Transcript Identification of liquid crystals phase

Identification of liquid crystals
phase-mesophase
characterisation
CHM3T1
Lecture- 7
Dr. M. Manickam
School of Chemistry
The University of Birmingham
[email protected]
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
Example of a compound that shows no LCs phase
Ice Cube
heat
Water
heat
liquid water
0 degrees of
order
solid crystalline
water; 3- (dimensional)
degrees of order
Steam
gaseous water
0 degrees of
order
Example of a compound that shows LCs phases
Crystals of a solid
organic compound
heat
3 degrees of order
in solid form
heat
Looks like milk
1 degree of order
3 degrees of order
Crystals of a solid
organic compound
Nematic liquid
crystals phase
heat
Smectic liquid
crystal phase
gooey material
2 degrees of order
Isotropic
liquid
0 degrees of order
heat
Isotropic
liquid
0 degrees of order
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)
Polarised Light and Unpolarised Light
(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.
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).
Light Travelling in a Vacuum
Electric Field
Light
Travelling inRadiation
a Vacuum
Electromagnetic
Electromagnetic Radiation
Invacuum
vacuum light
light travels
In
6 -1 -1
at 300
ms
at 300
X 10x 610
ms
x-Polarised Light
x
A
m
p
l
i
t
u
d
e
y-Polarised Light
x

Length
z
y
Ultra Violet
not visible
to the eye
Magnetic Field
A
m
p
l
i
t
u
d
e
y
Violet
Blue
420nm
470 nm
Length
z

Visible Light
 = 400-700 nm
Green
Yellow
Orange
Red
530 nm
610 nm
700
580 nm
Infra Red
not visible
to the eye
Light Travelling Through an Isotropic Medium
X andTravelling
Y polarised light
travelling through
an isotropicMedium
medium
Light
Through
an Isotropic
x-Polarised Light
x
Isotropic Medium

Water Glass NaCl
Crystal
z
One Values
Values for
for nn
Refractive Index = n1
y
x
y-Polarised Light
z
y

Refractive Index = n1
Light Travelling Through an Anisotropic Medium
X and Travelling
Y polarised light
travelling through
an anisotropic Medium
medium
Light
Through
an Anisotropic
x-Polarised Light
x
Anisotropic Medium

Quartz
Values for
fornn
Two Values
Calcite
Refractive Index =
n2
z
y
x
y-Polarised Light
Refractive Index = n3
z
y

Index of Refraction
~ 3 X 10 8 ms-1, however
Light travelling through a vacuum does so at a velocity of ~
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
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
Material
Air
Index of refraction, n
1.0003
Water
1.33
Glass
~ 1.5
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
Birefringence or Double Refraction
Refelected Beam
Ex, Ey
Ex, Ey


 
Ex Ey
Refracted Beams
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
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.
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
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 “fingerprint” texture of the mesophase.
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.
Refelected Beam
Ex, Ey
Ex, Ey


 
Ex Ey
Refracted Beams
Polarized microscopy of the mesophases
R= C5H11
OR
OR
Examples of
OPM images
R= C5H11
RO
ROCO
RO
ROCO
OR
OR
Optical texture of Ether at 50 0C
OCOR
OCOR
OCOR
OCOR
Optical texture of Ester at 70 0C
Mesophase textures
Schlieren texture of Nematic
Fan-shaped texture of smectic
Mesophase Textures
Focal conic textures of smectic
batonnets smectic
Mesophase Texture
B2
B1
Banana-shaped LC
Mesophase Textures
B4 phase
B3 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.
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, 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)
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)
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),
 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).
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
C16H33O
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.
O
O
O
O
N
N
OH
112.7oC
Cr
172.7oC
B7
HO
I
OC16H33
DSC Thermograms fo Anthraquinone-based
Discotic
The DSC runs were recorded at a heating / cooling rate
of 5 oC min-1
O
O
C7H15O
OC7H15
C7H15O
OC7H15
O
77.0
Cr
143.5
128.9
Colx transition
Colh
I
O
113.6 Colh
1,5-benzloxy-2,3,6,7Tetraalkyloxy-9,10anthraquinones
140.0 I
127.7
Colh
143.5 I
DSC thermograms for (i) the first heating; (ii) second
heating, and (iii) first cooling
X-Ray diffraction studies
OCOR
OCOR
OR
OR
ROCO
RO
ROCO
RO
R= C5H11
OR
R= C5H11
OR
intracolumnar
intracolumnar
alkyl
0
OCOR
OCOR
10
20
2 (deg)
alkyl
-
30
0
10
-
20
2 (deg)
The overall features observed are consistent with the structure of the Colh phase
30
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……., ) wavelengths
d= is the distance separating successive planes in the crystal
θ = is the angle which the incident beam X-rays makes with the
same planes
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.