Chapter 2: Liquid Crystals
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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.

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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.



Phase Transition first defined by Germany

Georges Freidel in 1922


The ordering parameter Germany

S=1/2<3cos2Q-1>

S=0, isotropic

S=1, Ordered

Nematic, S=0.5-0.6


Classification of Germany

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.









Advantages of Nematic Phase and Cholesteric Phase LC Germany

For Display Propose

Low Viscosity

Fast Response Time





Dielectric Constant molecules

ke0L = C = q/V


Flow of ions in the presence of electric field molecules

Internal Field StrengthE = E0 – E’



m molecules

m

Dielectric Anisotropy and Permanent Dipole Moment



Examples molecules



Light as Electromagnetic Wave molecules

Plane Polarized light can be resolved into Ex and Ey


Birefringence molecules


O moleculesrdinary 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 lighttravels in the crystal

with a speed v that varies with direction.

The refractive index n0=c/v also varies

with different direction







Designs of LC cell molecules

Electronic Drive

AM: active matrix; TFT: thin film transistor;

MIM: metal-insulator-metal





Optical Response of a Twisted Nematic (TN) Device molecules

Applied voltages and optical response


Super Twisted Nematic (STN) LC Device 1984 by Scheffer molecules

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




Black and White type)

RF-STN Device


Optical response of Nematic LC in a Phase-Change type)

Guest-Host Type Device (by G. Heilmeier)







Structure of a typical type)

LC Display


Hybrid Aligned Nematic (HAN) type type)

Fast response time,

Upto ms scale.



  • References type)

  • 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.


Structure of rigid rod like liquid crystal molecules type)

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 type)

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 type)

  • 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


L type)

D

L/D > 4 Ti > Tm (nematic)


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 interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.o

6-10 o

Flexible linker

D

L

Nematic phase could not be observed until L/D >4


67 interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.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


Other Options for the core group interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears..


Thermal Stability: interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

DT

TN-I

TC-N

Crystal

Nematic LC

Isotropic Liquid

Low TC-N; high TN-I

largerDT = 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.


  • Nematogenic interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.: 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.


Smectic Phase interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

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.


Terminal groups for smectic phase interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

  • Salts from RCO2H/RNH2

  • Terminal groups contain -CO2R, -CH=CHCOR, -CONH2, -OCF3, -Ph, -NHCOCH3, -OCOCH3


Terminal group for nematic interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

Short chain


For Smectic Phase interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

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 interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears..

  • 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.


  • At least interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.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.


No LC phase interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

Stereochemistry of alicyclic systems


Heteroatom interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

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 interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.


The T interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.CN and TNI orders: dicyclooctane > cyclohexane > phenyl


MM2 calculation interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

Linear

structure

Bent structure


Extending the number of the rings interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.


Linking group: interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.

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.


Odd number of CH interactions that lead to thermal stability of the nematic phase increases. When L/D goes over the critical value, nematic phase appears.2: 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.


Terminal Flexible Long Chain conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.:

The function of the terminal flexible long chain is to suppress the melting point without serious destroying the LC phase.


Lateral Substitution conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

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.


Not quite linear for some substituents conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.


Electronic effects arising from the lateral groups conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.


Mixing of two Components may generate a LC phase conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.


Mixture of two Components conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

A mixture of MBBA (60%) and EBBA (40%) would lead to LC at room temperature


Temperature Dependent Rotation of the Cholesteric Phase conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.


Lyotropic Liquid Crystal Polymers conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

Fairly rigid rod like polymers; but soluble in certain solvents to form a LC phase


Examples conjugation extended over the longer molecules. This could enhance the polarizability anisotropy.

Poly(p-phenylenebenzobisthiazole) PBT

Soluble in PPA or H2SO2 and could be fabricated as high tensile strength polymeric wires


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