Chapter 2: 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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Phase Transition first defined by Georges
Freidel in 1922
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The ordering parameter S1/2lt3cos2Q-1gt S0,
isotropic S1, Ordered Nematic, S0.5-0.6
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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 Viscosity Fast 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
ke0L C q/V
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Flow of ions in the presence of electric field
Internal Field Strength E E0 E
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Alignment of LC molecules in Electric Field
S 0 1 gt S gt 0
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Dielectric Anisotropy and Permanent Dipole Moment
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Dielectric Anisotropy and Induced Dipole Moment
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Examples
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Magnetic Susceptibility and Anisotropy
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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 the crystal with the
same speed v in all direction. The refractive
index n0c/v in all direction are identical.
Extraordinary light travels in the crystal with
a speed v that varies with direction. The
refractive index n0c/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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Circular Birefringence
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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
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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
NNumber of row for scanning Vs turn on
voltage Vnsturn off voltage
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Sharp change in the voltage-transmittance curve
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Electrically Controlled Birefringence (ECB)
Device (DAP type)
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Black and White RF-STN Device
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Optical response of Nematic LC in a
Phase-Change Guest-Host Type Device (by G.
Heilmeier)
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Phase Change (PC) in a Guest Host (GH) LC Device
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In-Plane Switching (IPS) type LC Device
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Polymer Dispersed Liquid Crystal (PDLC) Device
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Polymeric Nematic LC Materials
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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
Fast response time, Upto ms scale.
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Full color reflective display
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• References
• Liquid Crystals, P. J. Collings, Princeton
• Introduction to liquid crystals, P. J. Collings
  and M. Hird, Taylor and Francis
• Flat Panel Displays, J. A. Connor, RSC.

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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 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
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Common components for LC molecules
Core Group
Linker A, B -(CHN)- -(NN)- -(NNO)- -(O-CO)-
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
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• Character of LC molecules
• Rod like or Discotic
• 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)
• Having polar or highly polarizable moiety
• Large enough rigidity to maintain the rod or
  discotic like structure upon heating
• Chemically stable.
• Phase transition temperature is determined by DH
  and DS. At TCN or TNI, DGo DHo TDSo 0.
  Therefore TCN DHoCN/DSoCN and TNI DHoNI/DSoNI

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L
D
L/D gt 4 Ti gt Tm (nematic)
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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.
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67 o
6-10 o
Flexible linker
D
L
Nematic phase could not be observed until L/D gt4

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67 o
6-10 o
This type of linker group is more flexible.
Entropy gain is more effective in isotropic
liquid state. Therefore DSN-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
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Other Options for the core group.
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Thermal Stability
DT
TN-I
TC-N
Crystal
Nematic LC
Isotropic Liquid
Low TC-N high TN-I larger DT 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.
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• Nematogenic structures that lead to nematic
  phase as the only LC phase
• Smectogenic smectic phase is the only mesophase
  exhibited
• Calamitic Both nematic and smectic phases would
  exhibited.

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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 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.
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Terminal groups for smectic phase

• Salts from RCO2H/RNH2
• Terminal groups contain -CO2R, -CHCHCOR, -CONH2,
  -OCF3, -Ph, -NHCOCH3, -OCOCH3

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Terminal group for nematic
Short chain
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For Smectic Phase
NHCOCH3 gt Br gt Cl gt F gt NMe2 gt RO gt H gt NO2 gt OMe
For nematic Phase
NHCOCH3 gt OMe gt NO2 gt RO gt Br Cl gt NMe2 gt Me gtF
gt H
-CN,-NO2 -MeO are nematogen poor smectic/good
nematic -NHCOCH3, halogen, -NR2, good
smectic/nematic
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Nematic Phase.

• Due to its fast response time, the nematic LC
  phase is technologically the most important of
  the many different types of LC phase
• The smectic phases are lamellar in structure and
  more ordered than the nematic phase.
• The smectic phases are favored by an symmetrical
  molecular structure.
• 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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• At least two rings are required to enable the
  generation of LC phase.
• The nematic phase tends to be the phase
  exhibited when the conditions for the lamellar
  packing (smectic) cannot be met.
• Molecular features for nematic phase (a)
  breaking of the symmetry or (b) short terminal
  chain.

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Stereochemistry of alicyclic systems
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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.
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MM2 space-filling models
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The TCN and TNI orders dicyclooctane gt
cyclohexane gt phenyl
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MM2 calculation
Linear structure
Bent structure
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Extending the number of the rings
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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.
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Odd number of CH2 Bent
Even number of CH2 Linear
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(b) Linker groups that connect aromatic core
units with the conjugation extended over the
longer molecules. This could enhance the
polarizability anisotropy.
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Terminal Flexible Long Chain
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
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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Not quite linear for some substituents
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Electronic effects arising from the lateral groups
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Mixing of two Components may generate a LC phase
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Mixture of two Components
A mixture of MBBA (60) and EBBA (40) would lead
to LC at room temperature
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Temperature Dependent Rotation of the Cholesteric
Phase
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Lyotropic Liquid Crystal Polymers Fairly rigid
rod like polymers but soluble in certain
solvents to form a LC phase
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Examples
Poly(p-phenylenebenzobisthiazole) PBT Soluble
in PPA or H2SO2 and could be fabricated as high
tensile strength polymeric wires