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Chapter 2: Liquid Crystals
States 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
Phase Transition first defined by
Georges Freidel in 1922
C
CO2H
S
84.5o
N
128o
I
139.5o
The ordering parameter
S=1/2<3cos2Q-1>
S=0, isotropic
S=1, Ordered
Nematic, 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 Viscosity
Fast Response Time
Discotic Liquid Crystals
Response to Electric and Magnetic Fields
External Electric Field and Dielectric Properties of LC molecules
Dielectric Constant
ke0L = C = q/V
Flow of ions in the presence of electric field
Internal Field Strength E = E0 – E’
Alignment of LC molecules in Electric Field
S=0
1>S>0
Dielectric Anisotropy and Permanent Dipole Moment
m
m
Dielectric Anisotropy and Induced Dipole Moment
easily polarized
+
minduced is large
r//
eis large
-
+
r
Molecular axis
-
minduced is small
eis small
e
dielectric constant along the direction
perpendicular to the molecular axis
e
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, e = ee> 0 or ee
Positivee> 0 (10 to 20)
Negative e< 0 (-1 to -2)
For high electrical resistance materials, n is proportional
to e1/2
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
Examples
S
C N
C5H11
e = +33
C - N - I
76 98
O
N
C
O
C5H11
O
C7H15
e = - 4.0
C - N - I
45 101
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
Propagation of the light is hindered by the molecule
E field

e//
Induced dipole
by electromagnetic wave
Speed of the light is slowed down
 = C /e//
Propagation of the light parallel to the molecular axis
E field
Induced dipole
by electromagnetic wave
e
//
Change of the speed is relatively small
 // = C// /e
Circular Birefringence
Reflection of Circular Polarized Light
Devices for Liquid Crystal Display
Designs of LC cell
Electronic Drive
AM: 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 scanning
Vs: turn on voltage
Vns: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-Change
Guest-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 rigid
Linker: 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
Common components for LC molecules
Core Group
Linker A, B
-(CH=N)-; -(N=N)-(N=NO)-; -(O-C=O)Terminal Group X, Y
Non-polar flexible groups
-R, -OR, -O2CR
Polar rigid group
-CN, -CO2H, -NO2, -F, -NCS
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
like structure upon heating
(5) Chemically stable.
(6) Phase transition temperature is determined by H
and S. At TCN or TNI, Go = Ho –TSo= 0.
Therefore TCN= HoCN/SoCN and TNI= HoNI/SoNI
L
D
n
L/D > 4 Ti > Tm (nematic)
No. of Phenyl ring
2
3
4
5
6
L/D
2
3
3.9
4.8
5.5
Ti
77
213
320
445
565
Tm
388
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.
67 o
O
6-10 o
O
Flexible linker
O
O
L
n
D
Nematic phase could not
be observed until L/D >4
n
1
2
3
L/D
3 .8
5 .1
6 .4
Ti
132
254
464
Tm
176
220
6-10 o
67 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:
Crystal
TC-N
T
Nematic LC
TN-I
Isotropic Liquid
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.
(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.
O HO
R
R
OH O
R = C5H11
R = C8H17O
R = C10H21O
TCN = 88; TNI=126.5
TCS = 101; TSN = 108; TNI=147
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
C8H17O
N C
H
X
Terminal group for nematic
Short chain
MeO
N C
H
X
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
Tm
84
Ti
Ti
127
3.5
68
130 95
24
35
204
130 34
239
71 (52)
Stereochemistry of alicyclic systems
H
R
NC
H
H
H
NC
R
No LC phase
Tm
C
N
Ti
C5H11
CN
48
61
C5H11
CN
24
35
C5H11
CN
31
55
C5H11
CN
62
100
I
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.
Heteroatom
effects
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.
MM2 space-filling models
Flat molecule
TNI = 55 oC
Unsymmetrical
TNI = 19 oC
Symmetrical but
rings are
perpendicular
TNI = 28 oC
The TCN and TNI orders: dicyclooctane > cyclohexane > phenyl
MM2 calculation
Linear
structure
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.
R
(CH2)n
R
where
R=
N C
H
OCH3
n
0
1
2
3
4
5
Tm
266
171
156
-
Ti
>390
312
270
-
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.
O
C5H11
O
CN
Tm
Ti
48
79
30
51
C5H11
CN
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.
X
C8H17O
CO2H
Not quite linear for
some substituents
Electronic effects arising from the lateral groups
Mixing of two Components may generate a LC phase
RO
CO2H
R = Me or Et
Doesn't show LC properties
O HO
OR or R'
RO
OH O
LC
Mixture of two Components
RO
N
C4H9
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
Left
Cl
H
Right
CH3(CH2)12CO2
H
Main Chain Liquid Crystal Polymer
mesogenic unit
flexible linker
Side Chain Liquid Crystal Polymer
Polymer
Backbone
Polymer
Backbone
Laterally attached
Terminally attached
Combined Liquid Crystal Polymer
Lyotropic Liquid Crystal Polymers
Fairly rigid rod like polymers; but soluble in certain
solvents to form a LC phase
O
HN
O
O
NH
Kelver
HN
PBA
Dissolve and
LC formation
Fiber formation to give high tensile strength fibers
Common Components for Lyotropic Liquid Crystals
N
O
N
S
O
N
S
N
Examples
N
S
S
N
n
Poly(p-phenylenebenzobisthiazole) PBT
Soluble in PPA or H2SO2 and could be fabricated as high
tensile strength polymeric wires
N
O
O
N
n