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Chapter-2
Synthesis, Characterization & Measurement Techniques
As already mentioned in Chapter 1 (Section-1.6), the present work
is focussed on a systematic study related to the electrical, optical and
structural behaviour of PMMA-PAni blends. In this chapter, after a brief
description of PMMA, the experimental procedure followed for the
synthesis of different forms of Polyaniline (emeraldine salt) doped with
Dodecyl benzene sulfonic acid (DBSA), Camphor sulfonic acid (CSA),
Iron (Fe) and Copper (Cu), respectively has been discussed. Next, the
method adopted for the preparation of PMMA-PAni blends on mixing
the synthesized PAni in PMMA at various concentrations has been
presented. Thereafter, various characterization techniques, like UV-
Visible Spectroscopy (UV-VIS), Fourier Transform Infrared (FTIR)
Spectroscopy and Raman Spectroscopy, used in the present study for
characterizing PAni and PMMA-PAni blends, have been described.
Finally, a brief description regarding the Current-Voltage (I-V) and
dielectric measurements related to these blends is presented.
2.1 POLY(METHYL METHACRYLATE)
Poly(methyl methacrylate), generally named as PMMA and having
the monomer composition C5H8O2, is a linear, amorphous,
Ph.D. Thesis: A.K. Tomar-2012
46
thermoplastic polymer with high transparency in almost entire visible
range [Mark et al. 1985; Hong 2001; Leontyev 2001; Tatro et al.
2003; Hadjichristov et al. 2009; Lin 2010]. Due to this extraordinary
feature, it is sometimes referred to as acrylic glass. The chemical
structure of this polymer is as shown in figure 2.1 [Billmeyer 2005].
Figure 2.1: Chemical structure of Poly(methyl methacrylate)
Some of the important physical parameters of PMMA [Wunderlich
1975; Billmeyer 1984; Stobl 1997; Brandrup et al. 1999; Mark 1999]
are listed below:
Density : 1.17-1.20 g/cm3
Optical Energy Gap : 2.72 eV
Refractive index : 1.49
Glass transition temperature (Tg) : 105 0C
Melting point : 160 0C
Working temperature Range : -40 0C to +90 0C
Dielectric constant (at 1 MHz, 250 C) : 2.6
Dissipation Factor : 0.04-0.06
Resistivity : >1015 Ωcm
Thermal Conductivity : 0.193 Wm-1K-1
Water absorption : 0.3-0.4%
It is extraordinarily resistant to oxidative photodegradation
[Hawkins 1972], remarkably stable to sunlight [Benfor and Tipper
1975] and displays good environmental stability [Billmeyer 1971]. Due
Chapter-2: Synthesis, Characterization & Measurement Techniques
47
to such extraordinary features, in addition to its scientific and
technological applications [Kuo 2003; Leontyev et al. 2003; Kulish
2003; Koval 2004; Sousa 2007; Dorranian 2009; Abdelrazek 2010;
Fawzy 2011]; this polymer is widely being used in many day to day
applications.
2.2 SYNTHESIS OF POLYANILINE
Different forms of Polyaniline (Emeraldine Salt) doped with Dodecyl
benzene sulphonic acid (DBSA), Camphor sulphonic acid (CSA), Iron
(Fe) and Copper (Cu) salts, have been synthesized chemically using
oxidative polymerization method [Chen 2002]. The reactions were
carried out at low temperature below 10 0C, to get PAni with high
molecular weight [Macdiarmid et al. 1985; Arms and Miller 1988;
Austrias et al. 1989; Cao et al. 1989; Genies et al. 1990; Kricheldorf
1992; Kang et al. 1995; Mav et al. 1999; Vijayan and trivedi 1999; Xie
and Xiang 2000; Rao et al. 2001; Chen 2002; Negi and Adhyapak
2002]. The details of the procedure adopted are as follows:
2.2.1 Polyaniline Doped with DBSA (PAni.DBSA)
PAni.DBSA has been synthesized from aniline using DBSA as
dopant and ammonium persulphate (APS) as oxidant. The molar ratio
of aniline:DBSA:APS was kept as 1:1:1. First, the prepared milky white
solution of aniline and DBSA (70 weight % in 2-propanol) in deionized
water was stirred magnetically in ice-cooled environment and then, the
precooled solution of APS was added to it dropwise, resulting in change
in the colour of the solution from milky white to green. The so obtained
viscous solution was kept at low temperature (below ~10 0C) for 24
hours. Subsequently, acetone was added to this solution to decrease
its viscosity and favouring precipitation. This dispersion was filtered
and the residue was washed repeatedly in deionized water till the
filtrate became colourless. Then, the obtained powder was dried in
vacuum oven at 70 0C for 48 hours. Finally, a dark green powder of
PAni.DBSA was obtained.
Ph.D. Thesis: A.K. Tomar-2012
48
2.2.2 Polyaniline Doped with CSA (PAni.CSA)
For the synthesis of PAni.CSA, the monomer (aniline), CSA as
dopant and APS as oxidant were used in the molar ratio of 1:1:1. The
cooled solution of APS in deionized water was added dropwise in the
precooled transparent aqueous solution of aniline-CSA with continuous
stirring. The obtained clear solution was kept at low temperature for
24 hours, resulting dark green suspension, which was filtered,
repeatedly washed and dried in vacuum for 48 hours giving the dark
green powder of PAni.CSA.
2.2.3 Polyaniline Doped with Fe Salt (PAni.Fe)
To synthesize PAni.Fe, the greenish brown solution of aniline and
FeCl3, prepared in deionized water, was allowed to cool in ice-cooled
environment under stirring. The molarity ratio of aniline:FeCl3:APS
was kept as 1:1:1. Afterwards, the cooled APS solution in deionized
water was put drop by drop in the above prepared solution of aniline-
FeCl3. After 24 hours of constant cooling followed by filtration, washing
with deionized water and finally, on drying in vacuum oven for 48
hours, the dark green powder of PAni.Fe was obtained.
2.2.4 Polyaniline Doped with Cu Salt (PAni.Cu)
For the synthesis of PAni.Cu, aniline and CuCl2 were dissolved in
deionized water and a greenish coloured solution was obtained. The
precooled solution of APS in deionized water was added in the
prepared aniline-CuCl2 solution dropwise and a dark bluish solution
was resulted. The molarity ratio of aniline:CuCl2:APS was kept as
1:1:1. After the 24 hours of cooling, this solution was turned to dark
green suspension, which on filtration followed by washing with
deionized water and drying in vacuum oven, resulted in a dark green
powder of PAni.Cu.
In order to confirm the characteristics of prepared PAni powders
having different dopants, the same were subjected to UV-Visible, FTIR
and Raman spectroscopy. Their conduction and dielectric behaviour
was analysed using V-I and dielectric measurements. Afterwards, to
Chapter-2: Synthesis, Characterization & Measurement Techniques
49
make the blends of as prepared PAni with PMMA, the PAni salts were
mixed with PMMA at different concentrations by weight following the
procedure, as discussed in the next section.
2.3 PREPARATION OF PMMA-PANI BLENDS
The blends of PMMA with different weight % of PAni.DBSA,
PAni.CSA, PAni.Fe and PAni.Cu were prepared through solution casting
method by dissolving the as-prepared PAni (ES) powder at varying
concentrations in PMMA taking Chloroform as solvent. The prepared
blended solutions were put to glass petri-dishes, which after the
evaporation of the solvent at room temperature, were converted to
PMMA-PAni blends in the form of free standing films. In order to study
the optical, electrical and dielectric response of the blends, these were
subjected to UV-Visible spectroscopy, I-V measurements, and
Dielectric spectroscopy, respectively. Further, the structural analysis of
these blends was carried out through Fourier Transform Infrared
(FTIR) and Raman spectroscopic techniques.
The following sections are devoted to the short details of various
experimental and measurement techniques, as mentioned above, in
order to reveal the induced changes in PMMA as an effect of blending
with different weight % of synthesized PAni salts, as mentioned in
Section-2.2.
2.4 UV-VISIBLE SPECTROSCOPY
Ultraviolet-Visible spectroscopy is one of the most important
analytical techniques to characterize organic compounds, especially
conjugated organic materials as these absorb energy in UV or Visible
region [Pavia et al. 1994; Banwell and McCash 1995; Zerbi 1999].
This technique is most widely used due to its simplicity, versatility,
speed, accuracy and cost-effectiveness.
2.4.1 Theory and Instrumentation
The absorption of electromagnetic radiations in the UV-Visible
region generally involves the transition of an electron between the
Ph.D. Thesis: A.K. Tomar-2012
50
electronic energy levels of the molecule[Campbell and White 1989;
Kemp 1991; Pavia et al. 1994; Banwell and McCash 1995; Campbell et
al. 2000; Gauglitz and Dinh 2003; Hollas 2005; Yadav 2007; Wilhelm
2008]. The energy supplied by the incident radiations will promote
electrons from their ground state (bonding or non-bonding) orbitals to
excited state (anti-bonding) orbitals. Figure 2.2 represents different
kinds of electronic transitions involved in UV-Visible region, which are
briefly discussed as follows [Campbell and White 1989; Pavia et al.
1994]:
Figure 2.2: Different electronic transitions in UV-Visible region
(a) σ to σ* Transitions
These transitions are generally employed in the saturated
hydrocarbons having only σ-bonds. In these transitions, an electron in
a bonding s-orbital is excited to the corresponding anti-bonding orbital.
The energy required for these transitions is quite large and generally
does not appear in typical UV-Visible spectrum and falls in far
ultraviolet region. For example, in case of methane, where only C–H
bonds (σ-bonds) are present, the maximum absorption appears at
about 150 nm.
Chapter-2: Synthesis, Characterization & Measurement Techniques
51
(b) n to σ* Transitions
These transitions are favourable in saturated compounds
containing atoms with lone pairs (non-bonding electrons). These occur
at usually lesser energy than σ to σ* transitions. These can be initiated
in wavelength range ~150 - 250 nm. The organic molecules having
halogens, Nitrogen, Oxygen and Sulfur show absorption in this region.
(c) n to π* and π to π* Transitions
Mostly, the absorption spectroscopy of organic compounds is based
on transitions of n or π electrons to the π* excited state. This is
because the absorption peaks for these transitions fall in an
experimentally convenient region of the electromagnetic spectrum
(~200-700 nm). These transitions need an unsaturated group in the
molecule to provide the π electrons. The compounds containing
double/triple bonds, or aromatic rings in their structure show these
types of transitions.
The absorption of light in an organic molecule is generally due to
the presence of unsaturated groups, such as C=C, C=O, −N=N−,
−NO2, or the benzene ring. A compound is more likely to absorb visible
light when it contains a conjugated system of alternate C=C and C−C
bonds, with the π -electrons delocalized over a larger area. Some of
the compounds show two absorption bands. This is because the
excitation of the π electron in the C=O leads to the π to π* transition
and the excitation of one electron of the lone pair of electrons on the
oxygen atom results in n to π* transition.
In case of an isolated atom, the UV-Visible absorption spectrum is
expected to exhibit a single, discrete line corresponding to each
transition while for the case of a molecule, the UV-Visible absorption
usually occurs over a wide range of wavelengths because in a
molecule, each electronic energy level (either in ground state or in
excited state) is, generally, accompanied by a large number of
vibrational and rotational energy levels, which are also quantized.
Therefore, a molecule exhibits transitions in many states of rotational
Ph.D. Thesis: A.K. Tomar-2012
52
and vibrational excitation. The energy levels for these states are quite
closely spaced with energy difference smaller than those of electronic
levels. Since the energy required to induce electronic transition is
larger than the energy required to cause the vibrational and rotational
transitions, the vibrational-rotational levels are superimposed on the
electronic levels and a molecule may, therefore, undergo electronic
and vibrational-rotational excitations simultaneously. The
vibrational/rotational transitions are so closely spaced that the
spectrometer cannot resolve them; rather, the instrument traces the
envelope over the entire pattern. Thus, the combination of these
transitions results in the broad band of absorption centered near the
wavelength of major transition [Pavia et al. 1994].
Figure 2.3 presents a general layout [Campbell and White 1989] of
a UV-Visible spectrophotometer. Typically, a UV-VIS
spectrophotometer consists of a light source, a monochromator and a
detector.
Figure 2.3: General layout of UV-Visible spectrophotometer
A single source covering the full UV-Visible wavelength range
cannot be made available. Therefore, two independent sources, one for
ultraviolet region (deuterium lamp) and other for visible region
Chapter-2: Synthesis, Characterization & Measurement Techniques
53
(tungsten lamp) of electromagnetic spectrum are used. Incident
electromagnetic radiation from the source passes through
monochromator, consisting of a prism/diffraction grating, which makes
the spectral components spread out followed by slit permitting only the
desired wavelength to pass. Beam splitter splits the beam for sample
and reference cells held in standard holders. Light passing through the
sample is measured by a detector, commonly a photomultiplier tube
(PMT), which records intensity of transmitted light, thus, providing the
transmittance as a function of wavelength.
In the present work, we have used Shimadzu double beam
double monochromator UV-Visible Spectrophotometer (UV-2550)
equipped with integrating sphere assembly ISR-240A, covering the
electromagnetic range from 190 nm to 900 nm to characterize the
synthesized PAni salts. For PMMA and different PMMA-PAni (ES) films,
PerkinElmer Lambda 750 UV/VIS/NIR Spectrometer was employed.
From the recorded spectra for pure PMMA and PMMA-PAni (ES)
blends, the variation in optical energy gap with the change in the
concentration of PAni in PMMA has been studied.
2.4.2 Determination of Optical Energy Gap (Eopt)
From the obtained UV-Visible spectrum for any compound, several
optical parameters, e.g. refractive index, dispersion parameters,
extinction coefficient and optical energy gap may be determined, which
play a very imperative role in defining the optical properties of the
system. Out of these parameters, optical energy gap is most
extensively studied because it directly relates the optical response of
the material with its electrical and luminescent behaviour.
The optical energy gap in polymers has been determined through
optical absorption spectroscopy using the Tauc’s relation [Tauc 1974;
Das et al. 1999; Mathai et al. 2002; Sun et al. 2002; Raja et al. 2003;
Biederman 2004; Migahed and Zidan 2006]
( ) ( )
Ph.D. Thesis: A.K. Tomar-2012
54
where, is the absorption coefficient corresponding to the
fundamental absorption edge in UV-Visible spectrum, is the energy
of photon, is a parameter connected with the distribution of density
of states (DOS) in the transport gap (in the band tails) and B is a
proportionality factor that also provides information about the DOS
distribution. It is connected with the length of tails of the localized
states. Smaller values of B indicate longer tails of these states. The
value of [Fink 2004] can be taken to be 1/2 for allowed direct
transitions, 1 for transitions between density of states at band edges,
3/2 for forbidden direct transitions, 2 for indirect transitions with
phonons and 3 for transitions between valance and conduction band-
edge tails.
2.5 FOURIER TRANSFORM INFRARED (FTIR)
SPECTROSCOPY
Infrared (IR) Spectroscopy is a powerful tool for identifying the
types of chemical bonds in a molecule by recording its infrared
absorption spectrum. It is one of the most powerful spectroscopic tools
available for the analysis of polymeric systems [Kemp 1991; Pavia et
al. 1994; Banwell and McCash 1995; Koenig 1999; Zerbi 1999;
Campbell et al. 2000; Coates 2000; Chalmers and Griffiths 2002;
Gauglitz and Dinh 2003; Hollas 2005; Smith and Dent 2005; Wilhelm
2008]. IR spectroscopy is molecular specific with high sensitivity. It is
based on the absorption or attenuation of electromagnetic radiation by
specified motion of chemical bonds present in a molecule.
Theory and Instrumentation
When infrared radiations pass through the material, these are
absorbed only at frequencies corresponding to the molecular modes of
vibrations and a plot of transmitted radiation intensity versus
frequency shows absorption bands. Infrared spectroscopy measures
the vibrational energy levels of molecules. As the vibrational energy
levels are distinctive for each molecule (and its isomers), the
Chapter-2: Synthesis, Characterization & Measurement Techniques
55
vibrational spectrum has often been called the fingerprint of a
molecule [Pavia et al. 1994; Koenig 1999].
In the IR absorption process, not all bonds in a molecule, but only
those that have a dipole moment which changes as a function of time,
can absorb infrared energy. This changing electrical dipole moment on
interaction with the field of the incoming IR radiation is responsible for
absorption. Symmetric bonds, such as those of N2, O2 or Cl2, do not
absorb infrared radiations because they do not have permanent dipole
moment [Pavia et al. 1994; Koenig 1999; Kumar 2008].
The earlier infrared spectrometers were of the dispersive type.
These instruments separated the individual frequencies emitted from
the infrared source by using an infrared prism or grating. The detector
measured the intensity at each frequency, which had passed through
the sample and resulted in a plot of intensity versus frequency. But
slow scanning process and low signal-to-noise ratio were the major
limitations of the dispersive type IR-spectrometer.
Fourier Transform Infrared Spectrometer provides the
simultaneous measurements of all the infrared frequencies, rather than
individual ones. Figure 2.4 presents a basic layout of this
spectrometer. It makes use of the Michelson interferometer [Campbell
and White 1989; Silverstein et al. 1991], which has two mutually
perpendicular mirrors one of which remains stationary while the other
is movable. A beam splitter, constructed from an IR-transparent
material, divides the incoming IR beam into two beams of equal
intensity traversing towards the two mirrors of the interferometer.
These beams reflect off from the respective mirrors and recombine
when they meet back at the beam-splitter. With the continuous motion
of one mirror, the path length of one beam remains fixed while that of
other is constantly changing. The output signal of the interferometer is
the result of interference of these two beams. The resulting signal is
called an interferogram, which has the unique property that every data
point, making up the signal, has information about every infrared
frequency coming from the source. This interferogram is allowed to
Ph.D. Thesis: A.K. Tomar-2012
56
pass through the sample and the detector records the transmitted
signal with attenuated intensities.
As the data analysis requires a frequency spectrum (a plot of the
intensity at each individual frequency) for identification, therefore, the
measured interferogram signal, which is a time domain signal, cannot
be interpreted directly. Therefore, a method of decoding the individual
frequencies is required, which can be accomplished via a well-known
mathematical technique, called the Fourier transformation. With the
use of this method, the desired spectral information for analysis can be
obtained. From the analysis of the spectrum received from FTIR, the
presence of various bonds and their number can be estimated.
Figure 2.4: Schematic layout of FTIR spectrometer
Fourier transform infrared spectroscopy has some unique features
[Koenig 1999] like
It is a non-destructive technique.
Chapter-2: Synthesis, Characterization & Measurement Techniques
57
It provides precise measurements and requires no external
calibration.
It has high speed as it simultaneously records the full spectrum in
less than a second.
Spectral resolution can be adjusted according to the requirement.
It has high signal-to-noise ratio as a large number of scans can be
averaged.
It is mechanically simple with only one moving part.
It provides accurate wavelengths based on internal laser
calibration.
In the present work, PerkinElmer ABB FTIR has been used to
deduce the structural information of PMMA and the induced structural
changes on blending it with different salts of PAni.
2.6 RAMAN SPECTROSCOPY
Like Infrared (IR), Raman spectroscopy is also used to reveal the
molecular structure and composition of inorganic and organic materials
[Sibilia 1988; Banwell and McCash 1995; Campbell et al. 2000; Koenig
2001; Chalmers and Griffiths 2002; Smith and Dent 2005]. However,
the method of operation of these spectroscopic techniques is different.
In IR spectroscopy, infrared radiation covering a range of frequencies
is directed onto the sample. Absorption occurs where the frequency of
the incident radiation is compatible to that corresponding to vibrational
excited state of the molecule. Conversely, in Raman spectroscopy, the
radiation from a monochromatic source (generally a laser) is allowed
to scatter from the molecule. In this spectroscopy, it is the change in
frequency, rather than the incident frequency itself, which provides the
information of the vibrational states of the molecule. In Raman
scattering, the irradiated light interacts with the molecule and distorts
(polarizes) the cloud of electrons around the nuclei to form short-lived
virtual states. The transitions between these virtual states and the
normal states of the molecule cause the Raman scattering [Smith and
Dent 2005].
Ph.D. Thesis: A.K. Tomar-2012
58
Further, intense Raman scattering occurs from vibrations, which
cause a change in the polarizability of the electron cloud round the
molecule. Usually, this polarization is dominant in symmetric molecules
and thus, results in the maximum changes. This is in contrast with
infrared absorption where the most intense absorption is caused by a
largest change in dipole moment, which is caused by asymmetric
vibrations. Thus, the presence of a permanent dipole moment, which is
must for IR absorption, is not required for Raman spectroscopy.
Therefore, not all vibrations of a molecule need to be both infrared and
Raman active, and the two techniques usually give quite different
intensity patterns. As a result, the two are often assumed
complementary and used together to have a better view of the
vibrational structure of a molecule [Smith and Dent 2005].
Theory and Instrumentation
The Raman scattering occurs when a material is irradiated by
monochromatic light, causing a small fraction of the scattered radiation
to exhibit shifted frequencies that correspond to the vibrational
transitions of the molecule. Figure 2.5 shows the possible vibrational
transitions, which occur for a single vibration [Sibilia 1988; Banwell
and McCash 1995; Campbell et al. 2000; Smith and Dent 2005].
At room temperature, most of the molecules, but not all, are
present in the lowest energy vibrational level. Since the virtual states
are not real states of the molecule and are created when the incident
monochromatic radiation interacts with the electrons and causes
polarization and thus, the energy of these states is dependent on the
frequency of the light source used. The Rayleigh transitions are the
most intense transitions since most photons scatter this way. These do
not involve any energy change and consequently, the light returns to
the same energy state. The Raman scattering process from the ground
vibrational state (Eground) leads to absorption of energy by the molecule
and its promotion to a higher energy excited vibrational state (Eexcited)
by means of interaction with the irradiated monochromatic light. This
is called Stokes scattering. However, due to thermal energy, there
Chapter-2: Synthesis, Characterization & Measurement Techniques
59
may be some molecules present in an excited state (Eexcited) and
scattering from these states to the ground state (Eground) is called anti-
Stokes scattering and involves transfer of energy to the scattered
photon.
More precisely, ground-state molecules produce lines shifted to
energies lower than the source energy, while the slightly weaker lines
at higher frequency are due to molecules in excited vibrational states.
These lines as a result of the inelastic scattering of light by the sample
are called Stokes and anti-Stokes lines, respectively. Elastic collisions
resulting in Rayleigh scattering produce much more intense lines at the
same frequency as that of the incident light.
Figure 2.5: Raman scattering between different vibrational levels
The essential components of Raman spectrometer [Campbell and
White 1989] are shown in figure 2.6. The configuration for a Raman
dispersive experiment basically consists of five parts: laser, sample,
dispersing element, detector and computer. A monochromatic laser
beam (excitation radiation) is focused on the scattering sample. The
scattered light from the sample is focused on the entrance slit of the
monochromator and dispersed. The dispersion element discriminates
between the strong elastic scattering (Rayleigh scattering) and the
weak inelastically scattered light (Raman scattering) with different
Ph.D. Thesis: A.K. Tomar-2012
60
frequencies. The Raman spectrum is recorded by the detection of the
intensity of the scattered, frequency-shifted light by a photo-detector.
The resulting signal of the detector is amplified and converted to a
form appropriate for plotting as a function of frequency or
wavenumber. Different peaks in the spectrum correspond to different
bonds present in the molecule.
Figure 2.6: Schematic representation of Raman spectrometer
Raman spectroscopy [Smith and Dent 2005] has a number of
advantages, which enforce the use of this method as structural
informer:
Raman spectroscopy is a scattering process and therefore, samples
of any size or shape can be examined.
Glass and closed containers can be used for sampling.
Fiber optics can be used for remote sampling.
Aqueous solutions can be analyzed easily as water is weak Raman
scatterer.
A single instrument can be used to scan the region from
10-4000 cm-1.
Chapter-2: Synthesis, Characterization & Measurement Techniques
61
Small samples can be studied since the laser beam can be focused
down to about a spot with a diameter of 100 µm.
In the present study, the Jobin-Yvon HR-800 Raman spectrometer
(He-Ne laser, λ = 632.8 nm), available at UGC-DAE-CSR, Indore have
been used to record the Raman spectra of pure PMMA, PAni (ES forms)
and their blends, synthesized as discussed in Section-2.2.
2.7 ELECTRICAL MEASUREMENTS
2.7.1 DC Conductivity
Conductivity of a specimen depends upon the factors like, the
number of charge carriers, nature of charge species and temperature.
Various methods, such as two probe, four probe, Van-der Pauw and
circular ring electrode, etc. [Sze 2004] are used to determine the
conductivity of a material. The method used to determine the
conductivity, generally depends upon the nature of the material
[Blythe 1984].
Low-resistivity materials, for which the contact resistance being of
major importance, require four-terminal method for reliable
measurements.
For high-resistivity materials, contact resistance is of only minor
significance and leakage current, especially over surfaces, becomes
the more serious problem. For this reason, three-terminal method,
using a guard electrode, is commonly used to nullify the problem.
Circular Guard Ring Electrode Method
This method is of great importance for highly resistive materials,
e.g. polymeric materials. As the conductivity of these materials is very
small, hence, the main problem becomes the leakage current from the
high-voltage source to the ammeter via routes other than the intended
one through the specimen. The surface of the specimen provides a low
resistance path due to the accumulation of dirt and moisture on it. For
this reason, a third terminal, i.e. the extra guard ring electrode, is
introduced around the low-voltage electrode and is connected to the
Ph.D. Thesis: A.K. Tomar-2012
62
ammeter so as to intercept and divert any leakage currents, which are
excluded from the ammeter reading. The schematic diagram for the
resistivity measurement using three electrode method is shown in
figure 2.7 [Blythe 1979, 1984; Blythe and Bloor 2005].
Figure 2.7: Schematic representation of guard ring electrode method
Bulk resistivity is measured by applying a voltage across opposite
sides of the insulator specimen. From the measured values of the
resultant current through the specimen, the bulk resistivity (ρ) is
determined using the expression
where, KV is the effective area of the guarded electrode, t is average
thickness of the sample and R is resistance of the sample determined
through V-I measurements.
If the electrodes used are circular in dimensions, then
(
)
Chapter-2: Synthesis, Characterization & Measurement Techniques
63
with D1 as the outside diameter of guarded electrode, g is the distance
between the guarded electrode and the ring electrode and B is the
effective area coefficient.
Bulk dc conductivity (σdc) of any specimen can be found by taking
the reciprocal of the resistivity, calculated using above expression, and
can be represented as
In the present work, conductivity of PMMA and various PMMA-
PAni(ES) blends has been determined using the Keithley 6517A
electrometer attached with Model 8009 resistivity text fixture having
interface with computer.
For the present set-up, the value of is provided to be 22.9 for R
to be measured in ohm and t in cm. Accordingly, the resistivity of the
sample is given by
( )
( )
Thickness (t) of the samples has been measured using digital screw
guage, having least count 1 µm, and resistance (R) has been
estimated from the plotted V-I curves.
2.7.2 Dielectric Studies
Dielectric measurements include the study of dielectric constant,
dielectric loss, relaxation time, electric modulus, ac conductivity, etc.
This measurement technique is sensitive to dipolar species as well as
localized charges in the material [Boyd and Smith 2007; Te-Nijenhuis
2009]. As explained in Chapter 1, Section-1.5 that whenever a
dielectric is placed in an alternating electric field, depending on the
frequency of the field, it may polarize through different mechanisms
[Raju 2003; Kao 2004; Blythe and Bloor 2005; Boyd and Smith 2007].
As a result, polarization vector ⃗ (a measure of polarization in the
Ph.D. Thesis: A.K. Tomar-2012
64
material) and displacement vector ( ⃗⃗ ) lag behind the electric field
vector ( ⃗ ) in phase and a loss of energy takes place, which is
responsible for the complex nature of the dielectric constant [Mardare
and Rusu 2004; Bhargav et al. 2008]. The loss factor (tanδ) is the
primary criterion for the usefulness of a dielectric as insulator. To
attain the high capacitance in the smallest physical space, materials
with high dielectric constant and low loss factor are required. The
imaginary part of complex dielectric constant gives the dielectric loss.
To study the frequency dependent dielectric properties of PMMA
and PMMA-PAni blends, the dielectric measurements were carried out
in the frequency range from 20 Hz to 5 MHz using Aligent 4285A
Precision LCR meter. The values of capacitance (C) and tangent loss
(tan) of the polymeric dielectric material were recorded from the
instrument. From these values, the real part of dielectric constant (ɛ')
(component of energy stored in each cycle of the electric field) and
imaginary part of the dielectric constant (ɛ'') (component of energy
dissipated in each cycle of the electric field) were calculated using the
following relations [Kao 2004; Blythe and Bloor 2005]
where, d is the thickness of the sample, A is the area of the electrode
and ɛ0 (= 8.854 x 10-12 F/m) is the permittivity of the free space.
Another important parameter, which measures the rate of
movement of charge species present in the dielectric when an
alternating field is applied, is ac conductivity (σac). It is related to the
dielectric loss and frequency as [Kao 2004; Blythe and Bloor 2005]
From these calculated frequency dependent parameters, the complete
dielectric behaviour of PMMA and its blends with various emeraldine
salt form of PAni have been estimated.
Chapter-2: Synthesis, Characterization & Measurement Techniques
65
The experimental results related to the optical, electrical
and structural behaviour of PMMA-PAni blends and their
interpretation are presented in the subsequent chapters.
Ph.D. Thesis: A.K. Tomar-2012
66
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