Identification of liquid crystals phase-mesophase characterisation

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Identification of liquid crystals phase-mesophase
characterisation
CHM3T1 Lecture- 7

• Dr. M. Manickam
• School of Chemistry
• The University of Birmingham
• M.Manickam_at_bham.ac.uk

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Outline of Lecture

• Introduction
• Thermal Analysis
• Polarised Optical Microscopy
• Differential Scanning Calorimetry
• Mesophase Textures
• X-Ray diffraction

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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

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Examples
Example of a compound that shows no LCs phase
liquid water 0 degrees of order
solid crystalline water 3- (dimensional) degrees
of order
gaseous water 0 degrees of order
Example of a compound that shows LCs phases
Looks like milk 1 degree of order
3 degrees of order
0 degrees of order
0 degrees of order
3 degrees of order in solid form
gooey material 2 degrees of order
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Thermal Analysis
Thermal Methods of 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)

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Polarised Light and Unpolarised Light
(a)
(b)

1. 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.
2. Polarised light is characterised by only one
   plane of polarisation of the electric (and
   magnetic) field, which is represented by the
   vertical arrow.

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).
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Light Travelling in a Vacuum
Electromagnetic Radiation
In vacuum light travels at 300 X 10 6 ms-1
?
?
?
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Light Travelling Through an Isotropic Medium
X and Y polarised light travelling through an
isotropic medium
One Values for n
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Light Travelling Through an Anisotropic Medium
X and Y polarised light travelling through an
anisotropic medium
Two Values for n
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Index of Refraction

Light travelling through a vacuum does so at a
velocity of 3 X 10 8 ms-1, however this changes
in the presence of matter. The electric and
magnetic fields of a light wave affect the
charges in a material causing 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
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Index of Refraction
The index of refraction of all materials is
greater than one the following values are for
comparison
Indices of refraction for some common materials
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Reflection and Refraction of Light at the Surface
of an Isotropic Materials
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Reflection and Refraction of Light at the Surface
of an Anisotropic Materials
Birefringence or Double Refraction
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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.

• 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.

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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.
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Birefringence in LCs
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.
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.
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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
refracted light beams, as illustrated in the
figure.
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Polarized microscopy of the mesophases
Examples of OPM images
R C5H11
R C5H11
Optical texture of Ester at 70 0C
Optical texture of Ether at 50 0C
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Mesophase textures
Schlieren texture of Nematic
Fan-shaped texture of smectic
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Mesophase Textures
batonnets smectic
Focal conic textures of smectic
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Mesophase Texture
B2
B1
Banana-shaped LC
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Mesophase Textures
B4 phase
B3 phase
Banana-shaped LC
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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 (?T) that accompanies a phase
  transition, it is possible to measure the energy
  involved, as a variation of enthalpy (?H), 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 ?H.
• 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.

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Differential Scanning Calorimetry (DSC)

• 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, ?P (?P d?Q / 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 ?Q ? H).
• An endotherm peak (?Hlt 0) is involved when there
  is absorption of more power by the material under
  analysis respect to the reference, whilst an
  exothermic peak (?H gt 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.

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Differential Scanning Calorimetry (DSC)
Figure-a
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 (M?I) transition at 95.7oC. The
endothermic peaks go up, and exothermic ones go
down y, heat flow (mW) x, temperature (oC)
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Differential scanning calorimetry (DSC)

• 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),
• ? H enthalpy change of phase transition (by
  integration).
• The measurement of ? H is very useful to
  determine the entropy change (?S) 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 (? S ? H/ T).

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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
? indicates the differential signal
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DCS B7 phase of Banana-Shaped Achiral Mesogen
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.
112.7oC
172.7oC
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DSC Thermograms fo Anthraquinone-based Discotic
The DSC runs were recorded at a heating / cooling
rate of 5 oC min-1
77.0
143.5
128.9
I
Cr
Colx transition
Colh
113.6 Colh
140.0 I
1,5-benzloxy-2,3,6,7- Tetraalkyloxy-9,10- anthraqu
inones
127.7 Colh
143.5 I
DSC thermograms for (i) the first heating (ii)
second heating, and (iii) first cooling
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R C5H11
R C5H11
intracolumnar
intracolumnar
?-?
alkyl
Intensity (arb.units)
?-?
alkyl
0
10
20
30
2? (deg)
The overall features observed are consistent with
the structure of the Colh phase
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Bragg Equation

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

n? 2d sin?
n (1, 2, 3., ) wavelengths d is the distance
separating successive planes in the crystal ?
is the angle which the incident beam X-rays makes
with the same planes
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Final Comments

• Identification 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 scientists
• from many disciplines, the identification of
  mesophases requires a wide range of techniques
• to 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.