MEASUREMENT OF BIREFRINGENCE AND TRANSITION TEMPERATURE OF NEMATIC LIQUID CRYSTAL

PROJECT REPORT SUBMITTED IN THE PARTIAL FULFILLMENT OF THE ACADEMIC REQUIREMENT FOR THE AWARD OF

MASTER OF SCIENCE

IN

PHYSICS

BY

SOUVIK ROY

13PHMP09

IN GUIDANCE

Dr. SURAJIT DHARA

Associate Professor, School of Physics

DECLARATION

I, SOUVIK ROY, hereby declare that my project work on " MEASUREMENT OF BIREFRINGENCE AND TRANSITION TEMPERATURE OF NEMATIC LIQUID CRYSTAL" submitted to School Of Physics, University of Hyderabad, Hyderabad, for partial fulfillment of the Degree of MASTER OF SCIENCE IN PHYSICS has been carried out by me under the supervision of Dr. SURAJIT DHARA, School Of Physics, University Of Hyderabad. To the best of my knowledge, this work has not been submitted for any other degree in any university.

SOUVIK ROY

M.Sc., PHYSICS

Reg.No: 13PHMP09

UNIVERSITY OF HYDERABAD

CERTIFICATE

This is to certify that the project work on " MEASUREMENT OF BIREFRINGENCE AND TRANSITION TEMPERATURE OF NEMATIC LIQUID CRYSTAL " is a bonafide work done by Mr. SOUVIK ROY bearing the Reg.No:13PHMP09 under my guidance in partial fulfillment of the requirements for the award of degree of MASTER OF SCIENCE IN PHYSICS and submitted to School Of Physics, University Of Hyderabad. This work has not been submitted for any other degree in any University.

DR. SURAJIT DHARA

SUPERVISOR

School Of Physics

DEAN

School Of Physics

ACKNOWLEDGEMENTS

First and foremost I would like to thank the Almighty for his grace upon me, without which, I would not have been able to complete this project. I am indebted to my parents for their continuous encouragement.

I would like to express my sincere gratitude to my project supervisor Dr. SURAJIT DHARA for his generous nature, infinite patience and helpful guidance. I will always remember his suggestions and practice his advices in both my professional and personal life.

I am Highly thankfull to Mr. VENKATA SAI DASARI for constant help and encouragement during the project.

I take the opportunity to thank all my labmates like M.V RASNA, RASMITA SAHOO,ZUNAID AHMED for their co-operation and suggestions for my project work. SOUVIK ROY 13PHMP09

PLACE: DATE:UNIVERSITY OF HYDERABAD 6TH MAY, 2015 HYDERABAD-500046

CONTENTS

1. INTRODUCTION

2. ORDER PARAMETER

3 TYPES OF LIQUID CRYSTAL

3.1 CALAMITIC

a) NEMATIC

b) TWISTED NEMATIC

c) SMECTIC PHASES

i) SMECTIC A

ii) SMECTIC C

iii) SMECTIC C*

iv) ANTICLINIC

4. SOME OPTICAL PROPERTIES OF LIQUID CRYSTAL:

a) Refractive index.b) Birefringence.

5. MAKING OF LIQUID CRYSTAL CELL:

6. MEASUREMENT OF THICKNESS OF THE CELL:

7. LIQUID CRYSTAL SAMPLE FOR EXPERIMENT :

a) SAMPLE b) ORGANIC NAME c) TRANSITION TEMPERATURE

8. SOME TEXTURES OF LIQUID CRYSTAL UNDER NIKON POLARISING MICROSCOPE.

a) APPARATUS b) PROCEDURE c) TEXTURE IN DIFFERENT PHASE TRANSITIONS d) SOME TEXTURE WITH DIGITAL CAMERA WITH 10X AND 20X OPTICAL ZOOM AT TEMPERATURE 62 AND 54.

9. MEASUREMENT OF BIREFRINGENCE

a) INTENSITY MEASUREMENT TECHNIQUE i)APPARATUS. ii)PROCEDURE. iii)WORKING FORMULA.

b) CALCULATIONS AND GRAPH

c) RESULTS .

10. REFERENCES.

1. INTRODUCTION :

The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points (419K and 552K) .In this experiments, Reinitzer increased the temperature of a solid sample and observed the crystal changing into a hazy liquid. Further increasing of temperature that sample again changes to transparent liquid .they are found among organic compounds with molecular shape anisotropy.they are classified into two types namely thermotropic and lyotropic.thermotropic liquid crystal exhibit mesophases as a function of its concentration in solvent.conventional liquid crystals are mostly made of rod and disc type of molecules.

2. ORDER PARAMETER :

Liquid crystals possess some degree of orientational order To quantify how much order is present in a material an order parameter (S) is defined.Liquid crystal molecules possess orientational order. The direction of preferred orientation in a liquid crystal is called the director .The order parameter is given as follows S= ½ <3cos²θ – 1>where, θ is the angle made by each molecule with the director axis denoted by n.

3. TYPES OF LIQUID CRYSTAL :

Depending on the shape of the molecules thermotropic liquid crystal can be classified into three categories.i)calamitic.ii) Discotic.iii) bend core type. Which basically composed with rod like,disc like and bent core molecules.

3.1 calamitic liquid crystal:-

it consist of rigid cores with flexible side chains.there are many mesophases formed by calamitic mesogens.

a) Nematic phase: This phase is characterized by the molecules that have no positional order but tend to orient in the same direction i.e., along the director.

b) Twisted nematic phase: In this Cholesteric or twisted phase along with long range orientational order spatial variation of director is also exist which causes helical structure.

c) SMECTIC PHASES : These phases shows orientational order along with positional order.the molecules preferably pointing in one direction. Just like in the nematic phases.in the layer the centre of mass of the molecules are random i.e liquid like.there are several different types of smectic phases.

i) Smectic A : In smectic A (SmA) phases, on average, the molecules are parallel to one another and are arranged in layers, with the long axes perpendicular to the layer plane. Within the layers, the centers of gravity of the molecules are ordered at random. Thus, smectics A possess the one-dimensional quasi long-range positional order and within the layers molecules show a relatively high mobility. The layer thickness is equal to the molecule length. SmA LCs are optically positive and uniaxial with the optic axis parallel to the molecular long axes.

ii) Smectic c :. For The structure of the smectic C (SmC) liquid crystals is closely related to the structure of the SmA. The molecules are arranged in layers, but the long axes of the molecules are tilted to the layers planes.some materials the tilt angle is constant but for others it is temperature dependent. The centers of gravity of the molecules are randomly ordered and the moleculesare free to rotate around their long axes. SmC phases are optically biaxial.

iii) SMECTIC C* : The SmC* phase is similar to the SmC phase but consists of the chiral molecules, which rotate the direction of the director projection on the layer plane from one layer to the next. The twist axis of the SmC* is perpendicular to the layers. Therefore, these phases appear optically positive uniaxial, and show optical activity and selective reflection similar to the cholesterics.

iv) ANTICLINIC : Here the long axis of molecules make an angle with the layer normal .

4. SOME OPTICAL PROPERTIES OF LIQUID CRYSTAL :

a) REFRACTIVE INDEX : In liquid crystal the speed of light is parallel to the direction is different from the perpendicular direction.nematic phase has two principal refractive index for ordinary and extraordinary rays.

b) BIREFRINGENCE : Liquid crystals are found to be birefringent, due to their anisotropic nature. That is, they demonstrate double refraction (having two indices of refraction). In the following diagram, the blue lines represent the director field and the arrows show the polarization vector.

∆n= ne – no. Where ne and no are the refractive index for extraordinary and ordinary Rays .

5. MAKING OF A LIQUID CRYSTAL CELL :

a) First we cut the slides around 1.5 cm² in area with special diamond cutter .

b) Then we wash the slides with soap solution and start our solication process. First we solicate the slides with benzene for 30 mints, and then with asetone for 30 minutes and after that with distilled water for 30 mints.

c) After solication we dry up our slides with dryer and Nitrogen gas for bubble removing.d) After bubble removing we use our alignment layer SC7942 on the slides and put it in the spin

coater machine and put in a furnace at 180 ͦc for 45 mints, rubbing follows after the furnace work.

e) After rubbing sticking with 8µm sample with glue.and has to keep for some time under UV ray.

f) Cells are prepare for use.

6. MEASUREMENT OF CELL THICKNESS :

EQUIPMENTS NEEDEDSpin coater

RUBBING MACHINE

FURNACE

GLASS SLIDES

The liquid crystal cell is kept under spectrometer reflecltion probes, which consist of optical fibre.it is illuminated through outer six fibres of probes. And reflected light is collected by central fibre and fed to spectrometer.

Then view the spectrum in the computer using a software called spectrasuite .

FORMULA USED : -

d= (λn-λm)/ λn λm* (n - m)/2

where n & m are the peaks and λn, λm are the corresponding wavelength. We can get the values of λ’s accurately using the software.

7. LIQUID CRYSTAL SAMPLE FOR EXPERIMENT:

a) SAMPLE :- 8OCB

b) ORGANIC NAME :- 4-Octyloxy-4’-cyanobiphenyl

c) TRANSITION TEMPERATURE :-

i) CRYSTAL TO SMECTIC - 54 ͦ C

ii) SMECTIC(A) TO NEMATIC - 67 ͦ Ciii) NEMATIC TO ISOTROPIC - 80 ͦ C

8. SOME TEXTURES OF LIQUID CRYSTAL UNDER NIKON POLARISING MICROSCOPE :

we place the liquid crystal sample 8OCB under Nikon polarising microscope,and with varing the temperature and take some pictures of the sample at different transition temperature.

a) APPARATUS REQUIRED :

i) Liquid crystal sample holderii) Temperature controller.iii) Nikon porarising microscope.iv) Digital camera with 10x and 20x optical zoom.v) Computer Pc.

b) PROCEDURE:

i) First take the sample 8OCB in between the liquid crystal slide.ii) Stick it with temperature resistant cellotape with the sample holder.iii) Sample holder is connected with the temperature controller.

iv) Increase the tempreture beyond 80 ͦ , and then cool it down .v) Take the textures at 10x and 20x optical zoom by digital camera at the transition

temperatures.

c) TEXTURES AT DIFFERENT PHASE TRANSITION: Textures at isotropic phase : (at temperature 82 ͦ)

Textures from isotropic to nematic transition : (at temperature 78 ͦ )

Textures at nematic phase : ( at temperature 72 ͦ)

Textures from nematic to smectic transition : (at temperature 66 ͦ)

Textures at smectic phase : (at temperature 58 ͦ)

d) HERE IS SOME TEXTURES AT PURE SMECTIC PHASE : (AT TEMPERATURE 62 ͦ WITH 10X OPTICAL ZOOM)

e) SOME TEXTURES AT SMECTIC TO CRYSTAL TRANSITION (AT TEMPERATURE 54 ͦ WITH 20X OPTICAL ZOOM) :

9. MEASUREMENT OF BIREFRINGENCE : -

The birefringence of a uniaxial liquid crystal can be measured by two techniques.

Intensity measurement technique. Phase modulation technique.

In my experiment i use only intensity measurement technique.

a) INTENSITY MEASUREMENT TECHNIQUE :

Using dc intensity measurement technique we can measure the birefringence.the optical set up is shown in the figure.

i) APPERATUS :

a) He –Ne source.b) Optical bench on a shelf with a hole for passing the light.c) Glan-thompson polarizers.d) Detector.

ii) EXPERIMENTAL PROCEDURE :

The plane polarised light after the GT polariser passes through the sample, in which the rubbing direction is 45 ͦ with respect to the first polariser. The transmitted light then passes through the second polariser and after that goes to a detector.the detector output is connected to keithly dc voltmeter for measurement of output intensity.

the temperature of the sample then controlled by temperature controller called instec.

For measurement of temperature dependent birefringence,temperature is varied with finite step and the corresponding intensity is measured by the detector.

All the measurement is controlled by the computer and with the help of a software called LabVIEW.

We place the liquid crystal sample between two crossed polariser.and rubbing direction makes an angle φ with polariser.Assuming the analyser and polariser is in x and y direction and light propagates in z direction.

iii) WORKING FORMULA:

If the intensity of light after the first polariser is I0 , then intensity at the detector is given by,

I= I0 Sin²(2φ)Sin²(πΔn/λ)

Setting φ=45 ͦ , the intensity at the detector is given by,

Is = I0 Sin²(πΔn/λ)

And birefringence is given by,

∆n=(λ/πd)arcSin (√(Is/I0) .

b) Calculation & graph :-

measuring the temperature and corresponding intensity, Using the software LaBVIEW .

Calculate the birefringence for each intensity. Plot a graph in origin software with temperature is along x axis.

And birefringence along y axis .

THE GRAPH BETWEEN BIREFRINGENCE AND TEMPERATURE:

THE GRAPH FOR COMPAREING THE EXPERIMENTAL VALUE AND THEORITICAL VALUE:

c) RESULTS : After draw a plot between birefringence and temperature in origin software i can realise how the birefringence changes with temperature.with increases the temperature birefringence decreases.

When cooling from temperature 84, ͦI saw the first transition occurs in the graph at temperature near about 80.208 ͦ . it denotes some phase is changes.and we know the theoretical transition temperature for isotropic to nematic transition is near about 80 ͦ,which is nearer to our experimental value. So the transition of phase is isotropic to nematic.

The second transition occurs at temperature near about 65.947 ͦ. Which is close to our theoretical nematic to smectic transition temperature 67 ͦ. So the transition phase is nematic to smectic.

And also my graph is approximately the same to the graph which is plotted with the theoretical values.

EXPERIMENTAL VALUES OF TRANSITION TEMPERATURE:

FIRST TRANSITION TEMPERATURE : 80.208 ͦ (DENOTES ISOTROPIC TO NEMATIC TRANSITION)CORRESPONDING BIREFRINGENCE : 0.0823

SECOND TRANSITION TEMPERATURE : 65.947 ͦ (DENOTES NEMATIC TO SMECTIC TRANSITION )CORRESPONDING BIREFRINGENCE : 0.168

10. REFERENCES:

Liquid crystal binary mixtures 8CB+8OCB: critical behaviour at the smectic A–nematic transition M. B. SIED, D. O. LO´ PEZ*, J. Ll. TAMARIT and M. BARRIO Departament de FÌ´sica i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Polite`cnica de Catalunya, Diagonal 647, 08028 Barcelona, Catalonia, Spain (Received 16 December 2000; accepted 27 June 2001).

M. Cardona, in Solid State Physics, edited by F. Seitz, D. Turnbull, and H. Ehrenreich (Academic, New York, 1969), Suppl. 11.

Coupling between Orientational and Translational Order in a Liquid Crystal department of physics and Astronomy, State University of New York at &Buffalo, Amkerst, New York 14260, Khoon-Cheng Lim and John T. Ho.

Tharmal & spectrophotometric analysis of liquid crystal 8CB/8OCB mixtures.Sukrii ozgan,Mustafa okumus,sociedade brasileira de fisika 2011.

Order of phase transitions and tricriticality in mixtures of octyloxycyanobiphenyl and nonyloxycyanobiphenyl liquid crystals: A high-resolution study by adiabatic scanning calorimetry .

George Cordoyiannis, Chandra Shekhar Pati Tripathi, Christ Glorieux, and Jan Thoen Phys. Rev. E 82, 031707 – Published 22 September 2010.

Evolution of the isotropic to nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions A. Roshi and G. S. Iannacchione Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, USA P. S. Clegg and R. J. Birgeneau Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada (Dated: February 2, 2008)