Identification of liquid crystals phase-mesophase characterisation

Dr. M. ManickamSchool of Chemistry

The University of BirminghamM.Manickam@bham.ac.uk

CHM3T1

Lecture- 7

Outline of Lecture

Introduction

Thermal Analysis

Polarised Optical Microscopy

Differential Scanning Calorimetry

Mesophase Textures

X-Ray diffraction

Learning Objectives

After completing this lecture you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods.

Polarised Optical Microscopy (POM)

Reflection and Refraction

Index of Refraction

Birefringence

Mesophase Textures

Differential Scanning Calorimetry (DSC)

X-Ray Diffraction

Examples

Crystals of a solidorganic compound

Nematic liquidcrystals phase

Isotropic liquid

Crystals of a solidorganic compound

Smectic liquidcrystal phase

Isotropic liquid

heat heat

heat heat

Example of a compound that shows LCs phases

3 degrees of order

3 degrees of orderin solid form

Looks like milk1 degree of order 0 degrees of order

gooey material2 degrees of order

0 degrees of order

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

Thermal Analysis

The first step in the investigation of the liquid crystalline nature of materials is based upon thermal methods of analysis.

When a mesomorphic material in the crystal state is subjected to heating, the energy supplied disrupts the crystalline lattice leading to the LC phase.

As the temperature rises, the LC will absorb further energy becoming an isotropic liquid.

Thermal analysis allows the detection of this sequence of the phase transitions, using

Polarised optical microscopy (POM)

Differential scanning calorimetry (DSC)

Thermal Methods of Analysis

Polarised Light and Unpolarised Light

Polarised light (figure- a & b) is generated by the passage of unpolarised light (white light) through a polariser.

The polariser is a transparent anisotropic material, which selectively allows the transmission of light along one preferential plane of polarisation, which corresponds to the polariser optical axis. Examples of such kinds of materials are calcite prisms (e.g. Nicol prism) and polarising Filters (e.g. Polaroid).

(a) (b) (a) Representation of the unpolarised light, travelling in the direction perpendicular to the page. The electric (and magnetic) field vibrates in all the possible planes (represented by the arrows) perpendicular to the propagation of the light.

(b) Polarised light is characterised by only one plane of polarisation of the electric (and magnetic) field, which is represented by the vertical arrow.

Light Travelling in a VacuumElectric Field Magnetic Field

Visible Light = 400-700 nm

Ultra Violetnot visible to the eye

Infra Rednot visible to the eyeViolet

420nm

Blue

470 nm

Green

530 nm

Yellow

580 nm

Orange

610 nm

Red

700

x

y

z

x

y

z

x-Polarised Light y-Polarised LightIn vacuum light travels

at 300 x 106 ms-1

Amplitude Length Length

Amplitude

Electromagnetic Radiation

Light Travelling in a Vacuum

Electromagnetic Radiation

In vacuum light travelsat 300 X 10 6 ms-1

Light Travelling Through an Isotropic Medium

x

y

z

x

y

z

x-Polarised Light

y-Polarised Light

Water NaCl Crystal

Glass

Isotropic Medium

Refractive Index = n1

Refractive Index = n1

One Values for n

Light Travelling Through an Isotropic Medium

One Values for n

X and Y polarised light travelling through an isotropic medium

Light Travelling Through an Anisotropic Medium

x

y

z

x

y

z

x-Polarised Light

y-Polarised Light

Refractive Index = n2

Refractive Index = n3

Quartz Calcite

Anisotropic MediumTwo Values for n

Light Travelling Through an Anisotropic Medium

Two Values for n

X and Y polarised light travelling through an anisotropic medium

Index of Refraction

Light travelling through a vacuum does so at a velocity of ~ 3 X 10 8 ms-1, howeverthis changes in the presence of matter.

The electric and magnetic fields of a light wave affect the charges in a materialcausing them also to produce electric and magnetic fields.

The net effect of this is that the velocity of light passing through matter is less than that passing through a vacuum.

This retardation varies with the nature of the material, and each material is assigned a number that represents the factor by which the velocity of light is reduced. This is called the index of refraction, n, and is defined as:

~

n = c / v

Where: c = the velocity of light in a vacuum

v = the velocity of light in a material

Index of Refraction

Material Index of refraction, n

Air

Water

Glass

1.0003

1.33

~ 1.5

Indices of refraction for some common materials

The index of refraction of all materials is greater than one; the following values are for comparison

Reflection and Refraction of Light at the Surface of an Isotropic Materials

Refelected Beam

Refracted Beam

The path of the reflected or refracted light is independent of the polarization of light

Reflection and Refraction of Light at the Surface of an Anisotropic Materials

Refelected Beam

Ex, Ey Ex, Ey

Refracted BeamsEyEx

The path of the reflected light is indepenent of the polarization (Ex or Ey) of light

The path of the refracted light is dependent on the polarization of light

Birefringence or Double Refraction

Polarised optical microscopes are equipped with two polarisers (a polariser and an analyser), whose relative optical axis can be rotated from 0o to 900, changing from a parallel to a perpendicular arrangement respectively.

If the two polarisers are set up in series (at 0o) their optical axes are parallel, consequently light passes through both (figure a).

When they are in a crossed position (at 90o), their axes are perpendicular, therefore light from the first is extinguished by the second (figure b).

In order to investigate the mesophase behaviour of LCs, the most commonly

informative and used setting for the two polarisers is the crossed (90o) position.

Polarised Optical Microscopy (POM)

POM is employed to observe the mesophase textures of LCs, exploiting their anisotropic nature and, in particular, their birefringence when interacting with polarised light.

Polariser and Analyser

Figure : (a) When the polariser and analyser are in a parallel set up, their optical axes allow light transmission;

(b) when the polariser and analyser are crossed, the light from the polariser is absorbed by the analyser, resulting in dark condition.

Birefringence in LCs

As a consequence of the delay of one ray over the other, the two waves become out of phase.

Therefore, the plane of polarisation of the light is rotated.

Thus, when the polarised light reaches the analyser, there will be a component of it, which can go through its optical axis, and the light will be transmitted.

The preferential orientation of the molecules along the director, which forms an angle other than 0o or 90o with either the polariser or the analyser, is responsible for the rotation of the plane of polarisation and transmission of light with production of a bright field of view.

Hence, when a LC is placed between two crossed polarisers, it will shine bright interference colours, giving a characteristic pattern, which represents the “finger-print” texture of the mesophase.

When polarised light enters an anisotropic material (e.g. LC) it splits into two components, the ordinary and extraordinary rays, whose electric (and magnetic) fields vibrate in fixed planes at right angle to each other and propagate through the material at different velocities.

Birefringence

Birefringence is the term applied to the double refraction of nonpolarised light as it passes through an anisotropic material. This phenomenon occurs because the x-polarised and y- polarised component of the light interact differently with the anisotropic material, giving rise to two refractive indices, and therefore two refractedlight beams, as illustrated in the figure.

Refelected Beam

Ex, Ey Ex, Ey

Refracted BeamsEyEx

Optical texture of Ester at 70 0C

Polarized microscopy of the mesophasesPolarized microscopy of the mesophases

OROR

OROR

RO

RO

OCOROCOR

OCOROCOR

ROCO

ROCO

Optical texture of Ether at 50 0C

R= C5H11 R= C5H11Examples of OPM images

Mesophase textures

Schlieren texture of Nematic Fan-shaped texture of smectic

Mesophase Textures

Focal conic textures of smectic batonnets smectic

Mesophase Texture

B1B2

Banana-shaped LC

Mesophase Textures

B3 phase B4 phase

Banana-shaped LC

Differential Scanning Calorimetry (DSC)

Whenever a material undergoes a change in physical state, heat (Q) is either absorbed (e.g. melting) or liberated (e.g. solidification).

By monitoring calorimetrimetrically, the temperature change (ΔT) that accompanies a phase transition, it is possible to measure the energy involved, as a variation of enthalpy (ΔH), which is typical of the material for the transition under study.

Therefore, useful information for the characterisation of compounds is obtained by the calculation of ΔH.

DSC is one of the most widely used sophisticated methods to investigate samples behaviour over a range of programmed temperatures at constant pressure.

The term “differential scanning calorimetry” summarises the nature of the thermal

technique involved.

Calorimetry: the sample and an inert reference (commonly dry pre-heated alumina) are heated, simultaneously, at a defined rate, in an inert atmosphere at constant pressure over a programmed range of temperature

Scanning: the temperature of the system is scanned over a desired range as a function of time.

Differential: the difference in heat flow or power, ΔP (ΔP = dΔQ / dt) required to maintain the sample and the reference at the same temperature, is measured and plotted against temperature or time in a x y graph (since the thermal analysis is run under constant pressure, the measure of the heat corresponds to the enthalpy: ΔQ = Δ H).

An endotherm peak (ΔH< 0) is involved when there is absorption of more power by the material under analysis respect to the reference, whilst an exothermic peak (ΔH > 0) underlines absorption of more power by the reference, implying a liberation of energy by the analyzed material.

Plotting of the peaks upward or downward is a matter of convention.

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC)

Figure-a: DSC trace showing the typical pattern of a LC exhibiting a crystal to mesophase (K M) transition at 65.8oC, and a mesophase to isotropic liquid (MI) transition at 95.7oC. The endothermic peaks go up, and exothermic ones go down:y, heat flow (mW); x, temperature (oC)

Figure-a

From the DSC analysis it is possible to obtain the following quantitative data:

T: onset temperature of phase transition (by differentiation),

As: peaks area (by integration),

Δ H: enthalpy change of phase transition (by integration).

The measurement of Δ H is very useful to determine the entropy change (ΔS) associated with physical changes of LCs.

In fact at a transition temperature, any exchange of heat between the sample and the surrounding is reversible, because the two phase are in equilibrium.

Therefore, it is possible to calculate the change in entropy (Δ S = Δ H/ T).

Differential scanning calorimetry (DSC)

DSC Apparatus

The major parts of the system: 1. the DSC sensors plus amplifier, 2. the furnace and its temperature sensor, 3. the programmer or computer, 4. the recorder, plotter or data acquisition device

Δ indicates the differential signal

DCS: B7 phase of Banana-Shaped Achiral Mesogen

OO

OO

NN

OO HOC16H33

HC16H33O

112.7oC 172.7oCCr B7

I

The DSC thermogram obtained using heating and cooling modes (5 oC min-1)is shown in Figure

Only one mesophase is observed in both cyles.

DSC Thermograms fo Anthraquinone-based Discotic

O

O

OOC7H15

OC7H15O

C7H15O

C7H15O

DSC thermograms for (i) the first heating; (ii) secondheating, and (iii) first cooling

The DSC runs were recorded at a heating / cooling rateof 5 oC min-1

1,5-benzloxy-2,3,6,7-Tetraalkyloxy-9,10-anthraquinones

77.0

Cr Colx transition Colh

128.9I

143.5

127.7Colh

143.5 I

113.6 Colh

140.0 I

Int e

ns i

ty (

arb

.un

its)

0 10 20 30

2 (deg)

0 10 20 302 (deg)

Inte

nsi

ty (

arb

.un

its)

The overall features observed are consistent with the structure of the Colh phaseThe overall features observed are consistent with the structure of the Colh phase

X-Ray diffraction studiesX-Ray diffraction studies

intracolumnar

alkyl -

intracolumnar

alkyl-

OROR

OROR

RO

RO

OCOROCOR

OCOROCOR

ROCO

ROCO

R= C5H11 R= C5H11

Bragg Equation

When a beam of monochromatic X-rays of wavelength λ impinges on a crystal, strong scattering occurs in certain directions only: this is the phenomenon of X- Ray Diffraction

nλ = 2d sinθ

n= (1, 2, 3……., ) wavelengthsd= is the distance separating successive planes in the crystalθ = is the angle which the incident beam X-rays makes with the same planes

Final CommentsIdentification and systematic classification of scientication of scientific phenomena is vital in

any area of research.

Liquid crystals are no exception and many different liquid crystalline phases and other mesophases have been identified and classified according to their distinct phase structures.

Many liquid crystal phases (e.g., nematic, smectic A, smectic C and their chiral analogues) are commonly encountered in a wide range of compounds of varying molecular architectures.

Such liquid crystal phases are now easily identified by using optical polarising microscopy, usually in conjunction with differential scanning calorimetry.

However, some liquid crystal phases (e.g., antiferroelectric and ferrielectric phases ) are relatively recent discoveries and are more rarely encountered.

Although such novel LC phases can usually be identified by optical microscopy, their phase structures have not yet been fully elucidated and so other techniques such as X-ray analysis must be used.

Accordingly, just as the field of liquid crystals draws on the expertise of scientistsfrom many disciplines, the identification of mesophases requires a wide range of techniquesto identify and classify fully the different structures of the various mesophases.

As the identification techniques become more sophisticated, more novel mesophases will be discovered, possibly paving the way for the development of more technological applications.