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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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C11H23O CO2H
C S N I
84.5o
128o
139.5o
Phase Transition first defined by
Georges Freidel in 1922
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The ordering parameter
S=1/2
S=0, isotropic
S=1, Ordered
Nematic, S=0.5-0.6
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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 ViscosityFast 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
k0L = C = q/V
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Flow of ions in the presence of electric field
Internal Field StrengthE = E0E
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S = 0 1 > S > 0
Alignment of LC molecules in Electric Field
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m m
Dielectric Anisotropy and Permanent Dipole Moment
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Dielectric Anisotropy and Induced Dipole Moment
easily polarized
Molecular axis
minduced is large is large
minduced 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
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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 generaln is a very important parameter for a LC device.
Larger the n value, thinner the LC device and faster the
response time
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O
S C NC5H11
= +33C - N - I
76 98
O
O C7H15
C
N
C5H11 = - 4.0
C - N - I
45 101
Examples
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Magnetic Susceptibility and Anisotropy
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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 magneticfield. Therefore the magnetic susceptibility is negative
//
large
small
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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 thecrystal with the same speed v in
all direction.
The refractive index n0=c/v in
all direction are identical.
Extraordinary lighttravels in the crystalwith a speed v that varies with direction.
The refractive index n0=c/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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E field
Induced dipole
by electromagnetic wave
Propagation of the light is hindered by the molecule
Speed of the light is slowed down
= C / //
//
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E field
Induced dipole
by electromagnetic wave
Propagation of the light parallel to the molecular axis
Change of the speed is relatively small
// = C// /
//
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Circular Birefringence
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R fl ti f Ci l P l i d Li ht
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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
S T i t d N ti (STN) LC D i 1984 b S h ff
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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
N:Number of row for scanning
Vs: turn on voltage
Vns:
turn off voltage
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Sharp change in the voltage-transmittance curve
Electrically Controlled Birefringence (ECB) Device (DAP type)
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Black and White
RF-STN Device
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Phase Change (PC) in a Guest Host (GH) LC Device
In Plane Switching (IPS) type LC Device
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In-Plane Switching (IPS) type LC Device
Polymer Dispersed Liquid Crystal (PDLC) Device
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Polymer Dispersed Liquid Crystal (PDLC) Device
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Polymeric Nematic LC Materials
Active Matrix LCD
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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
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Hybrid Aligned Nematic (HAN) type
Fast response time,Upto ms scale.
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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
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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 bereduced. 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
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Linker A, B
-(CH=N)-; -(N=N)-
-(N=NO)-; -(O-C=O)-
Terminal Group X, YNon-polar flexible groups-R, -OR, -O2CRPolar rigid group
-CN, -CO2H, -NO2, -F, -NCS
Core Group
Common components for LC molecules
Side Branch
-F, -Cl, -CN, -CH3
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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 Hand S. At TCN or TNI, Go = HoTSo= 0.Therefore TCN= HoCN/SoCN and TNI= HoNI/SoNI
L
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n
L/D > 4 Ti > Tm (nematic)
D
No. of Phenyl ring L/D Ti Tm
2 2 773 3 213
4 3.9 320
5 4.8 445 388
6 5.5 565 438
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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.
O 6-10 o67 o
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O
O
O
O
n DL
n L/D Ti Tm
1 3 .8 1322 5 .1 254 1763 6 .4 464 220
Nematic phase could not
be observed until L/D >4
Flexible linker
6 10
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6-10 o67 o
This type of linker group is more flexible. Entropy gain ismore 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 254o
C
Other Options for the core group.
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Other Options for the core group.
Thermal Stability:
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Thermal Stability:
Low TC-N; high TN-IlargerT = 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 LCmolecules will increase. This will lead to higher thermal
stability.
Crystal Nematic LC Isotropic Liquid
TC-N TN-IT
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(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.
S ti Ph
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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 packwell 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.
RO
OH
RO
HO
R = C5H11 TCN = 88; TNI=126.5
R = C8H17O TCS = 101; TSN = 108; TNI=147
R = C10H21O TCS = 97; TSN = 122; TNI=142
Terminal groups for smectic phase
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Terminal groups for smectic phase
(1) Salts from RCO2H/RNH2(2) Terminal groups contain -CO2R, -CH=CHCOR, -CONH2, -OCF3, -Ph,
-NHCOCH3, -OCOCH3
N CH
C8H17O X
T i l f ti
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N CH
MeO XShort chain
Terminal group for nematic
F S Ph
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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
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Nematic Phase.
(1) Due to its fast response time, the nematic LC phase is
technologically the most importantof the many differenttypes 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.
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(1) At least two rings are requiredto enable the generation of
LC phase.
(2) The nematic phase tends to be the phase exhibited whenthe conditions for the lamellar packing (smectic) cannotbe met.
(3) Molecular features for nematic phase: (a) breaking of the
symmetry or (b) short terminal chain.
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C N TiTm Ti
24 35
130 239
84 127
68 130
71 (52)
204
95
3.5
34
Stereochemistry of alicyclic systems
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NC R
H
H
NC H
H
R
No LC phase
Stereochemistry of alicyclic systems
C N ITm Ti
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CNC5H11
CNC5H11
CNC5H11
CNC5H11
C N I
48
24
31
62
61
35
55
100
Change in the core structure of one phenyl ring for a range ofnon-aromatic rings only leads to increasing Tm and Ti, indicating
that packing effect is more important than the polarizability effectfor nematic phase. The ring functions in a space-filling manner,preventing the molecule form tumbling and maintaining theorientational ordering.
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H
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Heteroatom
effects
The heteroatoms enhances the polarity and higher melting point are seen. Nematicphase 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
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UnsymmetricalTNI = 19
oCFlat moleculeTNI = 55
oCSymmetrical butrings areperpendicularTNI = 28
oC
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The TCN and TNI orders: dicyclooctane > cyclohexane > phenyl
MM2 calculation
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Linear
structure
Bent
structure
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Extending the number of the rings
Linking group:
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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.
(CH2)nR R
R = N CH
OCH3
where
n Tm Ti
0 266 >390
1 - -
2 171 312
3 - -
4 156 270
5 - -
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Odd number of
CH2: Bent
Even number
of CH2: Linear
(b) Linker groups that connect aromatic core units with
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the conjugation extended over the longer molecules.
This could enhance the polarizability anisotropy.
Other common linker groups
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O
O
O O
O
e.g.
C5H11
O CN
O
C5H11
CN
Tm Ti
48
30
79
51Amide linker cannot be used due to the strong hydrogen bond
interactions that lead to high melting temperature
Terminal Flexible Long Chain:
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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
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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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X Not quite linear for
some substituents
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CO2HC8H17Osome substituents
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Electronic effects arising from the lateral groups
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g g p
Mixing of two Components may generate a LC phase
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CO2HRO
R = Me or Et
Doesn't show LC properties
RO
O
OH
OR or R'
O
HO
LC
Mixing of two Components may generate a LC phase
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Mixture of two Components
N C4H9
RO
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
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Cl
H
CH3(CH2)12CO2
H
Left
Right
Temperature Dependent Rotation of the Cholesteric Phase
i Ch i i id C l l
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Main Chain Liquid Crystal Polymer
mesogenic unit flexible linker
Side Chain Liquid Crystal Polymer
Polymer
BackbonePolymer
Backbone
Terminally attached
Laterally attached
C bi d Li id C t l P l
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Combined Liquid Crystal Polymer
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Lyotropic Liquid Crystal Polymers
Fairly rigid rod like polymers; but soluble in certainsolvents to form a LC phase
NHHN
O O
Kelver
HN
O
PBA
Dissolve and
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Dissolve andLC formation
Fiber formation to give high tensile strength fibers
Common Components for Lyotropic Liquid Crystals
N
ON
O N
SN
S
Examples
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N
SN
Sn
Poly(p-phenylenebenzobisthiazole) PBT
Soluble in PPA or H2SO2 and could be fabricated as hightensile strength polymeric wires
N
ON
On