Post on 15-Jan-2017
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
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
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R
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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 CrystalsLiquid crystals
Lyotropic Thermotropic
Calamitic Polycatenar Discotic Banana-shaped
Nematic (N)
Smectic (S)
Nematic Discotic(ND)
Columnar (Col)
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)
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.
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?
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