Liquid Crystals - Universität...
Transcript of Liquid Crystals - Universität...
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Liquid Crystals
2.2.2015
F.Kremer
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1. What are Liquid Crystals (LC); Experiment: LC are birefringent
2. Classification of liquid crystals
3. Orientation of LC
4. Structural and optical properties of LC; Experiment: How does a LC display work?
5. Textures of LC
Outline
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1. What are Liquid Crystals (LC)
A typical LC molecule:
rigid core, „mesogen“aliphatic tail
the core alone has a strong tendency to crystallize, but this is counterbalanced by the tendency of the aliphatic tails to form a glass.
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Thermotropic LC
Liquid crystals are an intermediate state between ordered three-dimensional crystalline arrays and isotropic disorder.
Temperature
solid hazy liquid clear liquid
The botanist Reinitzer observed 2 melting points in cholesteryl benzoate ( 1888)
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(Monatshefte für Chemie 9:421–41)
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Summary concerning the question, what are LC?
1. LC molecules are allways characterized by a rigid core (mesogen) and a typically aliphatic tail.
2. The mesogen may have very different shapes.
3. The structure of the LC molecule causes mesophases between the crystalline and the liquid state.
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Experiment: LC are birefringent
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Experiment: LC are birefringent
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2. Classification of liquid crystals
• Thermotropic LCs - obtained by heating up the crystalline solid or cooling the isotropic liquid
• Lyotropic LCs - obtained by dissolving the LC in an appropriate solvent under given concentration and temperature conditions
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Lyotropic liquid crystal
spherical micelles,
rod-like micelles packed in a hexagonal arrangement, a lamellar phase.
isolated molecules
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Shape of LC molecules
calamitic (rod-like molecules)
discotic (disc-like)
banana-shaped liquid crystal
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Calamitic liquid crystals
d
pitc
h
Nematic Cholesteric phase (chiral nematic) Smectic
n - director
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Mesophase polymorphism
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N
SmB
SmASmC
SmISmF
SmJ
SmGSmESmK
Solid crystal
orthogonal
Phase type Molecular orientation
Molecular packing
orthogonal
orthogonal
tilted
Tilt to apex of hexagon
Tilt to side of hexagon
Tilt to apex of hexagon
Tilt to side of hexagon
SmHTilted to side a
Tilted to side b
Orientational ordering
random
random
hexagonal
pseudo hexagonal
orthorombic
monoclinic
monoclinic
Positional ordering
Short range
long range
long range
Short range
Isotropicte
mpe
ratu
re
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Structures of discotic LC phases
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• special types of liquid crystal phases that appear in the temperaturerange between a chiral nematic phase and an isotropic liquid phase
• have a regular three-dimensional cubic structure of defects with latticeperiods of several hundred nanometers
• exhibit selective Bragg reflections in the wavelength range of light
Blue phases
Molecules
Double twist cylinder
Director
Molecules twisting out from the centre to form helical structure
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Typical DSC curve of LCs
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DSC curve of enantiotropic and mono-tropic liquid crystalline phases
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Pressure-temperature phase diagram
• Liquid crystalline phase can be induced by pressure.
H. Uehara et al.J. Phys. Soc. Jpn.,. 71, 2002
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The odd-even effect•The LC properties (e.g., the phase transition temperature, the order parameter, thetransition entropy show a pronounced alternation as the end-chain increases
The addition of an even-numbered carbon atome is along the major molecular axis.
Isotropic
Nematic
When the chains become longer their flexibility increases and the odd-even effect becomes less pronounced.
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Summary concerning the classification of LC
1. One separates between thermotropic and lyotropic LC
2. LC phases show a pronounced polymorphism
3. Besides temperature and concentration also pressuredetermines the phase sequence
4. The phase sequence depends in a subtle manner on the molecular structure, e.g. the even-odd effect
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3. Orientation of LC
1. LC can be oriented by magnetic fields if the mesogen has a strong magnetic moment.
2. LC can be easily oriented by an external electric field if the mesogen has an electric dipolemoment
3. LC can be easily oriented due to surface interactions
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Why can we order LC molecules by the external magnetic field ?• Most liquid crystals are diamagnetic and their magnetic anisotropy arises from the electronic structure of the mesogens (delocalisation of electronic charge enhances the diamagnetic susceptibility).
• Since the component of the diamagnetic susceptibility perpendicular to a benzene ring is greater than the in plane component, calamitic mesogens have a positive diamagnetic anisotropy.
The molecules align along the direction of an external magnetic field.
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Orientation due to surface interactionsHomeotropic orientation
Planar orientationThe polymer solution is spin coated on the surface
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Orientation of LC due to surface effects
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Summary concerning the Orientation of LC
1. LC can easily oriented by different means
2. Orientation on macroscopic scales (~100cm) was for a long time not possible, but is no problem in modern technology
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4. Structural and optical properties of LC
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Structural studies of LCs
•Peaks arise from the average end-to-end and side-to- side separations of the close-packedmolecules.
•The peaks are diffuse because positional correlations only extend over short distances, typically, a few molecular diameters.
•The widths of the diffuse maxima areinversely proportional to these correlation lengths.
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Structural studies of LCs
d
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Structural studies of LCs
0 5 10 15 20 25 300
200
400
600
800
1000
1200
1400
(210)(200)
(111)(110)
(003)(002)
Inte
nsity
/ a.u
.
2θ/deg
(001)
60oC
3BT
3BT 60°Ca = 8.00 ± 0,02 Åb = 5.35 ± 0,02 Åc = 17/36 ± 0,05Å
ca – distance between two moleculesb – diameter of the moleculesc –layer thickness
a = 5.02±0.2 °A,
c = 27.56 ± 0.05
BBOA
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Optical properties of LCs
Unpolarized light
ordinary ray
extraordinary ray
• Birefringence (double refraction) - the decomposition of a light beam into two rays (the ordinary and the extraordinary rays) when it passes through materials.
• Velocities of both components, are different and vary with the propagation direction through the specim and the waves get out of phase.
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Twisted nematic liquid crystal display
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• 1969 – George Heilmeier, RCA David Sarnoff Research Center, first liquid crystalline display (125 oC,)
•DSM (dynamic scattering method) :an electrical charge is applied which rearranges the molecules so that they scatter light)
The picture above shows George Heilmeier with the first dynamic scattering method-based liquid crystal display
First LCD
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Experiment: How does a LC display work?
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Experiment: How does a LC display work?
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Summary concerning the structural and optical properties of LC
1. The structural properties of LC are explored by X-ray and ν - diffraction
2. The pronounced optical bifringence is the basis for applications in displays
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5. Textures of LC
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• polarization colors result from the interference of the two components of light split by the anisotropic specimen and may be regarded as white light minus those colors that are interfering destructively
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Schlieren Defect Texture (Nematic)Distribution of directors in the neighborhood of 4 brushed defects
dark brush
Distribution of directors in the neighborhood of 2 brushed defects
dark brush
Crossed polarizers
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Fan shape focal-conic smectic A
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Schlieren Defects in Antiferroelectric LCs
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Textures
As the number defects increases the entropy (S) gets larger and free energy (G) decreases
G = E - TS
internal energy
Textures - delocalized topological defects
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Mosaic texture (smectic G)
Mosaic texture (smectic B)
Mosaic textures
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Summary concerning the textures of LC
1. The textures of LC are a „world“ for itself.
2. Under normal conditions a manifold of defects of different topology can be observed
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Final Summary for LC
1. LC form a special class of materials between the solid and the liquid state
2. LC show a manifold of mesophases which is determined by ist chemical structure
3. LC gained tremendous technological impact within the last 2 decades
4. LC can be incorporated into polymeric and elastomeric systems; by that novel materials are designed, e.g. artificial muscles or soft organic lasers
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Kontrollfragen zu der Vorlesung am 2.2.2015
153.Durch welche Eigenschaften sind „Flüssigkristalle“ ausgezeichnet?154.Welche molekulare Eigenschaft verursacht den flüssigkristallinen Charakter?155.Wie unterscheidet man zwischen thermotropen und lyotropen Flüssigkristallen?156.Wie werden die flüssigkristalline Mesophasen klassifiziert?157.Wie funktioniert ein Flüssigkristall-Display? Welchen Vorteil haben solche Anzeigeelemente? 158.Welche weiteren großtechnischen Anwendungen von Flüssigkristallen kennen Sie?