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

Phase Transition first defined by Georges Freidel in 1922

The ordering parameter S=1/2<3cos2Q-1> S=0, isotropic S=1, Ordered Nematic, S=0.5-0.6

Smectic Liquid Crystals 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

For Display Propose Low Viscosity Fast Response Time 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

Dielectric Anisotropy and Induced Dipole Moment

Examples

Magnetic Susceptibility and Anisotropy

Light as Electromagnetic Wave Plane Polarized light can be resolved into Ex and Ey

Birefringence

Extraordinary light travels in the crystal 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

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 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 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 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 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 o 6-10 o Flexible linker L D Nematic phase could not be observed until L/D >4

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

Other Options for the core group.

Thermal Stability: DT TC-N TN-I 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.

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.

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.

Terminal groups for smectic phase Salts from RCO2H/RNH2 Terminal groups contain -CO2R, -CH=CHCOR, -CONH2, -OCF3, -Ph, -NHCOCH3, -OCOCH3

Terminal group for nematic Short chain

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. 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 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.

Stereochemistry of alicyclic systems No LC phase

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

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.

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.

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.

Not quite linear for some substituents

Electronic effects arising from the lateral groups

Mixing of two Components may generate a LC phase

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

Temperature Dependent Rotation of the Cholesteric Phase

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

Examples Poly(p-phenylenebenzobisthiazole) PBT Soluble in PPA or H2SO2 and could be fabricated as high tensile strength polymeric wires