Synthesis, Characterization & Measurement Techniquesshodhganga.inflibnet.ac.in/bitstream/10603/32695/7/07_chapter 2.pdf · Synthesis, Characterization & Measurement Techniques As

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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.


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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


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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


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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.


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(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


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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


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(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]

( ) ( )


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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


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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


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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.


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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].


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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


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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


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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.


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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


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

(

)


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


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.


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.


66

REFERENCES Abdelrazek E.M., El-Damrawi G., Elashmawi I.S., El-Shahawy A., “The

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