LIQUID CRYSTALSLIQUID CRYSTALSProperties Synthesis ApplicationsProperties Synthesis Applications

By: By: BarnaliBarnali DuttaDutta

ContentsContents•• DefinitionDefinition•• Historical backgroundHistorical background•• Characteristic featuresCharacteristic features•• Phases of liquid crystalsPhases of liquid crystals•• Classification of liquid crystalsClassification of liquid crystals•• Chemical structureChemical structure•• Polymer liquid crystalsPolymer liquid crystals•• Main chain polymer liquid crystalsMain chain polymer liquid crystals•• Temperature range problemsTemperature range problems•• Side chain polymer liquid crystalsSide chain polymer liquid crystals•• ApplicationsApplications•• Liquid crystal displaysLiquid crystal displays•• ConclusionConclusion

What are liquid crystals ?What are liquid crystals ?

Positional orderPositional orderOrientational Orientational orderorderBond Bond orientational orientational orderorder

AA thermodynamically stable phase thermodynamically stable phase characterized by anisotropy of properties characterized by anisotropy of properties without the existence of a threewithout the existence of a three--dimensional crystal lattice, generally lying in dimensional crystal lattice, generally lying in the temperature range between the solid the temperature range between the solid and isotropic liquid phaseand isotropic liquid phase

Historical backgroundHistorical background18491849: W. : W. HeintzHeintz found that upon found that upon heatingheating (at 52°C), (at 52°C), stearinstearin, softened, softenedand became cloudy, and at 62.5° it became clear. Duffy describedand became cloudy, and at 62.5° it became clear. Duffy described this this phenomenon as the phenomenon as the existence of two melting pointsexistence of two melting points

18501850: : MettenheimierMettenheimier found myelin to be found myelin to be birefringentbirefringent, a property , a property previously known to be possessed by crystals onlypreviously known to be possessed by crystals only

18881888: : ReinitzerReinitzer observed the observed the twotwo--meltingmelting--pointspoints phenomenon, phenomenon, birefringencebirefringence and the and the occurrence of iridescent colors between the two occurrence of iridescent colors between the two melting pointsmelting points in a material that we now know was in a material that we now know was cholesterylcholesterylbenzoatebenzoate

18891889: Lehmann carried out detailed investigation and reasoned that : Lehmann carried out detailed investigation and reasoned that the the birefringentbirefringent portions of the liquid must be crystalsportions of the liquid must be crystals. He first referred . He first referred to this material as to this material as liquid crystalsliquid crystals

Historical backgroundHistorical background18901890: : GattermannGattermann synthesized the first liquid crystals of synthesized the first liquid crystals of azoxybenzeneazoxybenzene, , with fully known structureswith fully known structures

19081908: : D. D. VorlaenderVorlaender, the first to synthesize a , the first to synthesize a thermotropicthermotropic smecticsmecticcompound, related compound, related liquid liquid crystallinitycrystallinity to chemical structuresto chemical structures, detected , detected polymorphismpolymorphism of liquid crystal stateof liquid crystal state

19221922: : Freidel Freidel identified and named the various microscopic textures identified and named the various microscopic textures ((nematicnematic, , smecticsmectic, and , and cholestericcholesteric), observed the ), observed the effects of electric and effects of electric and magnetic fieldsmagnetic fields

19301930: : FreederickszFreedericksz studied the transition from a studied the transition from a homogeneous to a homogeneous to a deformed structuredeformed structure at some critical value of applied field strengthat some critical value of applied field strength

19651965: : The International Liquid Crystal SocietyThe International Liquid Crystal Society by late Glenn Brown at by late Glenn Brown at Kent State UniversityKent State University

Lehmann and Lehmann and ReinitzerReinitzer are known as the are known as the grandfathers of liquid grandfathers of liquid crystal sciencecrystal science

Characteristic featuresCharacteristic featuresMesogensMesogens point along a common axis, the point along a common axis, the directordirector

OrientationalOrientational orderorder is between the traditional solid and liquid is between the traditional solid and liquid phasesphases

AnisotropicAnisotropic i.e i.e the properties differ depending on what direction the properties differ depending on what direction they are measured relative to the directorthey are measured relative to the director

BirefringentBirefringent,, resulting in the two components of electric or resulting in the two components of electric or magnetic field traveling through it at different speeds hence oumagnetic field traveling through it at different speeds hence out of t of phase when they exitphase when they exit

StiffnessStiffness necessary for liquid necessary for liquid crystallinitycrystallinity results from results from restrictions restrictions on rotationon rotation caused by caused by stericsteric hindrancehindrance and and resonanceresonance

Axial ratio Axial ratio (x= L/d) (x= L/d) must be at least three to display the must be at least three to display the characteristics of liquid crystalscharacteristics of liquid crystals

Phases of liquid crystalsPhases of liquid crystalsNematicNematic phasephase: : molecules have no molecules have no positional order but tend to point in positional order but tend to point in the same direction along the directorthe same direction along the director

CholestericCholesteric phasephase: : composed of composed of chiralchiral nematicnematic mesogensmesogens, , intermolecular forces favor alignment intermolecular forces favor alignment between molecules at a slight angle between molecules at a slight angle to one another, directors form a to one another, directors form a continuous helical patterncontinuous helical pattern

Phases of liquid crystalsPhases of liquid crystalsColumnar phaseColumnar phase:: disc shaped, stacked columns of disc shaped, stacked columns of molecules, columns are packed together to form a molecules, columns are packed together to form a twotwo--dimensional crystalline arraydimensional crystalline array

SmecticSmectic phasephase:: molecules show a degree of molecules show a degree of translational order, tend to align themselves in layers translational order, tend to align themselves in layers or planes. Separate planes flow past each otheror planes. Separate planes flow past each other

SmecticSmectic A: A: director is perpendicular to the smectic plane, and there is no particular positional order in the layer

SmecticSmectic C:C: director is at a constant tilt angle measured normally to the smecticplane

Classification of liquid crystalsClassification of liquid crystals1.1. ThermotropicThermotropic liquid crystalliquid crystal

Thermally induced transition to the liquid crystalline stateThermally induced transition to the liquid crystalline stateThermotropicThermotropic mesophasesmesophases occur because of occur because of anisotropic anisotropic dispersion forcesdispersion forces between the molecules and between the molecules and packing packing interactionsinteractions

EnantiotropicEnantiotropic (lowering the temperature of a liquid (lowering the temperature of a liquid as well asas well asraising that of a solid)raising that of a solid)

MonotropicMonotropic (either an increase in the temperature of a solid (either an increase in the temperature of a solid oror a a decrease in the temperature of a liquid)decrease in the temperature of a liquid)

DiscoticsDiscotics flat discflat disc--like molecules with a core of adjacent like molecules with a core of adjacent aromatic rings forming two dimensional columnar orderingaromatic rings forming two dimensional columnar orderingRodRod--shaped moleculesshaped molecules molecules with an elongated, molecules with an elongated, anisotropic geometry aligned along one spatial directionanisotropic geometry aligned along one spatial direction

Classification of liquid crystalsClassification of liquid crystals2.2. LyotropicLyotropic liquid crystalsliquid crystals

SolventSolvent--induced aggregation induced aggregation of the constituent of the constituent amphiphilicamphiphilicmesogensmesogens into into micellarmicellarstructuresstructuresLyophobicLyophobic ends stay together ends stay together and the and the lyophiliclyophilic ends extend ends extend outward toward the solutionoutward toward the solutionSize of the micelles increase Size of the micelles increase with increase in concentration with increase in concentration and lowering of temperature.and lowering of temperature.They coalesce separating the They coalesce separating the liquid crystalline stateliquid crystalline state

Chemical structureChemical structureAll the known molecules that form liquid All the known molecules that form liquid

crystalline phases are crystalline phases are asymmetricasymmetric in their in their shapeshape

MonomersMonomers can be attached:can be attached:in a long single chain forming in a long single chain forming main chain main chain polymer polymer to polymer back bone forming to polymer back bone forming side chain side chain polymerpolymer

Monomers commonly usedMonomers commonly usedcholesterol estercholesterol esterphenyl benzoatesphenyl benzoatessurfactantssurfactantsparaffinesparaffinesglycoglyco lipidslipidscellulose derivativescellulose derivatives

Polymer Liquid CrystalsPolymer Liquid CrystalsMesophasesMesophases formed by polymeric molecules combine the formed by polymeric molecules combine the

unique unique properties of polymerproperties of polymer (high strength fiber, ultra thin (high strength fiber, ultra thin films) and anisotropic films) and anisotropic properties of liquid crystalsproperties of liquid crystals

Factors influencing the Factors influencing the mesomorphicmesomorphic behavior of polymersbehavior of polymerspresence of long flexible spacerspresence of long flexible spacerslow molecular weightlow molecular weightregular alternation of rigid and flexible units along the main cregular alternation of rigid and flexible units along the main chainhain

PolymerPolymer--Dispersed Liquid CrystalsDispersed Liquid CrystalsPolymerPolymer--Stabilized Liquid CrystalsStabilized Liquid Crystals

Main chain polymer liquid crystalsMain chain polymer liquid crystalsMonomers made up of Monomers made up of several several aromaticaromatic ringsringsMonomers are Monomers are stiff, rodstiff, rod--likelikeRestricted rotations due to Restricted rotations due to resonanceresonance and and stericsterichindrancehindranceLess prevalent due to very Less prevalent due to very high stiffnesshigh stiffness

MesogenicMesogenic units are made up of units are made up of two or more aromatic ringstwo or more aromatic ringsMesogenMesogen is directly incorporated in is directly incorporated in the chainthe chainFlexible Flexible spacerspacer and the stiff and the stiff mesogenmesogen3 times long as they are wide3 times long as they are wide

Temperature Range ProblemsTemperature Range ProblemsMethods of lowering the polymer melting temperaturesMethods of lowering the polymer melting temperatures::

minimizesdminimizesd interactions with reduced symmetry interactions with reduced symmetry

1.1. Introduction of Introduction of defectsdefects to disrupt the ability for neighboring polymers to disrupt the ability for neighboring polymers to line upto line up

2.2. Molecules are put together in Molecules are put together in random orientationrandom orientation (head(head--toto--tail, headtail, head--toto--head)head)

3.3. Random Random copolymerizationcopolymerization resulting in irregularity of polymer resulting in irregularity of polymer substituents substituents

Side chain polymer liquid crystalsSide chain polymer liquid crystalsThe backboneThe backbone

The spacer The The spacer The mesogenmesogenMore rigid More rigid the backbonesthe backbones higher is the higher is the

glass transition temperatureglass transition temperature

Rigid core of two or more aromatic rings Rigid core of two or more aromatic rings joined together by a functional group forms joined together by a functional group forms the the mesogenmesogen

The spacer The spacer consists of two to four consists of two to four methylenemethylene (CH2) groups. Longer spacers (CH2) groups. Longer spacers gives lower glass transition temperaturegives lower glass transition temperature

ApplicationsApplicationsThe liquid crystal displaysThe liquid crystal displays ((LCDsLCDs))Liquid Crystal Thermometers. Liquid Crystal Thermometers. CholestericCholesteric liquid crystals liquid crystals

reflect light with a wavelength equal to the pitch which is reflect light with a wavelength equal to the pitch which is dependent upon temperaturedependent upon temperature

High strength High strength fibresfibres.. Polymer liquid crystals are used in Polymer liquid crystals are used in applications calling for strong, light weight materials applications calling for strong, light weight materials e.g e.g kevlarkevlar

Optical imaging.Optical imaging. Generation of electric field as light falls on Generation of electric field as light falls on photoconductor platesphotoconductor plates

Drug delivery. Drug delivery. LyotropicLyotropic liquid crystals can coat a drug to keep it liquid crystals can coat a drug to keep it from being destroyed in the digestive tractfrom being destroyed in the digestive tract

Biological membranes.Biological membranes. LyotropicLyotropic liquid crystals act as biological liquid crystals act as biological membranes due to their membranes due to their amphiphilic amphiphilic naturenature

Liquid Crystal DisplayLiquid Crystal Display

Consists of an array of tiny segments (called Consists of an array of tiny segments (called pixelspixels) that can be manipulated to present ) that can be manipulated to present informationinformationLCD consists primarily of two glass plates with LCD consists primarily of two glass plates with some liquid crystal material between themsome liquid crystal material between themPlates are usually manufactured with Plates are usually manufactured with transparent electrodes (ITO),transparent electrodes (ITO), that make it that make it possible to apply an electric field across small possible to apply an electric field across small areas of the film of liquid crystalareas of the film of liquid crystalLCDsLCDs consume much less powerconsume much less power than their than their cathodecathode--ray tube (CRT) counterpartsray tube (CRT) counterpartsMost commonly used are the Most commonly used are the Twisted Twisted NematicNematic(TN) Displays(TN) Displays

Liquid crystalsLiquid crystals-- an interdisciplinary research areaan interdisciplinary research area

Chemistry.Chemistry. Creating new materials and using the Creating new materials and using the supramolecularsupramolecularsystems for synthesis, analysis and purifications of other matersystems for synthesis, analysis and purifications of other materials ials

Biology.Biology. Study of the cell membrane as a liquid crystal with respect Study of the cell membrane as a liquid crystal with respect to intercellular transport (drug delivery) and communicationsto intercellular transport (drug delivery) and communications

Physics.Physics. Measurement of unique properties of liquid crystals Measurement of unique properties of liquid crystals ((ferroelectricityferroelectricity, piezoelectricity, phase transitions) and developing , piezoelectricity, phase transitions) and developing new ways of new ways of electroopticselectrooptics

Engineering.Engineering. Creating Creating electroopticalelectrooptical displays (LCD, STN, TFT), displays (LCD, STN, TFT), optical information storage, optical information storage, switchableswitchable gearboxes and lubricants etc.gearboxes and lubricants etc.

Information technology.Information technology. Creating object oriented knowledge tools, Creating object oriented knowledge tools, using the color effects of liquid crystals for special effects using the color effects of liquid crystals for special effects

The Challenger disasterThe Challenger disaster

As demonstrated by Richard As demonstrated by Richard FeynmanFeynman, a member of the , a member of the presidential commission presidential commission appointed to investigate the appointed to investigate the accident, the accident, the elastic Oelastic O--ringring(a gasket)(a gasket) did not respond did not respond as expected as expected because of the because of the cold temperaturecold temperature (30°F/(30°F/--1°C) 1°C) at launch timeat launch time and thus and thus could not flex adequatelycould not flex adequately to to form a proper seal around form a proper seal around one of the two solid rocket one of the two solid rocket boostersboosters

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•• liquid crystal cell is placed between two layers of photoconductliquid crystal cell is placed between two layers of photoconductoror•• light is applied to the photoconductor, which increases the matelight is applied to the photoconductor, which increases the material's rial's

conductivity conductivity •• electric field corresponding to the intensity of the light develelectric field corresponding to the intensity of the light developsops•• the electric pattern transmitted by an electrode, enabling the ithe electric pattern transmitted by an electrode, enabling the image to mage to

be recordedbe recorded•• Positional order: Positional order: The extent to which the position of an average The extent to which the position of an average

molecule or group of molecules shows translational symmetry.molecule or group of molecules shows translational symmetry.•• Orientational Orientational order: order: Measure of the tendency of the molecules to Measure of the tendency of the molecules to

align along the director on a longalign along the director on a long--range basisrange basis•• Bond Bond orientational orientational order: order: Describes a line joining the Describes a line joining the centers centers of of

nearestnearest--neighborneighbor molecules without requiring a regular spacing molecules without requiring a regular spacing along that line. Thus, a relatively longalong that line. Thus, a relatively long--range order with respect to range order with respect to the line ofthe line of centerscenters but only short range positional order along that but only short range positional order along that lineline