC11H23
R
C
O
R
15
Discotics Liquid Crystals
n
n
Nematic discotic phase
Columnar, columns of molecules in
hexagonal lattice
16
Polymer Liquid Crystals
Combining the properties of liquid crystals and
polymers
Main Chain
Side Chain
mesogenic moieties attached as side chains on the
polymer backbone
mesogenic moieties are connected head-to-tail
rigid
semi-flexible
17
Polymer Liquid Crystals
forming nematic liquid crystal phases
n
side-chain
main-chain
18
Example of Side-Chain Polymer LCs
R1
-(-CH2-C-)X-
O
O C-O-(CH2)n-O
R2
C-O

• Too slow for display applications (switching
  times 0.5-1 s
• Useful for other applications such as
• Optical filters
• Optical memory
• Alignment layers for low molecular weight LCs
• Non-linear optic devices (optical computing)

19
The Order Parameter
n

q
no order perfect order
n
perfect crystal isotropic fluid
20
Maier-Saupe Theory - Mean Field Approach
Interactions between individual molecules are
represented by a potential of average force

n
y
q

• V minimum when phase is ordered (?-P2(cosq))
• V V0 when phase is disordered (?ltP2(cosq)gt)
• factor for intermolecular strength (? n)

f
From Statistical Mechanics (Self Consistency)
b(kT)-1
21
Maier-Saupe Theory - Mean Field Approach
1.0
Isotropic Fluid
Nematic Liquid Crystal
Order Parameter, S
0.0
-0.6
Temperature
22
Landau-de Gennes Theory
aao(T-T), ao, b, c, T, L are phenomenological
constants
G is a surface interaction strength
Good near NI transition
surface
Order Parameter, S
Predicts order near surface
Temperature
23
The Order Parameter How does it affects display
performance ?
The order parameter, S, is proportional to a
number of important parameters which dictate
display performance.
proportional to
Parameter Nomenclature
? Elastic Constant Kii S2 Birefringence
Dn S Dielectric Anisotropy De
S Magnetic Anisotropy Dc S Viscosity
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
24
Anisotropy Dielectric Constant
Off-axis dipole moment, angle b with molecular
axis
b
N number density h,f reaction field, reaction
cavity parameters S order parameter Da
anisotropy in polarizability m molecular
dipole moment kB Boltzman constant T
Temperature
For values of the angle blt54.7o, the dipolar term
is positive, and for values bgt54.7o, the dipolar
term is negative, and may result in a materials
with an overall -De.
25
Anisotropy Dielectric Constant
E

e


positive
- -
-
- -
e
De e - e gt 0
E
E

- - - -
negative
all angles in the plane ? to E are possible for
the -De materials
De e - e lt 0
26
Anisotropy Duel Frequency
high frequency, Delt0
low frequency, Degt0
MLC-2048 (EM Industries), Duel Frequency
Material Frequency (kHz) 0.1 1.0 10 50 100 D
ielectric Anisotropy (De) 3.28 3.22 0.72 -3.0 -3.
4
27
Dielectric Constants (_at_20oC, 1kHz)
Mixture Application De e?? e? BL038 PDLCs
16.7 21.7 5.3 MLC-6292 TN AMLCDs 7.4 11.1 3.7
ZLI-4792 TN AMLCDs 5.2 8.3 3.1 TL205 AM
PDLCs 5 9.1 4.1 18523 Fiber-Optics 2.7 7 4.
3 95-465 -De material -4.2 3.6 7.8
EM Materials
28
Dielectric Constants Temperature Dependence
4-pentyl-4-cyanobiphenyl
Temperature Dependence
Average Dielectric Anistropy
29
Magnetic Anisotropy Diamagnetism
Diamagnetism induction of a magnetic moment in
opposition to an applied magnetic field. LCs
are diamagnetic due to the dispersed electron
distribution associated with the electron
structure.
Delocalized charge makes the major contribution
to diamagnetism. Ring currents associated
with aromatic units give a large negative
component to c for directions ? to aromatic
ring plane. Dc is usually positive since
30
Magnetic Anisotropy Diamagnetism
Compound
31
Optical Anisotropy Birefringence
ordinary ray (no, ordinary index of refraction)
extraordinary ray (ne, extraordinary index
of refraction)
32
Optical Anisotropy Birefringence
ordinary wave
extraordinary wave
optic axis
q
For propagation along the optic axis, both modes
are no
33
Optical Anisotropy Phase Shift
f 2pdno,e/l Df fe - fo2pdDn/l
Dn ne - no 0 lt Dn lt 0.2 depending on
deformation 380 nm lt l lt 780 nm
visible light
analyzer
liquid crystal
polarizer
light
34
Birefringence (20oC _at_ 589 nm)
EM Industry Dn ne no
Application Mixture BL038 0.2720 1.7990
1.5270 PDLC TL213 0.2390 1.7660 1.5
270 PDLC TL205 0.2175 1.7455 1.5270 AM
PDLC ZLI 5400 0.1063 1.5918 1.4855 STN ZLI
3771 0.1045 1.5965 1.4920 TN ZLI
4792 0.0969 1.5763 1.4794 AM TN
LCDs MLC-6292 0.0903 1.5608 1.4705 AM TN
LCDs ZLI 6009 0.0859 1.5555 1.4696 AN TN
LCDs MLC-6608 0.0830 1.5578 1.4748 ECB 95-465
0.0827 1.5584 1.4752 -De devices MLC-6614 0.077
0 --------- --------- IPS MLC-6601 0.0763 ----
----- --------- IPS 18523 0.0490 1.5089 1.459
9 Fiber Optics ZLI 2806 0.0437 1.5183 1.4746
-De device
35
Birefringence Temperature Dependence
Average Index
Temperature Dependence
36
Birefringence Example 1/4 Wave Plate
circular polarized
What is minimum d for liquid crystal 1/4 wave
plate ?
linear polarized
Unpolarized
d
LC Dn0.05
polarizer
Takes greater number of e-waves than o-waves to
span d, use Dn0.05
37
Nematic Elasticity Frank Elastic Theory
Bend, K
Twist, K
Splay, K
33
22
11
38
Surface Anchoring
Alignment at surfaces propagates over macroscopic
distances
microgrooved surface - homogeneous alignment
(//) rubbed polyimide
ensemble of chains - homeotropic alignment
(?) surfactant or silane
39
Surface Anchoring
N
polar anchoring Wq
q
n
surface
f
azimuthal anchoring Wf
Wq,f is energy needed to move director n from
its easy axis
Strong anchoring 10-4 J/m2 Weak anchoring
10-7 J/m2
40
Creating Deformations with a Field and Surface -
Bend Deformation
E or B
41
Creating Deformations with a Field and Surface -
Splay Deformation
E or B
42
Creating Deformations with a Field and Surface -
Twist Deformation
E or B
43
Magnitudes of Elastic Constants
EM Industry K11 K22 K33 Mixture (pN) (pN) (
pN) Application BL038 13.7 ------ 27.7 PDLC TL2
05 17.3 ------ 20.4 AM PDLC ZLI
4792 13.2 6.5 18.3 TN AM LCD ZLI
5400 10 5.4 19.9 TN ZLI-6009 11.5 5.4 16.0 AM
LCD
Order of magnitude estimate of elastic
constant U intermolecular interaction
energy a molecule distance
44
Elastic Constant K22 Temperature Dependence
45
The Flexoelectric Effect
-
-
Undeformed state of banana and pear
shaped molecules
Polar structure corresponds to closer packing of
pear and banana molecules
Bend
Polar Axis
Splay
46
Effects of an Electric Field
n
y
E
x
q
e
e
Electric Free Energy Density
Electric Torque Density
Using De 5 and E0.5 V/mm
47
Effects of an Magnetic Field
y
n
B
x
q
c
c
Magnetic free energy density
Magnetic torque density
Using Dc 10-7 m3kg-1 and B 2 T 20,000 G
48
Deformation Torque
Orientation of molecules obeys this eq.
Free energy density from elastic theory
Torque density
49
Deformation Torque
Surface
q
x
d
Material Shear Modulus Steel
100 GPa Silica 40 GPa Nylon
1 GPa
Shear modulus ? Youngs modulus
50
Coherence Length Electric or Magnetic
E
Balance torque
Find distance d
Coherence length x
Using E 0.5 V/mm and De 20
51
Viscosity Shear Flow Viscosity Coefficient
h33
h11
h22
n
n
n
n?
n ??
n ??
n ?
Typically h22 gt h33 gth11
52
Viscosity Flow Viscosity Coefficient
LC specification sheets give kinematic viscosity
in mm2/s
Kinematic Viscosity (n) 1 m2/s
Dynamic Viscosity (h) 1 kg/ms 1 Pas 0.1
kg/ms 1 poise
Approximate density
53
Viscosity Flow Viscosity Coefficient
Typical Conversion Density
Conversion Flow
r 0.1 kg/ms 1
poise Viscosity
EM Industry Kinematic (n)
Dynamic (h) MIXTURE CONFIGURATION (mm2/s)
(Poise) ZLI-4792 TN AM LCDs 15
0.15 ZLI-2293 STN 20
0.20 MLC-6610 ECB 21
0.21 MLC-6292 TN AM LCDs (Tc120oC) 28
0.28 18523 Fiber Optics (no1.4599) 29
0.29 TL205 PDLC AM LCD 45
0.45 BL038 PDLCs (Dn0.28) 72 0.72
54
Viscosity Temperature Dependence
N
C4H9
H3CO
For isotropic liquids
E is the activation energy for diffusion of
molecular motion.
55
Viscosity Rotational Viscosity Coefficient
n
Rotation of the director n bv external fields
(rotating fields or static). Viscous torque's Gv
are exerted on a liquid crystal during rotation
of the director n and by shear flow.
n
Time
n
g1 rotational viscosity coefficient
56
Viscosity Rotational Viscosity Coefficient
n
n
n
EM Industry Viscosity Viscosity MIXTURE
CONFIGURATION (mPa?s)
(Poise) ZLI-5400 TN LCDs 109
1.09 ZLI-4792 TN AM LCDs 123
1.23 ZLI-2293 STN 149
1.49 95-465 -De Applications 185
1.85 MLC-6608 TN AM LCD 186
1.86
57
Viscosity Comparisons
Material Viscosity (poise) Air 10-7 Water
10-3 Light Oil 10-1 Glycerin 1.5 LC-Rotationa
l (g1) 1lt g1 lt 2 LC-Flow (hii) 0.2lt hiilt1.0
58
Relaxation from Deformation
field on state
E
Surface
x
Relaxation when field is turned off
Relaxation time t
Surface
zero field state
x
59
Relaxation from Deformation
Balance viscous/deformation torque
Assume small deformations
Solution
For 100 mm cell
For 5 mm cell
60
Freedericksz Transition - The
Threshold I
y
E
n
z
E
Ec
q
x
y
x
d
n
At some critical E field, the director rotates,
before Ec nothing happens
0
0
61
Freedericksz Transition - The
Threshold II
E-field free energy
total free energy
Minimize free energy with Euler
Equation
62
Freedericksz Transition - The
Threshold III
differential equation
soln. small q
threshold
mid-layer tilt (deg)
1.0 E/Ec
63
Defects
s1/2
s-1
s-1/2
s1
s1
s1
The singular line (disclination) is pointing out
of the page, and director orientation changes
by 2ps on going around the line (s is the
strength)
s2
s3/2
64
Estimate Defect Size
The simplest hypothesis is that the core or
defect or disclination is an isotropic liquid,
therefore the core energy is proportional to
kBDTc. Let M be the molecular mass, N
Avogadadros number and r the density of the
liquid crystal.
65
Microscopic Fluttering and Fluctuations

• Characteristic time t of
• Fluctuations
• Can see fluctuations with
• microscope
• Responsible for opaque
• appearance of nematic LC

Thermally induced Deformations
66
General Structure
Z
Z
Y
A
X

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

C N
CH3 - (CH2)4
4-pentyl-4-cyanobiphenyl (5CB)
67
Common Groups
Mesogenic Core
Linking Groups Ring Groups
biphenyl terphenyl diphenylethane stilbene tolane
schiffs base azobenzene azoxyben- zene phenylbenzo
ate (ester) phenylthio- benzoate
phenyl
N
pyrimidine
N
cyclohexane
68
Nomenclature
Mesogenic Core
terphenyl
biphenyl
phenyl benzyl benzene
phenylcyclohexane (PCH)
cyclohexane cyclohexyl
3
2
3
2
Ring Numbering Scheme
1
1
4
4
6
5
6
5
69
Terminal Groups (one
terminal group is typically an alkyl chain)
CH2
CH2
straight chain branched chain (chiral)
CH2
CH3
CH2
CH2
CH
CH3
CH3
Attachment to mesogenic ring structure Direct
- alkyl (butyl) Ether -O-
alkoxy (butoxy)
70
Terminal Groups
CH3-
methyl
CH3-O-
methoxy
CH3-CH2-
ethyl
CH3-CH2-O-
ethoxy
CH3-(CH2)2-
propyl
CH3-(CH2)2-O-
propoxy
CH3-(CH2)3-
butyl
CH3-(CH2)3-O-
butoxy
CH3-(CH2)4-
pentyl
CH3-(CH2)4-O-
pentoxy
CH3-(CH2)5-
hexyl
CH3-(CH2)5-O-
hexoxy
CH3-(CH2)6-
heptyl
CH3-(CH2)6-O-
heptoxy
CH3-(CH2)7-
octyl
CH3-(CH2)7-O-
octoxy
71
Second Terminal Group and Lateral Substituents (Y
Z)
H - F flouro Cl chlor
o Br bromo I iodo CH3 methyl CH3(CH2)n alky
l CN cyano NH2 amino N(CH3) dimethylamino NO2
nitro phenyl cyclohexyl
72
Odd-Even Effect
Clearing point versus alkyl chain length
O
CH3-(CH2)n-O
O-(CH2)n-CH3
C-O
18 16 14 12 10
clearing point
0 1 2 3 4 5 6 7 8 9 10 11
carbons in alkyl chain (n)
73
Nomenclature
Common molecules which exhibit a LC phase
4-pentyl-4-cyanobiphenyl
4-pentoxy-4-cyanobiphenyl
74
Structure - Property
vary mesogenic core
A
C N
CH3-(CH2)4
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
75
Structure - Property
vary end group
COO
X
CH3-(CH2)4
X C-N (oC) N-I (oC)
H F Br CN CH3 C6H5
87.5 92.0 115.5 111.0 106.0 155.0
114.0 156.0 193.0 226.0 176.0 266.0
76
Lateral Substituents (Z Z)
Z
Z
A
X
Y

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

77
Why Liquid Crystal Mixtures
Melt Temperature Liquid Crystal-Solid ln ci
DHi(Teu-1 - Tmi-1)/R DH enthalpies Teu
eutectic temperature Tmi melt temperature R
constant Nematic-Isotropic Temperature
TNI TNI S ciTNIi
E
Isotropic Liquid
Liquid Crystal
Temperature
eutectic point
Solid
0 50
100
Concentration (c2),
78
EM Industry Mixtures
S-N lt-40 C solid nematic transition (lt means
supercools) Clearing 92 C nematic-isotropic
transition temperature Viscosity (mm2
/s) flow viscosity, some materials may
stipulate the 20 C 15 rotational
viscosity also. May or may not give 0 C
40 a few temperatures K33/K11
1.39 ratio of the bend-to-splay elastic
constant De
5.2 dielectric anisotropy Dn
0.0969 optical birefringence (may or may not give
ne, no) dDn (mm) 0.5 product of dDn
(essentially the optical path length) dV/dT
(mV/oC) 2.55 how drive voltage changes as
temperature varies V(10,0,20)
2.14 V(50,0,20) 2.56 threshold voltage
( transmission, viewing angle, V(90,0,20)
3.21 temperature)
79
EM Industry Mixtures
Property ZLI 4792 MLC 6292/000 MLC
6292/100 S-N lt-40 C lt-30 C
lt-40 C Clearing 92 C 120 C 120
C Viscosity (mm2 /s) 20 C 15
28 25 0 C 40 95 85 -20 C
160 470 460 -40 C 2500 7000 7000
K33/K11 1.39 ------- ------ De
5.2 7.4 6.9 Dn
0.0969 0.0903 0.1146 dDn (mm)
0.5 0.5 0.5 dV/dT (mV/C)
2.55 1.88 1.38 V(10,0,20)
2.14 1.80 1.38 V(50,0,20)
2.56 2.24 2.25 V(90,0,20)
3.21 2.85 2.83
80
Summary of Fundamentals

• Thermotropic Liquid Crystal
• Anisotropy
• Nematic phase
• Chirality
• Order parameters
• Dielectric Anisotropy
• Diamagnetism
• Birefringence
• Elastic constants
• Surface Anchoring
• Viscosity
• Threshold
• Defects
• Eutectic Mixture