Muhammad Minhas Azeem Roll No 7616

Govt College University Faisalabad

Liquid Crystals and their Applications

Department of Applied Chemistry

Learning Objectives What are liquid crystals?

What are stuctures of liquid crystals?

What does Thermotropic and Lyotropic mean?

The different types of liquid crystal and their arrangement?

Working of LCDs?

Applications of Liquid crystals?

States of mattters 1: Solid2 : Liquid 3: Gass4:Liquid crystals

1 :Solid o All Soilds have fix shape, fix volume, ductile and high melting and boiling point.

oIt is due to solids have high attrective forces among the atoms and molecules which holds the atom together.

SiO² Quartz

Liquid Stucture of liquid

moleculeso Attractive forces between

the molecules is weak as compared to the soild but strong as compair to gass.

o The molecules of liquids have more empty spaces as compair to the soild , move aroud with each other but molecules remain close to each other so fluid donot have fix shape and adapted the shape of container.

Gas Stucture of gas

moleculeso Gases have less attractive

forces among the molecules is weak.

o The molecules are faraway from each.

o Gases move randomly so the molecules of gass will expand and fill the container.

Liquid CrystalsWhat is a liquid crystal?

What is so special about liquid crystals?

o A liquid crystals is a phase between solid and liquid states(phases)

Liquid Crystal

Melt

Solidify

Intermediate Phase

Heat

Cool

Heat

Cool

Examples Example of a compound that shows no LCs phase

Ice Cube Water Steamheat heat

solid crystalline water; 3- (dimensional)degrees of order

liquid water0 degrees of order

gaseous water 0 degrees of order

Example of a compound that shows LCs phasesCrystals of a solidorganic compound

Nematic liquidcrystals phase

Isotropic liquid

Crystals of a solidorganic compound

Smectic liquidcrystal phase

Isotropic liquid

heat heat

heat heat

3 degrees of orderLooks like milk1 degree of order

0 degrees of order

Phase change

A Brief History of LCsLiquid crystals was discoverd by Reinitzer and

Lehmann in 1888.cholesteryl benzoate showed two melting points each. The

crystal of this material melted at 145.5 oC into a cloudy fluid, which upon further heating to 178.5 oC became clear

Cholesterylbenzoate 145.5 0C 178.5 0Cheat heat

Lehmann observed by using polarised optical microscope “and observed crystal that were nearly liquid

RIGID FLEXIBLEFLEXIBLE

OO

R

O*****

*

*

*

R

O

History

In 1973 the discovery of the most technologically and commercial important class of liquid crystals , 4-alkyl-4-’cyanobiphenyl.This material found in calculators or mobile phones

NC C5H11

K N I35oC24oC

Reinitze

LCD: Multi Disciplinary Area of Research

Theory, lawand variousPhysicalproperties

Device (manufactures)Technological application

Preparation of varioustypes of liquid crystallinecompounds and characterisation

Crystals vs Liquid Crystalso A crystal is a highly ordered structure

which possesses long-range positional & orientational order

o For many substances these two types of order are destroyed simultaneously when the crystal melts to form a liquid

o For some substances, these orders are destroyed in stages. These are liquid crystals

Properties of liquid crystalso Liquid crystal can flow like a liquid, due to loss

of positional ordero Liquid crystal is optically birefringent, due to its

orientational ordero Transition from crystalline solids to liquid

crystals caused by a change of temperature – gives rise to THERMOTROPIC liquid crystals

o Substances that are most likely to form a liquid crystal phase at a certain temperature are molecules that are ELONGATED & have some degree of RIGIDITY

Typical chemical structures Cholesterol ester Phenyl benzoates Surfactants such as polyethylene-oxides, alkali soaps, ammonium salts, lecithin Paraffins Glycolipids Cellulose derivatives

Types of liquid crystals

Types of Liquid CrystalsLiquid crystals

Lyotropic Thermotropic

Calamitic Polycatenar Discotic Banana-shaped

Nematic (N)

Smectic (S)

Nematic Discotic(ND)

Columnar (Col)

Benzene-hexa-n-alkanoate derivatives

banana-shaped

 disk-like molecules

 calamitic LCs

Comparision betweenThermotropic and Lyotropic

o THERMOTROPICo Absence of solvento Rigid organic

moleculeso Depends on

Temperatureo Structures:

o Smectic o Nematico Cholesteric

LYOTROPICo In solvento Surfactantso Depends on

Temperature, Concentration, salt, alcohol

o Structures: Lamellar Hexagonal etc

LYOTROPIC LCsLyotropic LCs are two-component1: Hydrophilic polar“ head2: Hydrophobic “tailExamples molecules of soaps phospholipids (present in cell memberanes)

LYOTROPIC LCs

As temperature increases…o The first liquid crystal phase is the smectic A,

where there is layer-like arrangement as well as translational and rotational motion of the molecules.

o A further increase in temperature leads to the nematic phase, where the molecules rapidly diffuse out of the initial lattice structure and from the layer-like arrangement as well.

o At the highest temperatures, the material becomes an isotropic liquid where the motion of the molecules changes yet again.

The Arrangement of Molecules in the Nematic, Smectic, and Cholesteric Liquid Crystal Phases

Stuctural phases of liquid crystal

Nematico  Nematic drived from

the Greek  word, ‘nema’’ which means "thread".

o No possitional ordero Molecules in same

directiono When temperatures is

incrase molecules will be allign immediately

o In nematic crystal phase molecules are arranged paralell

Nematic liquid crystals are widely used in electro-optic display devices

The classical examples of LC displaying a nematic mesophase in the Cynobiphenyl

CNR

Cholesterico The first liquid crystal Cholestericthat was observed

through a polarising microscope is cholesteryl benzoate. Thus, CHOLESTERIC liquid crystal OR chiral nematic liquid crystal

E.g. cholesteryl benzoate: LC 147C, isotropic 186Co Cholesteric liquid crystals have great potential uses as

sensors Thermometer fashion fabrics that change colour with temperature display devices

In CHOLESTERIC phase, there is orientational order & no positional order, BUT, director is in HELICAL ORDER.

o The structure of cholesteric depends on the PITCH, the distance over which the director makes one complete turno One pitch - several hundred nanometers

o Pitch is affected by:- Temperature Pressure Electric & magnetic fields

SMECTICoSMECTIC phase occurs at temperature

below nematic or cholesteric

oMolecules align themselves approx. parallel & tend to arrange in layers

oNot all positional order is destroyed when a crystal melts to form a smectic liquid crystal

oChiral smectic C liquid crystals are useful in LCDS

Smectico SMECTIC phase occurs at

temperature below nematic or cholesteric

o Molecules align themselves approx. parallel & tend to arrange in layers

o Chiral smectic C liquid crystals are useful in LCDS

Eample: 4,4’’’ – Bis-nonyloxy-[1,1’;1’’;4’’,1’’’] quaterphenyl (2)

LIQUID CRYSTAL POLYMERS

o When liquid crystal polymers solidify, the liquid crystal structure ‘freeze in’

o This results in materials of high tensile strength & in some cases unusual electro-optical behaviour

o E.g. Kevlar aramid fibre – bullet-proof vest & airplane bodies (aromatic polyamide)

Technological Application

Liquid crystals can be found in the following devices: Digital watches Pocket TVs Gas pumps Parking meters Telecommunications Cell phones and pagers High-speed computing Digital signs Electronic games Personal digital assistants Electronic books Calculators Digital cameras and camcorders Fishfinders Thermometers

Where are liquid crystals used?

Applications Construction of LCD

Conclusion We know today that many chemical

compounds can exist in the liquid crystal state, such as cholesteryl benzoate. The world can focus on ways to make this product useful in society. Over the last century many applications such as the detection of hot points in microcircuits, the findings of fractures or tumors in humans and the conversion of infared images have become accessible due to the understanding of pitch in a liquid crystal.

References 1: P.G de Gennes, Port.j , 2010 “The Physics of Liquid Crystals”, ref 5. 2: M. J. Stephen, Excellent review of basic properties 3: J. P. Straley “Physics of liquid crystals”, Ref. [2]. 4: D. Fincham, rotational motion of linear molecules, 1984 , 47–48. 5: http://www.slideshare.net/Nawarajintermediate/liquid-crystal-and-its-application# 6: Stegemeyer H, Blumel T, Hiltrop K, Onusseit H and Porsch F, Liq. Cryst. 1986,1-28. 7: Tanimoto K and Crooker P,1985, Phys. Rev. A 32 1893-5. 8: Tsvetkov V, Acta Physicicochim (USSR) , 1942 (16- 132). 9: van der Meer B Wand Vertogen G, Phys. Lett, 1976. 59A( 279-81). 10: Wright D C and Mermin N D, Phys. Rev, 1985. A 31 3498-500. 11:Thoen J. 1988 Phys. Rev. A 37 1754-9 12 Tsvetkov V 1942 Acta Physicicochim (USSR) 16 132 13 Tanimoto K and Crooker P P 1984 Phys. Rev. A 29 1566-7 13 Tanimoto K,

Crooker P P and Koch G C 1985 Phys. Rev. A 32 1893-5