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Optical Characterization of Synthetic Liquid Crystals Compounds

Prof. M. Medhat Abd El-RahmanProfessor of optics

(Ain Shams University)

Ph.D Proposal Presented by/

Hassanein Shaban Hassanein The British University in Egypt (B.U.E.)

M.Sc. of Physics 2012 (Ain Shams University)

Supervised by/

Prof. S.Y. El–ZaiatProfessor of optics

(Ain Shams University)

Dr. Marwa S. SalemAss. Professor of organic chemistry

(Ain Shams University)

Dr. Amal KasryLecturer of Biosensors

(The British University in Egypt )

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Contents1. Introduction to liquid crystals 1. Description and Definition. 2. Types of liquid crystals. 3. Applications of liquid crystals.2. liquid crystals parameters 1. Refractive index (n) 2. Birefringence (Δn) 3.Phase retardation (ΔΦ) 4. Specific rotation (Ω) 5. Microscopic order (S) 6. Electric permittivity (ε) 7. Polarizability (α) 3. External effects on optical liquid crystal parameters

(Thermo-optic effect -Electro-optic effect - Magneto optic effect)4. Jones matrix 5. Experimental work 5.1. Chemical preparation

5.2. Construction of a cell is allowing the application of electric and magnetic fields under controlled temperature.

5.3. Construction of the optical setup.6. Literature survey7. Plan of work and Objectives

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Liquid crystals

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Liquid crystals (LC) are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. Hence LC shows anisotropy.

i.e. Liquid crystals can flow like liquid and at the same time, its molecules has certain periodicity.

>> its molecules has short range positional order (describes the molecule periodicity according to its position)

>> its molecules has long range orientational order (describes the molecule periodicity according to its orientation)

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Crystal Phase

Fixed orientation Fixed position

Liquid Crystal Phase

Long range orientational order Short range positional order

Liquid Phase

Random orientation Random position

Temperature

Mesophase

In Greek means (in between)

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i. NEMATIC LC

II. LYOTROPIC LC

ii. CHOLESTERIC LC iii. SMECTIC LC

I. THERMOTROPIC LC III. METALLOTROPIC LC

LIQUID CRYSTALS

I. Thermotropic liquid crystalsWhich are composed of organic molecules and its formation is temperature dependentEnantiotropic: Mesophases appears with heating and cooling.

“reversible process”Monotropic: Mesophases appears with cooling only.

II. Lyotropic liquid crystals

III. Metallotropic liquid crystals

Which are composed of organic molecules and its formation is solvent and concentration dependent

Which are composed of organic and inorganic molecules and its formation depends on temperature, concentration and inorganic/organic composition ratio

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Thermotropic liquid crystals can be classified according to its positional and orientational order as: Nematic liquid

crystal

Thermotropic liquid crystals

Smectic liquid crystal

Cholesteric liquid crystal

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Note:Mesogen:It is the fundamental unit of a liquid crystal that

induces structural order in the crystals. 12

Calcification of liquid crystalsAccording to molecule shape

Banana shaped moleculeDiscotic or Disk like

molecule

Calamitic or Rode like molecule

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Liquid Crystal Applications

• Liquid crystal displays – LCD.• Polymer dispersed liquid crystal – PDLC.• Liquid crystal lens.• Liquid crystal tunable filter- LCTF.• Retarders (Waveplates).• Thermometers.• Thermography in medicine and electronic

circuits.

Liquid crystal display - LCD

Flexible liquid crystal display15

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Smart glass

Polymer Dispersed Liquid Crystals

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Liquid crystal lens

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d4=2d3=2d2=4d1

Lyot filter is a type of optical filter that uses birefringence to produce a narrow band pass of transmitted wavelengths.

Liquid crystal tunable filter

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WAVEPLATES (RETARDERS)

WAVEPLATE

Δφ

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Liquid crystals in medicine

Liquid cholesteric crystal solution is sprayed on black plastic sheet covering the area of investigation. Because LCs are affected by sub-surface temperature, variations in color appear.

The resultant thermogram shows the much larger extent of a tumor on the right hand than one might previously have predicted on the basis of the clinical appearance.

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Blue(warm) fingers for normal blood flow

Green (cooler) fingers from nicotine in cigarettes that restricts flow to extremities

Cholesteric liquid crystal thermography was promoted as a cost-effective way for doctors to see beneath the skin

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Liquid crystals in electronic circuits

Using the color response of thermochromic liquid crystals (TLC) for the purpose of temperature measurement..

It is used for accurate temperature measurement on hot spots on mini and micro circuits and locating defects.

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Phase retardation

ΔΦ

Birefringence Δn

Specific rotationΩ

Microscopic order parameter

SPolarizability

α

Absorption coefficient

a

Electric permittivity

ε

BirefringenceAnisotropic crystals, such as quartz, calcite, and tourmaline, have crystallographic distinct axes and interact with light by a mechanism that is dependent upon the orientation of the crystalline lattice with respect to the incident light angle.

Halite (optically isotropic)

Calcite (optically anisotropic)

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when light ray enters a non-equivalent axis crystal structure, it is refracted into two rays (Ordinary & extra ordinary), each one is polarized with the vibration directions oriented at right angles (mutually perpendicular) to one another and traveling at different velocities.

Δn is the difference in the refractive indices of the crystal for these two components

e -no

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Phase RetardationRetarders are devices which modify the polarization of an incident wave changing the relative phase of two orthogonally polarized waves that form the incident wave, and birefringence can be used to produce the desired phase change

These waves are the ordinary and extraordinary waves in the birefringent crystal , one wave is retarded relative to the other. If the retarder plate has a thickness d, these two waves emerge from the crystal with a phase

difference Δφ

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Optical Modulation

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Optical activity (Ω) When a certain organic liquids, solutions (sugar) or quartz crystals are placed in the path of plane polarized light, the plane of polarization is rotated

a – observed angle of rotationL – length in decimetersC - grams of substance in 100ml of solution

ΩSpecific rotation

This orientational order allows us to define an average direction of the molecules called the director and denoted by the vector n. An important variable in nematic liquid crystals is the order parameter which measures how molecules aligned with the director. The usual measure of this order is,

Microscopic order parameter(S)

where <> denotes a thermal averaging and θ is the angle between each molecule and the director. If the molecules are very well aligned with the director then S=1 and if the molecules are randomly oriented about n i.e. isotropic the S=0.

𝑺=𝟏𝟐 ⟨𝟑𝒄𝒐𝒔𝟐𝜽−𝟏 ⟩

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Polarizability (α) Is defined as “The number of collective molecular electric dipoles moments per unit volume.

Also is defined as “The ratio of the induced electric dipole moment to the applied electric field that produces this dipole moment”.

Elongated molecules have electrons that are easily moved increasing their polarizability and thus strengthening the dispersion forces. In contrast, small, compact, symmetrical molecules are less polarizable

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Vuks derived an equation for anisotropic medium which relates ordinary and extraordinary refractive indices to the corresponding polarizabilities:

where αe and αo are molecular polarizabilities, N is a number of molecules per unit volume and <n>2 is the mean value of the square of refractive index given by the following equation

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Absorption Coefficient (a)

Io I

IT= Io e-aX

IT Transmitted intensityIO Incident intensitya Absorption coefficientX Material thickness

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ELECTRO-OPTIC EFFECT

MAGNETO-OPTIC EFFECT

THERMO-OPTIC EFFECT

THERMO-OPTIC EFFECT

Note:Mesogen:It is the fundamental unit of a liquid crystal that

induces structural order in the crystals. 32

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ELECTRO-OPTIC EFFECT

Some materials its refractive index (n) is a function of the applied electrci field (E) on it.

Linear electro-optic effect (Pockels effect, 1893)

Quadratic electro-optic effect (Kerr effect, 1875)

n(E)

E0

0

n(E)

E

PERMANENT ELECTRIC DIPOLE Many liquid crystals molecules are composed of neutral atoms and not charged. However, it is possible for the bonding between the atoms of a molecule to be such that a permanent electric dipole is produced. The result is that the molecule bears a positive charge at one end and a negative charge at the other.One example is a common calamitic liquid crystals template, Alkoxy cyanobiphenyls

RO C N

RO C N

..

+ -

INTERACTION WITH ELECTRIC FIELDSIn the case of non-polar molecules, the induced electric dipoles are created by an applied electric field causing the slight separation between positive and negative charges in the molecules. The induced electric dipoles are much weaker than permanent electric dipoles. However, they experience the same forces in an electric field.

++ +

---

-

++ +

-- --

Applying an E- field on LC molecules

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Magneto optic effect Faraday effect

• The linear polarized light that is seen to rotate in the Faraday effect can be seen as consisting of the superposition of a right- and a left- circularly polarized beam. In circularly polarized light the direction of the electric field rotates at the frequency of the light, either clockwise or anticlockwise.

• In a material, this electric field causes a force on the charged particles comprising the material. The affected motion will be circular, and circularly moving charges will create their own (magnetic) field in addition to the external magnetic field.

• The created field will be parallel to the external field for one (circular) polarization, and in the opposing direction for the other polarization direction

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= +Linearly polarized R-Circular

polarizedL-Circular polarized

ɵ

B- Field

Phase difference

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θ=VBL

• The net B field is enhanced in one direction and diminished in the opposite direction. This changes the dynamics of the interaction for each beam and one of the beams will be slowed down more than the other, causing a phase difference between the left- and right-polarized beam. When you add the two beams after this phase shift, the result is again a linearly polarized beam, but with a rotation in the polarization direction.

θ is the angle of rotation B  is the magnetic flux density in the direction of propagation L  is the length of the path (in meters) where the light and magnetic field interactV  is the Verdet constant for the material. 

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1-Chemical Preparation

3-Design of test cell

4-Optical Setup

DSCFTIR2-Detection of liquid

crystals phase

NMRMass

Spectroscopy

Polarized Light Microscope

Experimental work

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POLARIZED LIGHT MICROSCOPE

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PHASE TRANSITIONS

Nematic-Isotropic phase transition under the X polarizers

Isotropic Nematic

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Test Cell Design

Temperature (T)

Electric filed (E) Magnetic field (B)

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thermocouple

x .

Two similar glass plates filled with LC sample

Heater

Two crossed polarizers

Hot Stage

White light source

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thermocoupleTwo similar glass plates + spacer + LC sampleELECTRIC HEATER

Two crossed polarizers

Hot Stage

White light source

Copper jacketTeflon jacket

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Transverse E – field

Longitudinal E – filed

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Longitudinal B – field Transverse B – field

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Edser Butler fringes

Polarized white light

interference fringes

Optical Setup

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Edser Butler Fringes

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Polarized white light interference fringes

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Literature survey

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Plan of work1. Previous work survey.2. Preparation of liquid crystal (L.C.) compound samples.3. Diagnosing the chemical composition using: 1. FTIR 2.HNMR 3. Mass spectroscopy.4. Investigation of L.C. phases with the change of temperature using: 1. Polarized light microscope. 2- DSC. 5. Designs of several cells for L.C. samples are provided with : 1. An adjustable heater to get certain temperature (T).

2. A controlled electric filed (E).3. A controlled magnetic field (B).6. Construction of optical setup for producing: 1. Edser Butler fringes. 2. Polarized white light interference fringes.

7. Investigation of L.C. samples using optical methods and calculating its parameters.

8. Deduction of L.C. parameters under the effect of: 1. Temperature (T). 2. Electric field (E). 3. Magnetic field (B).

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