electrochemical synthesis of novel polyaniline...
TRANSCRIPT
Electrochemical Synthesis of Novel Polyaniline-
Montmorillonite Nanocomposites and Corrosion
Protection of Steel
von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz
genehmigte Dissertation zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat)
vorgelegt von
MSc. Hung Van Hoang
geboren am 08.12.1973 in Hanoi, Vietnam
eingereicht am 02 Sep 2006
Gutachter: Prof. Dr. Rudolf Holze Prof. Dr. Stefan Spange
Dr.habil. Karin Potje-Kamloth
Tag der Verteidigung: 08 Januar 2007
Bibliographische Beschreibung und Referat
Bibliographische Beschreibung und Referat
Hung Van Hoang
“Elektrochemische Synthese neuartiger Polyanilin-Montmorillonite nanocomposite und
Korrosionsschutz von Stahl”
Diese Dissertation beschreibt eine neue elektrochemische Synthese neuartiger Compositmaterialien basierend auf dem Tonmineral Montmorillonite (MMT) und intrinsisch leitfähigem Polyanilin (PANI). Die Elektropolymerisation von Aniliniumionen, welche in die Tonmineralschichten eingebaut sind, wurde bei einem konstanten Potenzial durchgeführt. Das resultierende organisch-anorganische Hybridmaterial PANI-MMT wurde mit verschiedenen physikochemischen Methoden charakterisiert. Die Ergebnisse der Elementaranalyse zeigen, dass nur 10 % des Nanocompositmaterials aus leitfähigem PANI bestehen. Die Vergrößerung des Zwischenschichtabstandes von MMT, die bei Röntgendiffraktometrieuntersuchungen beobachtet wurde, lässt auf die Bildung von PANI innerhalb der Tonmineral-Taktoide schließen. IR-spektroskopische Untersuchungen deuten auf das Vorhandensein von Wechselwirkungen physikochemischer Art, wahrscheinlich Wasserstoffbindungen zwischen dem Tonmineral und Polyanilin, hin. Untersuchungen mit zyklischer Voltammetrie zeigten, dass die Anwesenheit von elektroinaktivem Tonmineral die elektrochemische Aktivität von PANI nicht beeinflusst. Das elektrochrome Verhalten von PANI-MMT Nanocompositen wurde mit UV-Vis-Spektroskopie untersucht, wobei sich herausstellte, dass das elektrochrome Verhalten vom PANI im Compositmaterial erhalten bleibt. Eines der technologischen Hauptanwendungsgebiete von leitfähigen Polymeren, insbesondere von PANI, ist der Korrosionsschutz von aktiven Metallen. PANI-MMT Nanocomposite die mit der angegebenen Methode (elektrochemisch) synthetisiert wurden und chemisch synthetisiertes in organischen Medien lösliches PANI wurden zum Korrosionsschutz von C45 Stahl eingesetzt. Die Korrosionsuntersuchungen wurden mit Hilfe von elektrochemischen Impedanzmessungen (EIM) und anodischen Polarisationsuntersuchungen durchgeführt. Der von PANI-MMT und von in organischen Medien löslichem PANI gebotene Korrosionsschutz ist wahrscheinlich auf die Zunahme des Ladungsdurchtritts widerstandes der beschichteten Stahloberfläche zurückzuführen. Die anodische Verschiebung des Korrosionspotenzials, eine Verringerung der Korrosions-geschwindigkeit und eine deutliche Zunahme des Polarisationswiderstandes sind eindeutige Hinweise für das Antikorrosionsvermögen von PANI-MMT und auch von in organischen Medien löslichem PANI, welche auf der zu schützenden Stahloberfläche abgeschieden wurden.
1
Abstract
Abstract
Hung Van Hoang
“Electrochemical Synthesis of Novel Polyaniline−Montmorillonite nanocomposites and
Corrosion Protection of Steel”
Chemnitz University of Technology, Faculty of Natural Science
This dissertation describes a new electrochemical synthesis of novel composite materials
based on montmorillonite (MMT) clay and intrinsically conducting polyaniline (PANI).
PANI was successfully incorporated into MMT galleries to form PANI−MMT
nanocomposites. Electropolymerization of anilinium ions which are intercalated inside the
clay layers have been carried out at a constant applied potential. The synthetic conditions
have been optimized taking into account the effect of concentration of aniline, magnetic
stirring and potential cycling. The resulting organic-inorganic hybrid material, PANI-
MMT has been characterized by various physicochemical techniques. Results of elemental
analysis show that nanocomposite contains only 10 % of conducting PANI. Formation of
PANI inside the clay tactoid has been confirmed by the expansion of inter layer distance of
MMT as revealed by X-ray diffraction studies. Relatively lower interlayer expansion for
PANI-MMT than that of anilinium-MMT indicates the higher stereoregularity in PANI-
MMT which has strong influence on electrical properties of nanocomposites. Infrared
spectroscopy studies reveal the presence of physicochemical interaction, probably
hydrogen bonding, between clay and polyaniline. Cyclic voltammetry studies indicate that
presence of electroinactive clay does not influence the electrochemical activity of PANI.
Electrochromic behaviour of PANI-MMT nanocomposites have been studied using in situ
UV-Vis spectroscopy which reveals that electrochromism of PANI in the composite
material has been retained.
One of the main technological applications of conducting polymers, particularly PANI, is
in the area of corrosion protection of active metals. PANI-MMT nanocomposites
synthesized using the present method and a chemically synthesized PANI which is soluble
in organic solvents have been used to protect C45 steel surface against corrosion.
Corrosion studies have been performed using electrochemical impedance measurements
2
Abstract
(EIM) and anodic polarization studies. Electrochemical impedance data has been analyzed
using a suitable equivalent circuit. Corrosion protection of steel offered by both PANI-
MMT and organically soluble PANI is evident form the increase in the value of charge
transfer resistance of the coated steel surfaces. Time dependent EIM measurements reveal
that charge transfer resistance gradually decreases with time, however, the values are much
higher than that of uncoated surfaces. Two capacitive loops, one at higher and another at
lower frequencies, observed in the Nyquist plots have been assigned to the electrical
properties of coating material (in the present case, PANI-MMT or soluble PANI) and
electrochemical process at the interface, respectively. An anodic shift in the corrosion
potential, a decrease in the corrosion rate and a significant increase in the polarization
resistance indicate a significant anti-corrosion performance of both PANI-MMT
nanocomposite and organically soluble PANI deposited on the protected steel surface.
3
Zeitraum, Ort der Durchführung
Die vorliegende Arbeit wurde in der Zeit von November 2002 bis Januar 2005 unter
Leitung von Prof. Dr. Rudolf Holze am Lehrstuhl für Elektrochemie der Technischen
Universität Chemnitz durchgeführt.
4
Dedication
Dedication
To my parents
To my teachers
To my sisters and brothers
To whom I love
Hung Van Hoang
5
Table of contents
Table of contents
BIBLIOGRAPHISCHE BESCHREIBUNG UND REFERAT ............................................... 1
ABSTRACT ................................................................................................................................. 2
DEDICATION ............................................................................................................................. 5
ACKNOWLEDGEMENT .......................................................................................................... 9
LIST OF ABBREVIATIONS................................................................................................... 10
1. INTRODUCTION ................................................................................................................. 12
1.1 Intrinsically conducting polymer ....................................................................................... 13
1.1.1 Polyaniline ...................................................................................................................... 13
1.1.2 Synthesis of PANI .......................................................................................................... 14
1.1.3 Conductivity of PANI..................................................................................................... 15
1.2 Montmorillonite (Clay minerals) ....................................................................................... 16
1.3 Organic-inorganic hybrid materials .................................................................................. 17
1.3.1 Polyaniline-montmorillonite (PANI-MMT)................................................................. 17
1.3.2 Characterization of PANI-MMT .................................................................................... 18
1.3.2.1 Cyclic voltammetry ................................................................................................. 18
1.3.2.2 X-ray diffraction..................................................................................................... 20
1.3.2.3 FT-IR spectroscopy ................................................................................................ 21
1.3.2.4 UV-Vis spectroscopy.............................................................................................. 21
1.3.2.5. In situ conductivity measurements ......................................................................... 22
1.4 Corrosion.............................................................................................................................. 23
1.4. 1 Corrosion protection of PANI ....................................................................................... 23
1.4.2 Techniques used in corrosion studies ............................................................................. 24
1.4.2.1 Electrochemical impedance measurements (EIM) .................................................. 24
1.4.2.2 Polarization measurements ...................................................................................... 29
6
Table of contents
1.5 Soluble PANI........................................................................................................................ 32
1.6 Synthesis of PANI-MMT..................................................................................................... 33
1.7 Aim and scope ...................................................................................................................... 34
2. EXPERIMENTAL................................................................................................................. 36
2.1 Chemicals and materials ..................................................................................................... 36
2.2 Preparation of anilinium montmorillonite ........................................................................ 36
2.3 Synthesis of PANI-MMT nanocomposites ....................................................................... 37
2.3.1 Electrochemical synthesis of PANI-MMT nanocomposites ......................................... 37
2.3.2 Chemical synthesis of PANI-MMT nanocomposites.................................................... 37
2.4 Synthesis of soluble PANI ................................................................................................... 38
2.5 Characterization of PANI-MMT nanocomposites .......................................................... 38
2.5.1 X-ray diffraction............................................................................................................ 38
2.4.2 FT-IR spectroscopy ....................................................................................................... 38
2.5.3 Cyclic voltammetry ........................................................................................................ 39
2.5.4 In situ UV-Vis spectroscopy ......................................................................................... 39
2.5.5 In situ conductivity measurements ................................................................................. 40
2.6 Corrosion studies ................................................................................................................. 40
2.6.1 Impedance and polarization measurements.................................................................... 41
2.6.2 Polarization measurements ............................................................................................. 41
3. RESULTS AND DISCUSSION............................................................................................ 43
3.1 Synthesis of PANI-MMT ................................................................................................... 43
3.2 Elemental analysis ............................................................................................................... 45
3.3 X-ray diffraction................................................................................................................. 46
3.4 FT-IR analysis..................................................................................................................... 48
7
Table of contents
3.5 Cyclic voltammetry.............................................................................................................. 50
3.6 In situ UV-Vis spectroscopy............................................................................................... 52
3.7 In situ conductivity measurements..................................................................................... 54
3.9 Corrosion studies ................................................................................................................. 55
3.9.1 The anti-corrosion properties of PANI-MMT................................................................ 56
3.9.1.1 Electrochemical impedance measurements ............................................................. 56
3.9.1.2 Polarization measurements ...................................................................................... 60
3.9.2 The anti-corrosion properties of soluble PANI .............................................................. 62
3.9.2.1 Electrochemical impedance measurements ............................................................. 62
3.9.1.2 Polarization measurements ...................................................................................... 66
4. SUMMARY............................................................................................................................ 69
5. REFERENCES ...................................................................................................................... 71
8
Acknowledgement
Acknowledgement
I would like to take this opportunity to express my deep gratitude to people who have
helped me in my research over the past 4 years.
First of all, I would like to send special thanks to my supervisor Prof. Dr. Rudolf Holze for
giving me a chance to study in Germany and invaluable guidance throughout this course. I
would also like to thank Prof. Dr. Stefan Spange and Dr. Ing. habil. Karin Potje Kamloth
for being as examiners and evaluating my thesis.
The financial support from The Ministry of Education of Vietnam is gratefully
acknowledged.
I also wish to thank to Subbu, Susanne, Anwar and all other members, former and present,
at department of electrochemistry−institute of chemistry−TU Chemnitz for their friendship,
help and care during these years.
To my teachers in Vietnam Prof.Dr. Tran Thanh Hue, Prof.Dr. Nguyen Duc Chuy, Dr.Tran
Hiep Hai, Dr. Nguyen Thi Thu, to my friends Nguyen Ngoc Ha, Nguyen Tien Dung, Tran
Thi Hoa, Tong Duy Hien and to my students Duong, Huyen, Long, Ngan, Hoang. Thank
you very much for your encouragement, love and care.
I am very grateful to Mr. M. Kehr in physics department for his help to record X-ray
diffraction.
Finally, the most grateful words are expressed to my parents, my sisters and brothers for
their moral support and love that they have given me.
9
List of abbreviations
List of abbreviations
A Surface area
AC Alternative current
AE Auxiliary electrode
Aw Molecular weight
ba Anodic slope
bc Cathodic slope
CC Coating capacitance
CDL Double layer capacitance
CE Counter electrode
CEC Cation exchange capacity
CR Corrosion rate
CV Cyclic voltammogram
d Density of metals or alloys
DBSA Dedocylbenzene sulfonic acid
DC Direct current
EB Emeraldine salt form of polyaniline
EC Equivalent circuit
Ecorr Corrosion potentials
Eeq Equilibrium potential
EIM Electrochemical impedance measurements
EM Emeraldine form of polyaniline
EQ Equivalent weight
ES Emeraldine base form of polyaniline
ESCE Potential versus saturated calomel electrode
η Overpotential
ηa Anodic overpotential
ηc Cathodic potential
FT-IR Fourier transform infrared spectroscopy
i0 Exchange current density
ia Anodic current density
10
List of abbreviations
ic Cathodic current density
icorr Corrosion current density
ITO Indium tin oxide coated glass
LE Leucoemeraldine form of polyaniline
MMT Montmorillonite
MPY Milliinche per year
OCP Open circuit potential
PANI Polyaniline
PN Pernigraniline form of polyaniline
RCT Charge transfer resistance
RF Film resistance
Rp Polarization resistance
RPM Rotations per minute
RS Solution resistance
SEM Scanning electron microscopy
TEM Transmission electron microscopy
TIP-5 Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 5:1
TIP-6 Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 7:1
TIP-7 Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 10:1
UV-Vis Ultraviolet visible
WE Working electrode
XRD X-ray diffraction
Z Impedance
'Z Real part of impedance
"Z Imaginary part of impedance
11
Introduction
1. Introduction
Generally composite materials can be defined as materials consisting of two or more
components with different properties and distinct boundaries between the components. The
idea of combining several components to produce a new material with new properties that
are not attainable with individual components has been used intensively in the past.
Correspondingly, the majority of natural materials that have emerged as a result of
prolonged evolution process can be treated as composite materials [1, 2].
Nanocomposites are generally defined as composites in which the components have at
least one dimension (i.e., length, width or thickness) in the size range of 1-100 nm.
Nanocomposites differ from traditional composites in a sense that interesting properties
can result from the complex interaction of the nanostructured heterogeneous phases. In
addition, nanoscopic particles of a material differ greatly in the analogous properties from
a macroscopic sample of the same material [3, 4, 5].
Conducting polymers are a class of polymer with conjugated double bonds in their
backbones. They display unusually high electrical conductivity and become highly
conductive only in their doped state. Due to the excellent electrical and electronic
properties and plastic nature of conducting polymers, they have been proposed for
application such as antistatic coating, corrosion protection, electrochromic display, sensors,
light-emitting diodes, capacitors, light weight batteries and gas permeation membranes,
etc. They are also believed to be promising alternatives to the environmentally hazardous
chromate conventional coating. There are many published reports focusing on the design,
preparation and characterization of novel organic-inorganic nanocomposites consisting of
conducting polymer with various layered materials, such as FeOCl [6], MoO3 [7-8], V2O5
[9] and clay minerals (montmorillonite (MMT)) [10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
20 , 21 ]. Since the advent of the nano-technology era, nanocomposites composed of
conducting polymers and inorganic particles have aroused much interest in the scientific
community. In order to improve the interesting properties possessed by conducting
polymers and to generate new properties, researchers are formulating organic-inorganic
hybrid materials based on conducting polymers.
12
Introduction
1.1 Intrinsically conducting polymer
1.1.1 Polyaniline
Polyaniline (PANI) has been known for more than one hundred years in its 'aniline black'
form, an undesirable black deposit formed on the anode during electrolysis involving
aniline. Among the conducting polymers, polyaniline (PANI) is the most promising
polymer due to its simple synthesis, controllable electrical conductivity, and good
environmental stability. PANI is a typical phenylene-base polymer having a chemically
flexible –NH– group in the polymer chain flanked on either side by a phenylene ring. The
protonation and deprotonation and various other physico-chemical properties of PANI can
be traced to the presence of the –NH– group [22]. It is well known that PANI exists in
three different oxidation states (leucoemeraldine, emeraldine, and pernigraniline); only
polyemeraldine is electrically conductive. The electronic transport properties of PANI can
be changed by doping electrochemically or chemically with some anions as shown in
Figure 1 [23].
NH
NH
NH
NH
n
n
NH2
+NH
NH
NH
Leucoemeraldine base Leucoemeraldine salt
NH
NH
N Nn
NH
NH
+ NH
NH
+
n
Emeraldine base Emeraldine salt
N N N Nn
Pernigraniline
Figure 1 The different polyaniline forms
13
Introduction
In the last decades, PANI has been one of the most extensively investigated of the
conducting polymers due to its electronic, electrochemical, and optical properties. In
addition, PANI has thermal stability, particularly in the conducting emeraldine salt form
and is a candidate for potential commercial application, such as in light-emitting diodes,
lightweight battery electrodes, sensors, electro-optics, electromagnetic shielding materials,
biochemical capacitors, and anticorrosion coating [24, 25, 26, 27, 28]. In recent years, due
to the development of nanotechnology, PANI has been employed for studying
nanocomposite materials in order to get new desired properties for practical applications.
1.1.2 Synthesis of PANI
PANI can be easily synthesized by both chemical method and electrochemical methods
[24] at ambient temperature. Chemical synthesis of PANI is carried out by direct oxidation
of aniline using an appropriate chemical oxidant such as hydrogen peroxide, ammonium
persulfate, in acidic medium, in particular sulfuric acid at a pH between 0 and 2. However,
chemical synthesis of PANI can also be carried out in neutral and basic media (in
acetonitrile or in aqueous solution) at pH values in the range of 9 to 10. The concentration
of aniline employed varies between 0.01 and 1 M [29].
In electrochemical synthesis of PANI, anodic oxidation of aniline is carried out on an inert
metallic electrode using two main modes: potentiostatic or galvanostatic. However, several
studies have been carried out with other electrode materials such as iron [30, 31, 32, 33,
34, 35, 36], aluminum and aluminum alloys [37]. In the case of electrochemical method of
synthesis, the potential is fixed or cycled with the value of the applied potential being in
order of 0.7 to 1.2 V (versus saturated calomel electrode potential, SCE) and that of cycled
potential being –0.2 to 0.7 – 1.2 V. The scan rates most commonly used are in the range of
10 to 100 mV s-1. The electrochemical synthesis of PANI offers some advantages over the
chemical methods. The resulting product is clean, does not need to be extracted from the
initial monomer/oxidant/solvent mixture. This method offers the possibility of coupling
with physical spectroscopic techniques for in situ characterization such as UV-visible,
Raman spectroscopy and conductometry [29].
14
Introduction
1.1.3 Conductivity of PANI
As mentioned earlier, PANI exists in three oxidation states (leucoemeraldine, emeraldine
and pernigraniline forms) that differ in chemical and physical properties [25, 29, 38]. Only
the green protonated emeraldine has conductivity on a semiconductor level of the order of
100 S cm-1, many orders of magnitude higher than that of common polymers (<10-9 S cm-1)
but lower than that of typical metals (>104 S cm-1). Protonated PANI converts to a
nonconducting emeraldine base when treated with alkali solutions (Figure 2) [38].
NH
+ NH
NH
NH
+
n
A A
N NH
N NH
n
AA-2n H+ +2nH+Emeraldine salt
Emeraldine base
Figure 2 Emeraldine salt is protonated in the alkaline medium to emeraldine base. A- is
arbitrary ion, e.g., chloride.
The conductivity of PANI can be changed by doping, and spans a very wide range (<10-12
to ∼ 105 S cm-1) depending on doping [22]. The changes in physicochemical properties of
PANI occurring in response to various external stimuli are used in various applications,
e.g., in sensors and actuators [38]. Other uses are based on the combination of electrical
properties typical of semiconductors with materials properties characteristic of polymers,
like the development of “plastic” microelectronics, electrochromic devices. The
establishment of the physical properties of PANI reflecting the conditions of preparation is
thus of fundamental importance.
15
Introduction
1.2 Montmorillonite (Clay minerals)
Among the large amount of layered solids such as graphite, layered double hydroxides,
transition metal dichalcogenides, metal phosphates and metal phosphonates, clay minerals
especially the members of smectite group are the most suitable candidates for synthesis of
polymer nanocomposites, because they possess a unique structure and reactivity together
with high strength, stiffness and high aspect ratio of each platelet. In particular,
montmorillonite [Mx(Alx-2Mgx)Si4O10(OH)2.nH2O] and hectorite [Mx(Mg3-x Lix)Si4O10
(OH)2.nH2O], where M indicates exchangeable monovalent ions, are most widely used in
this field. Montmorillonite is a hydrophilic mineral and belongs to the general family of
2:1 (so-called smectite) phyllosilicates (Figure 3) composed of stacked layers of
aluminum octahedron and silicon tetrahedrons. Substitution of aluminum with magnesium
will create an overall negative charge which is compensated by exchangeable metal cations
such as Na+, K+, Ca2+, Mg2+ [4, 39].
Figure 3 Schematic representation of the structure of montmorillonite (adapted from
reference 40)
16
Introduction
Characteristics of clay minerals which make them important are their particle size and
shape, cation exchangeability, adsorption properties and large surface area. The unique
structure, low negative charge per unit cell and weak van der Waals forces between
adjacent layers can allow the interlayer space of clay minerals to expand upon the
intercalation of organic cations, organic solvent or polymer. Clay minerals especially
montmorillonite have widely been employed to synthesize polymer-clay nanocomposites
due to their swelling behaviour, ubiquity and low cost.
1.3 Organic-inorganic hybrid materials
1.3.1 Polyaniline-montmorillonite (PANI-MMT)
Recently, accompany with advancement of nanotechnology and potential applications,
organic-inorganic nanocomposites have received considerable attention due to the special
nature of both components. Conducting polymers including PANI themselves could find
their niche in electronics, pharmaceutical, biomedical industries etc. However, there are
intrinsic problems with these materials that prevent them from wide commercial
applications such as poor processability due to rigid chemical structure and porosity of
their coating. Some of these limitations can be overcome by reinforcing the conducting
polymers with nanosized filler such as clay particles [5]. The incorporation of clay and
conducting polymers may provide characteristics which cannot be attained from pristine
conducting polymer such as processability [41]. Among organic-inorganic
nanocomposites, PANI-MMT nanocomposites are the most prevalent and interesting due
to the special properties as well as wide uses of polyaniline, the nature, abundance, low
cost of MMT and attractive features such as a large surface area and ion-exchange
properties [41, 42].
17
Introduction
1.3.2 Characterization of PANI-MMT
1.3.2.1 Cyclic voltammetry
Cyclic voltammetry is the most widely used electrochemical technique acquiring
qualitative information about electrochemical reactions. The power of cyclic voltammetry
results from its ability to rapidly provide considerable information on the thermodynamics
of redox processes, on kinetic of heterogeneous electron-transfer reaction, and on coupled
chemical reaction or adsorption processes. In a typical cyclic voltammetry, a solution
component is electrolyzed (oxidized or reduced) by placing the solution in contact with an
electrode surface, and then imposing sufficiently positive or negative potential on that
surface using a triangle potential waveform to force electron transfer. In simple cases, the
electrode surface is started at a particular potential with respect to a reference. The
electrode potential is swept to a higher or lower value at a linear rate, and finally, the
potential will sweep back to the original value at the same linear rate (Figure 4).
Switching potential
ReverseScan
ForwardScan
EFinal
E initial
Cycle 1
Pote
ntia
l
Time
Figure 4 Potential-time signal in cyclic voltammetry experiments
The electrochemical reaction of interest takes place at the working electrode (WE).
Electrical current at the WE due to electron transfer is termed as faradaic current. An
auxiliary (AE), or "counter" electrode is driven by a potentiostatic circuit to balance the
18
Introduction
faradaic process at the WE with an electron transfer of opposite direction. The process at
AE is typically not of interest, and in most experiments the small currents observed mean
that the electrolytic products at AE have no influence on the processes at the WE [43].
Depending on the information sought, single or multiple cycles can be applied. During
potential sweep, the potentiostat measures the faradaic current at the WE resulting from the
applied potential. The resulting plot of current versus potential is called cyclic
voltammogram, which is a complicated, time-dependent function of a large number of
physical and chemical parameters.
-0.4 -0.2 0.0 0.2 0.4
-0.8
-0.4
0.0
0.4
0.8
reverse scan
forward scan
R O
OR
cath
odic
c
urre
nt/m
A
a
nodi
c
Potential/V Figure 5 Typical cyclic voltammogram for a reversible O + ne ⇔ R redox processes.
Figure 5 shows one expected response of a reversible redox couple during a single cycle. It
is supposed that only oxidized form O is present initially. A negative-going potential scan
is chosen for the first half-cycle, starting from the value where no reduction occurs. A
cathodic current begins to increase, until a peak is reached. After traversing the potential
region where the reduction process takes place, the direction of the potential sweep is
reversed. During the reverse scan reduced molecules are re-oxidized to O and an anodic
peak results [44, 45]
19
Introduction
The cyclic voltammogram is characterized by several important parameters. Four of these
observable parameters, the two peak currents and two peak potentials, provide the basis for
the diagnostics in order to analyze the cyclic voltammetric response.
1.3.2.2 X-ray diffraction
X-ray diffraction is a versatile, non-destructive technique used for identifying the
crystalline phases present in solid materials and powders and for analyzing structural
properties (such as stress, grain size, phase composition, crystal orientation, and defects) of
the phases. The method uses a beam of X-rays to bombard a specimen from various
angles.
The X-rays are diffracted from successive planes formed by the crystal lattice of the
material, according to Bragg's law: nλ = 2dsinθ with n is an integer, λ is the X-ray
wavelength, d is the distance between crystal lattice planes and θ is diffraction angle
(Figure 6). By varying the angle of incidence, a diffraction pattern emerges that is
characteristic of the sample. The peak positions, intensities, widths and shapes in the
resultant X-ray pattern provide important information about the structure of the material.
Figure 6 Schematic representation of diffraction of X-rays in a crystalline material.
20
Introduction
Structure of PANI-MMT is generally elucidated using X-ray diffraction (XRD) and
Fourier transform infrared (FT-IR) spectroscopy. XRD, in particular wide angle XRD, is
the most commonly used technique for exploring the structure of PANI-MMT as well as
polymer-MMT [46]. Monitoring the position, shape and intensity of the basal reflections;
intercalated nanostructure can be easily identified. The layer expansion of an intercalated
nanocomposite is associated with appearance of a new basal reflection corresponding to
the larger gallery height. However, such technique cannot provide information about the
formation of PANI (or other polymer) inside MMT as well as the interaction between
PANI and MMT.
1.3.2.3 FT-IR spectroscopy
Infrared spectroscopy is one of the most powerful techniques available for analytical
chemists. Fourier transform infrared (FT-IR) spectroscopy is an interferometry-based IR
technology offering a faster, more sensitive means of analysis than traditional dispersive
IR spectroscopy. FT-IR spectroscopy that has been widely used in laboratory and industry
for several years is a non-destructive technique for determination of chemical compounds
in liquids, gases, powders and films. FT-IR spectroscopy is used within a broad range of
applications including; biomedical research, foodstuff analysis, gas and solid surface
analysis. The advantages of FT-IR method include multi-component analysis capability,
good sensitivity, excellent specificity, speed and simplicity of calibration. Infrared
spectroscopy is applicable to both qualitative and quantitative analysis. The FT-IR
spectrum provides information about the molecules present in a given sample. Thus, FT-IR
spectroscopy can provide information about presence of PANI inside MMT and any
interaction between them.
1.3.2.4 UV-Vis spectroscopy
Ultraviolet and visible (UV-Vis) spectroscopy is a reliable and accurate analytical
laboratory assessment procedure that allows for both qualitative and quantitative analysis
of a substance. Specifically, UV-Vis spectroscopy probes the electronic transitions of
molecules as they absorb light in the UV and visible regions of the electromagnetic
21
Introduction
spectrum. Any species with an extended system of alternating double and single bonds will
absorb UV light, and anything with colour will absorb visible light, making UV-Vis
spectroscopy applicable to a wide range of samples (molecules and inorganic ions or
complexes in solution) in different fields such as forensic science, pharmaceuticals, food,
biochemistry and analytical chemistry [47].
When sample molecules are exposed to light having energy that matches a possible
electronic transition within the molecule, some of the light energy will be absorbed as the
electron is promoted to a higher energy orbital. An optical spectrometer records the
wavelengths at which absorption occurs, together with the degree of absorption at each
wavelength. The resulting spectrum is presented as a graph of absorbance versus
wavelength. The peaks in a UV-Vis spectrum are commonly due to n → π* and/or π→ π*
transitions. Both the shape of the peak(s) and the wavelength of maximum absorbance
(λmax) in spectrum give information about the structure of the compounds. The
combination of electronic absorption spectroscopy and electrochemistry provides valuable
information regarding electrochromic properties of materials. For PANI-MMT, in situ UV-
Vis spectroscopy can provide information about electrochromism properties,
electrochromic stability and pH sensitivity etc.
1.3.2.5. In situ conductivity measurements
For over two decades, conducting polymers have been studied in great detail due to
their potential applications. One of the most important factors results in the potential
application is their conductibility. The conductivity of polymers depends upon how the
polymers were processed and manipulated [ 48 ]. So conductivity measurement is an
important step to characterize conducting polymers. Conductivity measurements of
polymer can be both operated by ex situ (two or four-probe method) and in situ
measurements. However, in situ electrical conductivity measurements of polymer using a
bandgap electrode are greatly simplified for polymer film deposited on this electrode [49].
In addition, the differences of doping state or electrolyte solution composition can be seen
through changes in electrical conductivity for many polymer films prepared
electrochemically. The relative conductivity changes of polymers let us to understand the
22
Introduction
characteristic material property and provide helpful knowledge for the development of
mechanistic conduction models for conducting polymers.
1.4 Corrosion
1.4. 1 Corrosion protection of PANI
Metal surfaces undergo corrosion when they are exposed to air, water or other corrosive
media, resulting in unwanted waste of materials and catastrophic failure of structures. It is
estimated that corrosion and its consequences cost developed nations between 3 to 5 % of
their gross domestic product amounting to over 100 billion dollars per year [37]. This
statistic gives an idea on what an important task it is to prevent corrosion. To prevent
metals from corroding, one of the most commonly practiced techniques is to apply organic
coatings. The commonly used organic coatings are formulated from thermosetting resins
such as epoxy, polyester and polyurethane. The present coating technology requires the
presence of corrosion inhibitors such as chromate compounds to provide sufficient
protection. However, the strict EPA environmental regulation requires the elimination of
the heavily used chromate inhibitors by the year 2007. Therefore, an environmentally
friendly and effective corrosion inhibitor is needed urgently for the shipping, aerospace,
and automobile industries [5].
Conducting polymers, especially PANI have been extensively investigated for their ability
to protect metals against corrosion in aqueous media since the work of Deberry [50] who
observed effective passivation of iron by PANI layer which had been electrode deposited
in perchloric acid. Due to the chemical stability and environmental viably, PANI is a
promising material for anti-corrosion purposes and can replace conventional coatings (e.g.,
chromate coating), which have adverse effects on environment [36, 51, 52, 53]. Protective
layers of PANI can be either electrodeposited or spin/drop coated on metal substrate.
However, the mechanism of corrosion protection is still not fully understood. So far,
several mechanisms have been proposed to explain the nature of protection of steel by
PANI. Some authors believe that the PANI coating layer protects steel simply by
producing some sort of barrier effect [37, 54]. But others say that PANI coating layer aid
23
Introduction
to form a passive oxide film on the metal surface through an oxidation-reduction process
[ 55 ]. PANI oxidizes iron to Fe2+ and itself is reduced to leucoemeraldine. Further
oxidation of Fe2+ leads to Fe2O3 and oxygen re-oxidizes leucoemeraldine form of PANI to
emeraldine salt [56]. There is still some conjecture as to the exact mechanism of corrosion
protection of PANI due to variations in experimental procedures used (coating type,
substrate preparation, corrosive environment, test method). Therefore, more study on study
corrosion protection of PANI is needed. This is also the reason of our choice of PANI in
this research.
1.4.2 Techniques used in corrosion studies
1.4.2.1 Electrochemical impedance measurements (EIM)
Electrical resistance is the ability of a circuit element to resist the flow of electrical current.
Ohm's law defines resistance in terms of the ratio between voltage U (Volt) and current I
(Ampere).
R = U/I (1)
However, this relationship is limited to one circuit element, resistor. In the real world,
many systems (systems consist of other elements such as capacitor, inductor) exhibit a
much more complex behaviour and the elements force us to abandon the simple concept of
resistance. In its place we use impedance, Z, which is a measure of circuit's tendency to
resist the flow of an alternating electrical current. The expression of Z is:
Z = Uac/Iac (2)
The electrochemical impedance measurement is a method to study electrochemical
processes at the electrode surfaces [57]. A small AC voltage perturbation (from 1 to 10
mV) is applied to an electrode/solution interface, the resulting alternate current is
measured, and corresponding electrochemical impedance is obtained as a function of the
AC frequency (at one or several DC potential values) and an equivalent circuit (EC) is
deduced from the measurements in order to analyse impedance data.
The EC is an electrical circuit composed of resistors, capacitors, inductors, and other
special components such as constant-phase elements (CPE), which serve as an electrical
24
Introduction
model of the physical interface. The components of the EC are then related to physical
features and/or processes at the electrode/solution interface through suitable modelling.
Thus EIM gives in a relatively straightforward way an electrical characterization of the
electrochemical system and has been applied to the study of faradic electrode reactions,
characterization of rough and porous electrodes, and partially or totally blocked electrodes.
This field includes problems such as adhesion of particles and scale deposits, passivation
and corrosion of metals, and performance of protective coatings. Latter two topics are the
most important practical applications of EIM, allowing the investigation of protection
methods such as corrosion inhibitors, conversion and barrier coatings, oxide layers, and
cathodic protection. The adhesion performance and the water uptake of protective coatings
can also be tested using EIM, which is nearly the only non-destructive technique available
to study those problems [57, 58].
The AC voltage perturbation, expressed as a function of time, has the form:
Ut = U0 sin(ωt) (3)
Ut is potential at time t; U0 is amplitude of signal; ω is the radial frequency. The
relationship between radial frequency ω (expressed in radians/second) and frequency
(expressed in hertz) is:
ω = 2πf (4)
In a linear system, the response current intensity, It, is shifted in phase (θ), and has
different amplitude, I0.
It = I0 sin (ωt + θ) (5)
An expression analogous to Ohm’s law allows us to calculate the impedance of the system
as:
)sin()sin(
)sin()sin()(
0
0
t
tθω
ωθω
ωω+
=+
==t
tZtI
tUI
UZ (6)
The impedance is therefore expressed in terms of a magnitude, Z0, and a phase shift, θ.
It is possible to express response current and perturbation potential as complex functions:
It = I0 sin (jωt - jθ) (7)
Et = E0 sin(jωt) (8)
With Euler’s relation: exp (jθ) = cos (θ) + jsin(θ), the impedance now becomes:
''')sin(cos)exp()sin(
)sin()(0
0
t
t jZZjZjZjtjI
tjEIEZ +=+==
−== θθθ
θωωω (9)
25
Introduction
Where 'Z , "Z are real part and imaginary part of impedance, respectively. In the complex
plane, the impedance of a single frequency ca be represented by a vector of length (Figure
7) with argument θ (angle between this vector and the x-axis)
Z
θ
Z''
Z'
Im (Z)
Re (Z)
Figure 7 The impedance plotted as a planar vector using rectangular coordinate.
The modulus Z and phase angle θ are related to 'Z and "Z by equations:
22 )"()'( ZZZ += (10)
θ = atan ( 'Z / "Z ) (11)
The obtained data can be represented in two types of plot: Nyquist plot ( "Z versus 'Z ) and
Bode plot (log Z or phase angle θ versus log frequency).
Figure 8a shows the Nyquist plot of impedance of the simple reaction. The impedance was
almost entirely created by the ohmic resistance, RS. The frequency reaches its high limit at
the leftmost end of semicircle, where semicircle touches the x-axis. At low frequency limit,
the impedance also approximates a pure resistance, but now the value is (RS + RCT). The
frequency reaches its low limit at the rightmost end of the semicircle. In Nyquist plot, it is
easy to see the effects of ohmic resistance.
26
Introduction
a
"Z (ω)max
Increasing ω
RS RS + RCT'Z
"Z
b
RS CDL
RCT
Figure 8 (a) Nyquist plot of impedance for a simple electrode reaction and (b)
corresponding equivalent circuit. RS is solution resistance, CDL is double layer
capacitance and RCT is charge transfer resistance.
If the frequency is sufficiently high, it is easy to read the ohmic resistance by extrapolating
semicircle toward the left, down to the x-axis. In addition, it is possible to compare the
results of two separate experiments which differ only in the position of reference
electrodes. However, frequency does not appear explicitly in Nyquist plot and the electrode
capacitance can be calculated only after the frequency is known.
In a typical Bode plot (Figure 9), the absolute impedance,⏐Z⏐, and phase angle are
presented as a function of frequency. Bode plot has some distinct advantages over Nyquist
plot. It is easy to understand from plot how the impedance depends on the frequency due to
appearance of frequency, f, or angular frequency (ω = 2πf) on one axis. The values of RS
and RCT can be obtained from Bode plot. At high frequency, the solution resistance
dominates the impedance and log(RS) can be read from the high frequency horizontal
plateau. At the lowest frequency, charge transfer resistance also contributes to impedance,
and log (RS + RCT) can be read from low frequency horizontal plateau. At the intermediate
frequency, curve should be a straight line with a slope of –1. Extrapolating this line to the
27
Introduction
Zlog axis at log ω = 0 (ω =1) yields the values of CDL from relationship Z = 1/ CDL
(Figure 9).
RS + RCT
RS
Z = 1/CDL
log Z
θω max
-900
00
θ
logω
Figure 9 Bode plot for the same data with the Nyquist plot in Figure 8.
Corresponding to impedance plot, a suitable equivalence circuit (EC) related to the
electrochemical reaction at the electrode should be constructed for explanation. The choice
is based on the understanding of electrochemical cell in study. The values of circuit’s
elements are obtained by fitting procedure. However, an impedance plot can be simulated
by several ECs. Figure 10 shows an example (see next page). The Nyquist plot shows two
semi circles and the corresponding ECs shown below. However, there is no a unique EC
that can be used to simulate a specific impedance plot. The choice of EC and elements of
EC should be based on the understanding of electrochemical system and the condition of a
fit with a minimal deviation between the measured and simulated results.
28
Introduction
a
RS RS + RCT 'Z
"Z
b
Figure 10 Nyquist plot impedance (a) and corresponding ECs (b) can be used to simulate
the plot.
1.4.2.2 Polarization measurements
When two complementary processes: such as those illustrated in Figure 11 and given
below occur over a single metallic surface:
Anodic reaction: M → Mn+ + ne−
Cathodic reaction:
2H+ + 2e− → H2 (in acidic medium)
4H+ + O2 + 4e−→ 2H2O (in acidic medium containing dissolved oxygen)
4H+ + O2 + 2H2O → 4OH− (in neutral or basic medium)
The potential of materials will no longer be at an equilibrium value. This deviation from
equilibrium potential is called polarization. Electrodes can also be polarized by the
application of an external voltage. The magnitude of polarization is usually measured in
terms of overpotential η, which is a measure of polarization with respect to the equilibrium
potential Eeq of an electrode. This polarization is said to be either anodic, when the anodic
processes on the electrode are accelerated by changing the specimen potential in positive
direction, or cathodic, when the cathodic processes are accelerated by moving the potential
in negative direction. Overpotentials corresponding to the anodic and cathodic polarization
29
Introduction
are called anodic (ηa) and cathodic (ηc) overpotential, respectively. Overpotential can be
expressed as follows: η = E - Eeq (E is applied potential) [57, 58, 59, 60, 61]
nH+
M n+
ne-
Figure 11 Simple model describing the electrochemical nature of corrosion processes
During polarization, reduction and oxidation reactions occuring on the surface of metal
produce a net electric current on the surface of metal. The sum of current density of these
reactions is related to overpotential by Butler-Volmer equation :
⎭⎬⎫
⎩⎨⎧
⎥⎦⎤
⎢⎣⎡ −−−⎟
⎠⎞
⎜⎝⎛= η
RTnFη
RTnFii )(1expexp0 αα (12)
Where: i0 = exchange current density (anodic or cathodic current density at the equilibrium
potential Eeq)
α = charge transfer barrier or symmetry coefficient for the anodic or cathodic reaction,
close to 0.5
η = overpotential and equal with Eapplied − Eeq
n = number of participating electrons
R = gas constant
T = absolute temperature
F = Faraday constant
Under anodic and cathodic polarization individually, the Butler-Volmer reduces to:
30
Introduction
⎥⎦⎤
⎢⎣⎡= a0a exp η
RTαnFii (for ia >> ic , ηa >> ηc ) (13)
⎥⎦⎤
⎢⎣⎡ −−−= c0c
)1(exp ηαRT
nFii (for ci >> ia , cη >> ηa ) (14)
Solving these two equations for the overpotential yields:
ηa = - (RT/αnF)lni0 + (RT/αnF)lnia (15)
c0c lnln i)nF-(1
RTi)nF-(1
RTαα
η −= (16)
Both of these equations can be written in a short form called Tafel equation:
η = a ± b log i (17)
where: b is known as Tafel slope of the anodic or cathodic reaction (anodic overpotential
ba = (RT/αnF), cathodic overpotential bc = RT/(1-α)nF, the ± sign uses for anodic and
cathodic overpotential, respectively.
A plot of applied potential (or overpotential) versus logarithm of current density is called
Tafel plot in which the values of Tafel slopes, corrosion potential Ecorr, and corrosion
current density icorr can be determined using extrapolation (Figure 12).
Since, polarization resistance and corrosion rate will be obtained by using Stern-Geary
equation: [54, 57, 59, 62, 63, 64, 65]
Aibbbb
R⋅
⋅+
⋅=
corrca
cap
1)(303.2
(18)
And corrosion rate can be calculated using equation: [66, 67, 68, 69]
dAEWi
C⋅⋅⋅
=)(129.0 corr
R (19)
Where: CR = corrosion rate
icorr = current density of corrosion
ba, bc = Tafel slope of anodic and cathodic reactions
Rp = polarization resistance
A = corroded area
d = density of the materials
EW = equivalent weight.
31
Introduction
CR has a unit in milliinch per year (MPY) if A is measured in cm2, d in g cm-3, EW in g
equivalent -1, icorr in μA cm-2.
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
ba= tanα'
bc= tanα''
α''
α'
anodic reaction
cathodic reaction
logicorr
EcorrA
pplie
d po
tent
ial/
V
Log(i/mA.cm-2)
Figure 12 Schematic polarization curve showing Tafel extrapolation.
1.5 Soluble PANI
The application of PANI in different fields depends on the processability of PANI; this
results in seeking a new synthesis of polyaniline in order to achieve processable PANI such
as soluble PANI which may increase the applicability of PANI [25]. Several research
groups reported enhanced solubility of the parent PANI emeraldine salt when they use
bulky acidic groups as a dopant or when they apply a different synthetic route [70, 71].
Laska and Widlarz [72] have reported synthesis of water-soluble polyaniline with various
32
Introduction
phosphonic and sulfonic acids as dopants. PANIs in this case are directly produced as
dispersions in water and they are stable only for a few days. Kinlen et al. [73] have
reported a synthetic protocol for PANI doped with dinonylnaphthalenesulfonic acid
(DNNSA). An emulsion polymerization pathway is employed to prepare PANI-DNNSA in
the organic phase, which can be purified and extracted as a suspension in organic solvent.
However, authors report that reprecipitated PANI has very low solubility. Ito et al. [70]
have prepared sulfonated polyaniline that can be dissolved up to 88 g/L in water. They
adapted a difficult procedure to sulfonate the emeraldine base and the solubility of the
resulting material depends on the S/N ratio. However, the conductivity of the PANI is low
in the range of 0.02 to 1×10-5 S cm-1. Recently, Athawale et al. [ 74 ] synthesized
polyaniline codoped with acrylic acid but the PANI synthesized is soluble only in NMP
and m-cresol. Ruckenstein and co-worker [71] reported PANI codoped with HCl and
DBSA and the resulting material is soluble in chloroform. Conductivity of PANI prepared
is as high as 7.9 S cm-1 but the solubility is not clearly defined. Recently, Sathyanarayana
and co-workers [75, 76] successfully used benzoyl peroxide as the oxidizing agent for the
polymerization of aniline with many organic and mineral acids as dopants in good yield
and conductivity. However, the resulting polymers have low solubility and the authors
have not studied PANI doped with DBSA. Further works on synthesis of soluble PANI
should be continued in order to increase the applicability of PANI.
1.6 Synthesis of PANI-MMT
As in the case of PANI, composite of PANI-MMT can also be synthesized either
chemically or electrochemically. Several reports have also been published on the chemical
synthesis of PANI-MMT nanocomposites from intercalated anilinium-MMT. Kim et al.
[77] used DBSA as surfactant to intercalate anilinium ions into MMT layers by mixing the
emulsion of DBSA + aniline with aqueous sodium-MMT under continuous stirring. In this
case DBSA acts also as dopant and chemical polymerization of aniline was initiated using
ammonium persulfate at room temperature. A similar approach was employed by Wu et al.
[78] but 1.5 M HCl was used in the place of DBSA. Similarly Lee et al. [79 , 80 ]
synthesized PANI−MMT using DBSA. However, the synthesis was carried out at 00C
rather than at room temperature.
33
Introduction
Electrochemical polymerization of monomers on an electrode surface offers many
advantages over chemical methods. The resulting solid product does not necessarily be
extracted from the initial monomer/oxidant/solvent mixture and is easily amenable to
numerous techniques for characterization such as UV-Visible, infrared and Raman
spectroscopies, ellipsometry and in situ conductometry [29]. Inoue et al. [ 81 ] have
reported the electrochemical synthesis of PANI-MMT as follows. A clay-coated electrode
was dipped in liquid aniline for 3 h followed by drying in air. This electrode was then
employed as working electrode and the electropolymerization was carried out
galvanostatically at 20 µA cm−2 in 2 M HCl up to 20 mC cm-2. However, this method does
not yield a homogeneous composite because the intercalation is not homogeneous and
excess of aniline could not be removed.
Feng et al. [39] have also electropolymerized anilinium in MMT potentiostatically at
EAg/AgCl = 0.80 V in a pre-treated mixture of aniline-MMT and HCl where the final
concentration of HCl was 1 M under magnetic stirring. However, the PANI-MMT
composite was obtained in the dispersion and not on the surface of the electrode. To obtain
homogeneous and clean PANI-MMT nanocomposites on metallic substrates, further work
is required on its synthesis and characterization, especially using electrochemical methods.
1.7 Aim and scope
The purpose of this study is to electrosynthesize intercalated PANI−MMT
nanocomposites and offer a better understanding of the intercalated structure of
nanocomposites. In order to take advantages of electrochemical synthesis, the resulting
PANI-MMT nanocomposites have been characterized with physical spectroscopic
techniques such as in situ UV-Vis spectroscopy and in situ conductometry. Furthermore,
anti-corrosion properties of PANI-MMT on C45 steel have been studied using impedance
measurements and the polarization method.
The synthesis of soluble PANI doped with dodecylbenzenesulfonic acid (DBSA)
via an inverse emulsion pathway has recently been reported from our laboratory [25]. In
34
Introduction
this dissertation, a pilot attempt towards the application of this soluble PANI has been
carried out. PANI-DBSA dissolved in chloroform was drop-coated onto a steel electrode
surface and its anti-corrosion performance is studied using electrochemical impedance
measurements (EIM) and anodic polarization measurements.
35
Experimetal
2. Experimental
2.1 Chemicals and materials
Sodium montmorillonite (Na+-MMT) was prepared from clay mineral bentonite
(purchased from ABCR GmbH, Germany) by cation exchanging with saturated sodium
chloride solution under stirring for 12 hours. The resulting product was then washed with
excess of deionized water, filtered and dried in an oven at 500C for 8 hours to get sodium
montmorillonite (Na+-MMT). For X-ray diffraction and FT-IR spectroscopy use, Na+-
MMT was then kept in vacuum and ground using a mortar and pestle.
Aniline (purchased from Merck, Germany) was distilled under reduced pressure and stored
under nitrogen prior to use. 18 MΩ water (Seralpur pro 90C) was used for washing
experimental equipments and diluting solutions, and all others chemicals were purchased
as analytical grade reagents and used as received.
Indium doped tin oxide (ITO) coated glass sheets of surface resistance 20 Ω cm-2
purchased from Merck.
2.2 Preparation of anilinium montmorillonite
It is proposed that the weakly polar aniline finds it more difficult to penetrate into the clay
galleries than a polar anilinium cation. Therefore, cation exchange reaction between Na+
and anilinium cation was carried out in aqueous solution. A given amount of sodium
montmorillonite (1 g) was first dispersed in the 50 mL of 0.5 M sulfuric acid containing
0.1 M aniline. The mixture was then purged with a stream of nitrogen gas for few minutes
and stirred for 24 hours at room temperature. The dispersion was filtered and washed with
excess deionized of water in order to remove free anilinium ions which were not
exchanged with sodium ions inside Na+-MMT. The resulting wet solid (anilinium-MMT)
was dispersed in 20 mL of deionized water using magnetic stirring to form dispersion of
anilinium-MMT. This dispersion was used for all experiments to synthesize PANI-MMT
nanocomposites.
36
Experimetal
For X-ray diffraction use, anilinium-MMT was dried in vacuum for 4 days, then ground
using a mortar and pestle.
2.3 Synthesis of PANI-MMT nanocomposites
2.3.1 Electrochemical synthesis of PANI-MMT nanocomposites
The PANI-MMT nanocomposites were synthesized electrochemically as follows. 20 mL of
dispersion of anilinium-MMT was diluted to 50 mL by 0.5 M sulfuric acid solution where
the final concentration of sulfuric acid is 0.3 M. The whole solution was transferred to a
three-compartment cell, purged with a stream of nitrogen gas for 10 minutes. The
electrochemical polymerization was carried out on a gold sheet electrode at a constant
potential of ESCE = 700 mV at room temperature. Another gold sheet electrode was used as
counter electrode and a saturated calomel electrode (SCE) was used as reference electrode.
The solution around the working electrode was kept under slow magnetic stirring (RPM =
80) in order to maintain the homogeneity of the dispersion around working electrode. The
resulting PANI-MMT deposited on working electrode was washed with deionized water
and dried at room temperature. For elemental analysis and FT-IR spectroscopy, the
resulting PANI-MMT deposited on electrode was washed with deionized water, separated,
and dried in vacuum for 4 days.
2.3.2 Chemical synthesis of PANI-MMT nanocomposites
In order to compare the expansion of MMT’s layers in PANI-MMT, the nanocomposites
were also synthesized chemically from aniline free anilinium-MMT dispersion in 0.3 M
sulfuric solution. The 0.1 M ammonium persulfate solution was used as oxidizing agent.
The polymerization was carried out at room temperature under magnetic stirring. The
resulting PANI-MMT was filtered, washed with excess deionized water, dried in vacuum
for 4 days. The final PANI-MMT nanocomposites were ground using a mortar and pestle
for X-ray diffraction use.
37
Experimetal
2.4 Synthesis of soluble PANI
Polymerization of aniline in an inverse emulsion medium composed of toluene + 2-
propanol (2:1) and water in the presence of dodecylbenzenesulfonic acid (DBSA) using
benzoyl peroxide as oxidant has recently reported from our laboratory [25]. PANIs with
different mole ratios of DBSA to aniline have been prepared by keeping constant the
oxidant-to-monomer ratio and by varying the concentration of DBSA. The polymer
samples were labeled as TIP-5, TIP-6 and TIP-7 where the mole ratios of DBSA/aniline in
the feed were 5:1, 7:1 and 10:1, respectively.
2.5 Characterization of PANI-MMT nanocomposites
2.5.1 X-ray diffraction
The powder samples of Na+-MMT, anilinium-MMT, and PANI-MMT nanocomposites
were dried as before elemental analysis. X-ray diffraction measurements were carried out
on a Seifert FPM/XRD7 diffractometer with Ni-filtered Cu-Kα radiation (λ = 0.154 nm)
operated at 40 kV and 30 mA.
2.4.2 FT-IR spectroscopy
Infrared spectra were recorded on a BioRad FTS-40 FT-IR spectrometer with a liquid-
nitrogen cooled MCT detector using the KBr pellet technique. All powder samples of Na+-
MMT, PANI and PANI-MMT nanocomposites were dried in vacuum for 4 days before
measurements.
38
Experimetal
2.5.3 Cyclic voltammetry
The deposited PANI-MMT gold sheet electrode was used for cyclic voltammetry
measurements. All cyclic voltammorgams were recorded by a custom built potentiostat
connected to computer using AD/DA converter. Measurements were carried out under
nitrogen atmosphere in a three-compartment cell containing 0.5 M sulfuric acid solution.
Another gold sheet electrode was used as counter electrode and a saturated calomel
electrode was used as reference electrode.
2.5.4 In situ UV-Vis spectroscopy
For in situ UV-Vis measurements PANI-MMT was deposited on an indium doped tin
oxide (ITO) coated glass sheet electrode as follows. 20 mL of dispersion solution of
anilinium-MMT was diluted to 50 mL by 0.5 M sulfuric acid solution where the final
concentration of sulfuric acid is 0.3 M. The whole solution was then transferred to a three-
compartment cell, purged with a stream of nitrogen gas for 10 minutes. The
electrochemical polymerization was carried out on a ITO sheet electrode at a constant
potential of ESCE = 800 mV at room temperature. A gold sheet electrode was used as
counter electrode and a saturated calomel electrode was used as reference electrode. The
solution around the working electrode (ITO electrode) was kept under slow magnetic
stirring (RPM = 80) in order to maintain the homogeneity of the dispersion around
working electrode. The resulting PANI-MMT deposited on working electrode was washed
with deionized water.
In situ UV-Vis spectra were recorded with a Shimadzu model UV-2101 PC spectrometer.
Experiments were carried out in a 1 cm path length quartz cuvette arranged with a PANI-
MMT deposited ITO electrode that was used as working electrode, installed
perpendicularly to the light path. A platinum wire was used as counter electrode. A
saturated calomel electrode connected via a salt bridge served as reference electrode. In the
reference channel of spectrometer a quartz cuvette filled with 0.5 M sulfuric acid solution
39
Experimetal
containing an identical ITO glass electrode was placed. In situ measurements were carried
out under ambient conditions.
A custom built potentiostat was used. All potential values are reported with respect to the
saturated calomel electrode, filled with 0.5 M sulfuric acid.
2.5.5 In situ conductivity measurements
For in situ conductivity measurements, a double-band gold electrode was used as working
electrode as described elsewhere [49]. Another gold sheet electrode and saturated calomel
electrode were used as counter and reference electrodes, respectively. Electrodeposition of
PANI-MMT on Au double band electrode was carried out potentiostatically as described in
section 2.3.1 for 2 h.
The conductivity measurements were carried out in 0.5 M sulfuric acid supporting
electrolyte solution. The current flowing across the band was measured with an I/V
converter with an amplification factor (Fac) ranging from 102 to 106. The film resistance
Rx (ohm) is related to the measured voltage Ux and the amplification factor Fac according
to Rx = (0.01×Fac) / Ux. Electrode potential was increased stepwise by 100 mV and after
approximately 5 min the electrochemical cell was cut off from the potentiostat.
2.6 Corrosion studies
In corrosion studies, a working electrode was made of mild steel (steel C45). Chemical
composition of the C45 steel (wt %): C = 0.46, Si = 0.40, Mn = 0.65, Cr = 0.40, Mo =
0.10, Ni = 0.40 and others = 0.63. It was manufactured as cylinder of 10 mm height in a
way to function as disk electrode with exposed area of 1.13 cm2, surrounded with Teflon
tape. Before using, steel electrode was first polished on sand paper of 1000 grade and then
on a polishing cloth with alumina slurry (13 μm).
40
Experimetal
For deposition of PANI-MMT on steel electrode, a saturated calomel electrode (SCE,
saturated in KCl) and sheet gold electrode were employed as reference and counter
electrodes, respectively. Before electropolymerization, steel electrode was cathodically
cleaned in 0.5 M oxalic acid solution for 10 min at ESCE = -900 mV; it was subsequently
passivated in two steps, a fast potentiodynamic rise up to ESCE = 1000 mV, followed by a
potentiostatic polarization at ESCE = 700 mV during 30 min. Electrodeposition of PANI-
MMT on C45 steel electrode was carried out potentiostatically as described in section
2.3.1. Resulting PANI-MMT coating steel electrode was washed with deionized water.
In the case of soluble PANI-DBSA, the PANIs dissolved in CHCl3 were drop coated on the
C45 steel discs which were previously polished with fine emery paper (P 1000) and with γ-
Al2O3 (13 µm).
2.6.1 Impedance and polarization measurements
Electrochemical impedance measurements were carried out in a one-compartment cell
containing 3.5 wt.% sodium chloride solution at open circuit potential (OCP). C45 steel
electrode (coated and uncoated) was used as working electrode. A saturated calomel and
gold sheet electrode were used as reference and counter electrodes, respectively. A
combination of a Solartron SI1287 potentiostat and a SI1255 frequency response analyzer,
both connected to a PC via IEE488.2 connections, was used to record electrode impedance
data with modulation amplitude of 5 mV in the frequency range between 0.1 Hz and 100
kHz. Evaluation of the impedance data was performed assuming equivalent circuit with
Zview software.
2.6.2 Polarization measurements
Anodic polarization measurements were carried out under ambient conditions in a three-
electrode single compartment cell containing 3.5 % NaCl. C45 steel electrode (coated and
uncoated) and a saturated calomel electrodes were used as working, reference, counter
41
Experimetal
electrodes, respectively, using a Solartron SI1287 potentiostat connected to a computer via
IEE488.2 connections. Measurements were carried out at a scan rate of 5 mV/s.
42
Results and discussion
3. Results and Discussion
3.1 Synthesis of PANI-MMT
Anilinium-MMT was obtained after 24 hours cation exchanging between 1 g sodium-
MMT and 50 mL of 0.5 M sulfuric acid containing 0.1 M aniline, filtering and washing,
respectively. The MMT can be dissolved in the acidic solution. However, the dissolution
rate of MMT is very small (10-10 mol m-2 s-1) even if the pH of the acidic solution is 1 or 2
at room temperature [82, 83]. The dissolution rate increases with increasing temperature
and decreasing pH. However, in the intercalation method we have used and within the
given time frame, the dissolution rate of MMT has no significant effect on stability of the
anilinium-MMT solution. During intercalation, anilinium ions can also be absorbed on the
surface of the montmorillonite tactoids as it is structurally the same as the interlayer-
oxygen basal plane with exchangeable cations. However, any such absorbed ions were
most likely removed during elution with excess of deionised water. As mentioned in
experimental section, soon after the intercalation procedure, anilinium-MMT was eluted
with excess of water to remove any such surface absorbed ions. Furthermore, presence of
any such absorbed ions will be freed during electropolymerization which significantly
enhances the polymerization. In the present study we did not observe any enhancement in
the electropolymerization. In contrary, significantly longer (2-3 h) polymerization times
were necessary to get good adherent films of PANI-MMT.
Chemical polymerization of anilinium-MMT takes place easily when a moderately strong
oxidizing agent such as ammonium persulfate is used [80]. For comparison we have also
polymerized aniline free anilinium-MMT dispersion using 0.1 M ammonium persulfate.
Chemical oxidation commenced within 5 minutes and the colour of the dispersion changes
from ash to blue-green. In case of the electrochemical polymerization the colour change
could be seen only after 30 minutes and a thick film is obtained after 2.5 h. The
electrochemically inactive clay particles hinder the formation of a film on the electrode
surface. Formation of a good adherent film on the electrode surface depends on several
parameters such as method of synthesis, magnetic stirring, electrolyte used and the
concentration of anilinium ions in the clay. Initially, we tried to synthesize PANI−MMT
43
Results and discussion
composites electrochemically by cycling the potential between –200 and 900 mV at
different scan rates. However, no film formation was observed on the electrode surface
even after some hours. Alternatively, the PANI−MMT was deposited on the electrode
surface with constant potential electrolysis. When the anilinium-MMT dispersion near
working electrode was stirred with higher speed (≥ 100 rotation per minute), the PANI-
MMT nanocomposites formed was not adhered to the electrode surface. Therefore we have
used very slow stirring with 80 rotations per minute which is well enough to maintain the
homogeneity of the dispersion. We have also noticed that with the use of HCl as electrolyte
there was no electropolymerization of anilinium-MMT. This may be due to the strong
adsorption of Cl− ions on the surface of the gold electrode. PANI-MMT was successfully
synthesized using other acids such as H2SO4, HClO4 and oxalic acid. Electro-
polymerization does not take place when we use lower concentrations of aniline (< 0.1 M)
whereas higher concentrations of aniline yield free anilinium ions which were removed
during washing. Schematic representation of intercalation of anilinium ions into MMT and
electropolymerization of anilinium inside layers of MMT is shown in Figure 13.
Na+
Na+
Na+
NH3+
NH3+
NH3+
NH3+
NH
NH2
+NH2
+
MMT
MMT
MMT
MMT
MMT
Eox
MMT
Figure 13 Schematic representation of intercalation of anilinium ions into MMT and
electropolymerization of anilinium inside layers of MMT
44
Results and discussion
3.2 Elemental analysis
The elemental analysis was carried out on the dried powder samples of PANI-MMT in
order to calculate the percentage composition of PANI in the nanocomposite. The results
are shown in Table 1.
The percentage composition of PANI in the PANI-MMT nanocomposite was calculated to
be 10.22 wt. %. PANI content in PANI-MMT nanocomposites generally varies in the
range of 2 to 12.30 wt.% [10, 78, 79, 80]. Higher contents of PANI have been reported (up
to 74.7 wt.%) but based on experimental evidence PANI is deposited on the outside of
MMT and not only intercalated [80]. Such deposits may be the result of surface absorption
of anilinium ions on the clay. In the procedure employed here formation of PANI on the
outside of the nanocomposite is highly unlikely because of the strong interaction between
the intercalated anilinium cations and the MMT layers which make egress of the anilinium
cations very unlikely.
Table 1 Elemental analysis results of PANI-MMT samples
Samples
Total amount
of PANI-
MMT
(mg)
Amount of
nitrogen
(mg)
Amount of
carbon
(mg)
Amount of
hydrogen
(mg)
Percentage
of PANI
(%)
1 3.9160 0.0420 0.3090 0.0580 10.45
2 3.4980 0.330 0.2540 0.0530 9,69
3 3.5720 0.0380 0.2850 0.0530 10.53
The percentage of anilinium (approximate to the percentage of polyaniline) in PANI-
MMT nanocomposite can be evaluated via cation exchange capacity (CEC). The
montmorillonite is known to have a charge density of 0.25 to 0.60 charges per half-unit
cell. Considering a mean molar mass of 370 g per half-unit cell, the charge per unit cell
corresponds to a CEC of 67 to 162 mmol per 100 g sodium−montmorillonite [84]. The
45
Results and discussion
expected amount of anilinium (approximate to the amount of polyaniline) cations
intercalated into MMT layers can be calculated using CEC following expression:
Anilinium (wt. %) = )()(10100
)(10
sodium w,anilinium w,3
anilinium w,1
AACECACEC
−⋅⋅+⋅⋅
−
−
(20)
Where: CEC = cation exchange capacity of the montmorillonite per 100 g
Aw, anilinium = molecular weight of anilinium cation
Aw, sodium = atomic weight of sodium cation
The values of the expected anilinium that can be intercalated in to the layers of MMT is in
the range of 6 to 13.6 wt.%. In our case, the actual PANI content of 10.22 wt. % in PANI-
MMT nanocomposites is in good agreement either with calculation using CEC or with
values reported elsewhere [10, 78, 79, 80], is near the maximum value of PANI content
which can remain inside layers of MMT [79, 80].
3.3 X-ray diffraction
The d-spacing of the materials was calculated from the angular position 2θ of the observed
peaks using the Bragg's equation: nλ = 2dsinθ, where λ is the wavelength of the incident
X-ray beam, θ is the diffraction angle and n is an integral. Figure 14 shows X-ray
diffraction patterns of Na+-MMT, anilinium-MMT, PANI-MMT oxidized form and
PANI-MMT reduced form (reduced form of PANI-MMT was obtained by applying a
constant potential of ESCE = -200 mV on a freshly synthesized oxidized sample of PANI-
MMT for 10 min).
As shown in Figure 14 (see next page), the reflection peak of the Na+-MMT sample at 2θ
= 8.80 is shifted towards lower angles for anilinium-MMT and PANI-MMT
nanocomposites (both oxidized and reduced forms). The d-spacing of materials are 12.8
Å, 12.6 Å, 12.5 Å for anilinium-MMT, the PANI-MMT oxidized form, and the PANI-
MMT reduced form, respectively. The average d-spacing of the PANI-MMT
nanocomposites were found to be 12.55 Å, which is little bit smaller than that of
anilinium-MMT. Such a smaller d-spacing for PANI-MMT may due to the higher stereo
46
Results and discussion
regularity of PANI staked inside clay layers than that of anilinium ions staked inside clay
layers.
Due to the insertion of PANI, d-spacing is expanded from 10 to 12.55 Å, that is increased
by 2.55 Å. Generally, in the case of PANI−MMT nanocomposites, the d-spacing is
expanded in the range of 0.7 to 6.0 Å [10, 11, 16, 39, 78, 79, 80]. Thus, the expansion in
the d-spacing observed in this study is comparable to the data reported by other groups
[41, 42]. The diffraction peak of Na+-MMT in Figure 14 is broader than with PANI-
MMT whereas the peak anilinium-MMT is intense and sharp. The sharpness of the peaks
can be influenced by crystallinity or clay-layer stacking order. Thus the broader peak of
Na+-MMT indicates less crystallinity and order of clay-layer stacking than the other
samples [78, 85, 86].
4 6 8 100
20
40
60
12
dc
b
a
Inte
nsity
2θ/degree
Figure 14 X-ray diffraction patterns of Na+- MMT (a), anilinium - MMT (b), oxidized
form (c) and reduced form of PANI - MMT (d) synthesized by
electrochemical method.
47
Results and discussion
3.4 FT-IR analysis
The characteristic peaks observed in the FT−IR spectrum of polymer−MMT
nanocomposites gives valuable information regarding to the conformation of polymer in
the clay and possible interaction between clay and polymer [87, 88].
1000 1500 2000 2500
0
60
120
1637
1637
1637
918
918
918
795
795
795
1040
1040
1040
1311
1296
1296
1579
1579
1579
1447
1447
1447
d
c
b
a
Tran
smita
nce
Wavenumber/cm-1
Figure 15 FT-IR spectra of PANI (a), electrochemically synthesized PANI-MMT (b),
mechanical mixture of PANI and MMT (c) and Na+-MMT (d).
The spectra in the Figure 15 show the presence of characteristic peaks of Na+-MMT,
PANI, mechanical mixture of PANI and Na+-MMT (i.e., an unintercalated system), and
electrochemically deposited PANI-MMT (Table 2). The characteristic vibrations of Na+-
MMT and the emeraldine salt are known to be in the region of 700 cm-1 to 1700 cm -1 [79].
The bands of Na+-MMT are observed at 1637 cm-1 (H-O-H bending of water molecule),
1040 cm-1 (Si-O stretching), 918 cm-1 and 795 cm-1 (Al-O bending) [11, 78]. For PANI,
the characteristic absorption bands appear at 1296 cm -1 (C-N bending) 1447 cm-1 and
1579 cm-1 (C=C stretching of benzenoid and quinoid rings, respectively) [89].
48
Results and discussion
Table 2 Infrared band assignments of Na+-MMT, PANI, and PANI-MMT
nanocomposites.
Samples Wavelength (cm-1) Band assignment
Na+ -MMT 1637
1040
918
795
H-O-H bending of water
Si-O stretching
Al-O bending
Al-O bending
PANI
1579
1447
1296
C=C stretching of quinoid ring
C=C stretching of benzenoid ring
C-N bending
PANI-MMT
1579
1447
1311
1040
918
795
C=C stretching of quinoid ring
C=C stretching of benzenoid ring
C-N bending with physicochemical
interaction between –NH group of PANI
and –O of silicate
Si-O stretching
Al-O bending
Al-O bending
PANI-MMT
mechanical mixture
1579
1447
1296
1040
918
795
C=C stretching of quinoid ring
C=C stretching of benzenoid ring
C-N bending
Si-O stretching
Al-O bending
Al-O bending
FT-IR spectra of PANI-MMT composites exhibit bands characteristic of PANI as well as
of MMT which confirms the presence of both components in the PANI-MMT composite.
FT-IR spectra of the mechanical mixture of PANI and MMT are slightly different from
49
Results and discussion
the spectra of electrochemically synthesized PANI-MMT. The band at 1296 cm-1 in the
spectrum of a mechanical mixture of PANI and MMT is shifted to 1311 cm-1 in the spectra
of the intercalated nanocomposites (Figure 15).
This shift is due to the physicochemical interaction (hydrogen bonding between –NH
group of PANI and –O of silicate) in the intercalated PANI-MMT [79, 80] whereas
mechanical mixtures of PANI and MMT lack such an interaction. A similar trend was
observed by Stutzmann and Siffert [90] for the acetamide-MMT system. They found that
C–N stretching vibration of acetamide which was observed at 1380 cm-1 was shifted to
higher wavenumbers (1400 cm-1) after adsorption onto a clay surface and they have
attributed this shift to the hydrogen bonding between NH2 groups of acetamide and oxygen
atoms of the basal surface of the clay.
3.5 Cyclic voltammetry
Cyclic voltammograms (CVs) of PANI-MMT nanocomposites, deposited on a gold
electrode, were recorded in an aqueous solution of 0.5 M sulfuric solution with different
thickness as obtained after different times of electropolymerization of the PANI-MMT
films (Figure 16). CVs of PANI exhibit two pairs of redox waves with the first one
observed at ESCE = 200 mV indicating the transformation of leucoemeraldine form to
conducting emeraldine form and the second one at ESCE = 810 mV which is due to the
conversion of emeraldine into the pernigraniline form. A pair of humps in the region of
ESCE = 0.30 to 0.50 V has been assigned to overoxidation products [91, 92]. The shape of
the CVs of PANI-MMT is similar to those of PANI. This indicates that clay layers do not
influence the electrochemical properties of PANI nor does the intercalation favour a
polymer with different properties (such as e.g., molecular weight) as could evidenced with
this electrochemical technique. There is only a minor shift of the reduction peak associated
with the pernigraniline-emeraldine transition which might indicate some not yet
understood interaction between PANI and MMT.
50
Results and discussion
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
0
2
-3
0
3
6
PA N I-M M T
I/mA
E SCE/V
PA N I
Figure 16 Cyclic voltammogram of PANI and PANI-MMT nanocomposites in 0.5 M
H2SO4 at scan rate of 100 mV s-1.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
0
2
4
-1
0
1
2
PANI-MMT
PANI b
I/mA
ESCE/V
10th
20th
1st
a
Figure 17 CV of (a) PANI-MMT (1st, 10th and 20th cycles) recorded in 0.5 M H2SO4,
(b) PANI and PANI-MMT recorded in 0.5 M KCl at scan rate of 100 mV s-1.
51
Results and discussion
It was also observed that the PANI-MMT film was stable as it was not damaged/peeled
off from the surface of the electrode even with continuous potential cycling for up to 20
cycles (Figure 17a); changes in the CVs implying degrading or loss of active material are
minor only. The electrochemical activity of the nanocomposite was also checked in a
neutral unbuffered aqueous solution of 0.5 M KCl by recording CVs in the range of ESCE =
- 0.20 to + 0.85 V (Figure 17b). The figure demonstrates that electroactivity of the
nanocomposite is retained even at neutral pH. However, PANI reversibly looses its redox
activity and only one degenerated redox wave is observed [93].
3.6 In situ UV-Vis spectroscopy
Figure 18 shows in situ UV-Vis spectra of PANI-MMT on an ITO electrode at various
electrode potentials recorded in aqueous 0.5 M H2SO4.
400 600 800
0.8
1.0
1.2
1.4
hg
fe
d
b
c
a
0.2 V
Abs
orba
nce
/ -
Wavelength/ nm
Figure 18 In situ UV-Vis spectra of PANI-MMT recorded in 0.5 M H2SO4 solution at
different positive going potentials (ESCE / V): -0.20 (a), 0.0 (b), 0.10 (c), 0.20
(d), 0.30 (e), 0.50 (f), 0.70 (g), 0.80 (h).
52
Results and discussion
PANI exhibits three electronic absorption bands at 320, 430 and ~800 nm which originate
from the π→π* transition, radical cations and polarons respectively [ 94 , 95 , 96 ].
Electronic absorption spectra of PANI-MMT, like PANI, exhibit bands at 430 and 870 nm
but the band at 320 nm could not be seen. Absorbance of the band at 430 nm reaches a
maximum at ESCE = 0.20 V which indicates higher concentration of radical cations at this
applied potential. At this applied potential (ESCE = 0.20 V) the first oxidation wave in the
CV of PANI-MMT which corresponds to the leucoemeraldine to emeraldine transition has
a maximum peak current (Figure 16). By shifting the electrode potential to higher values,
the intensity of this band diminishes. When the applied potential is increased from ESCE = -
0.20 to 0.70, maximum positions of the band at 870 nm (polaronic transition) are shifted
into the near-infrared (NIR) region and at ESCE = 0.70 this band becomes more flattened. A
similar trend was observed by Malinauskas et al. [97] for potentiostatically (ERHE = 1.20
V) synthesized PANI.
-0.2 0.0 0.2 0.4 0.6 0.81.0
1.2
1.4
1.6
1.8
2.0
forward backward
Abs
orba
nce
/ -
ESCE / V
Figure 19 Plot of absorbance at 670 nm versus applied electrode potential for PANI-
MMT nanocomposites.
53
Results and discussion
When the applied potential is increased further to ESCE = 0.80 V, the polaronic band in the
NIR (near infrared) region disappears and a new band at 670 nm appears which is
attributed to the blue non-conducting pernigraniline state of PANI. The CV of PANI-
MMT has a second oxidation wave at ESCE = 0.80 V corresponding to the emeraldine to
pernigraniline transition.
In situ electronic absorption spectra of PANI-MMT were also recorded during a stepwise
cathodic potential sweep. Figure 18 shows a plot of absorbance at 670 nm versus applied
potential recorded with the electrode potential going into the positive and negative
directions. Both traces are very close to each other in the potential range of ESCE = 0 to
0.80 V indicating a good electrochemical reversibility of the PANI-MMT nanocomposite.
Figure 18 and 19 also reveal that electrochromism of PANI in the PANI-MMT
nanocomposite is almost retained.
3.7 In situ conductivity measurements
Resistance values of PANI and PANI-MMT deposited at ESCE = 700 mV were measured in
an aqueous solution of 0.5 M H2SO4 in the range of - 0.20 < ESCE < 0.9 0V in the anodic
direction and then in the reverse cathodic direction. The log R values of both PANI and
PANI-MMT against the applied electrode potential are displayed in Figure 20. Two
transitions can be observed in the resistivities of both PANI and PANI-MMT. The first
transition appears at around ESCE = 0 V where the resistivity values start to decrease and
the second transition appears at around ESCE = 0.60 V where again the resistivity begins to
increase. Thus in the potential range of ESCE = 0.0 to 0.60 V, PANI as well as PANI-MMT
is highly conducting, this is the potential range where PANI is in the emeraldine state.
When the potential sweep direction was reversed from ESCE = 0.90 to –0.20 V, almost
similar trends were observed, however, the conductivities are lower than in the anodic
sweep. This loss of in situ conductivity in the reverse cathodic sweep was attributed to
partial degradation of PANI at ESCE = 0.90 V [96]. The apparent resistivity of PANI-MMT
is higher than that of PANI. In the absence of data enabling the conversion of resistivities
into specific resistivities a quantitative comparison is impossible. The slightly smaller
relative change of resistivity in case of the nanocomposite may be due to the high fraction
(90 %) of inert MMT.
54
Results and discussion
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
1.0
1.5
2.0
2.5
blog(
R/Ω
)
EESC / V
anodic sweep cathodic sweep
0.0
0.5
1.0
1.5
a anodic sweep cathodic sweep
Figure 20 Plot of log(R) versus applied electrode potential for (a) PANI and (b) PANI-
MMT in an aqueous solution of 0.5 M sulphuric acid.
3.9 Corrosion studies
The newly developed controlled electropolymerization technique for the synthesis of
PANI-MMT nanocomposites has been described in the previous sections. The
electrodeposited PANI-MMT composites, for the first time, have been used in corrosion
protection of steel. The corrosion protection performance of these nanocomposites as
studied using two electrochemical tools: anodic polarization measurements and EIM.
55
Results and discussion
3.9.1 The anti-corrosion properties of PANI-MMT
3.9.1.1 Electrochemical impedance measurements
As described in section 1.5.2.1, electrochemical impedance measurement is a very useful
method in characterizing an electrode corrosion behavior. The electrode characterization
includes the determination of polarization resistance, corrosion rate and electrochemical
mechanism [57]. The electrochemical impedance data is interpreted in terms of the
equivalent circuit composed of resistors, capacitors and sometimes including some other
elements. The sample preparation and measurement conditions have been described in
section 2.6.1.
0 20 40 60 80 100 120
0
-20
-40
-60
-80
-100
0.1 Hz
0.1 Hz1.67 Hz
0.1 Hz
4 6 8
Z "/Ω
Z '/Ω
Figure 21 Nyquist plots of the bare C45 steel electrode ( ), passivated ( ) and PANI-
MMT (○) coated C45 steel electrodes recorded at OCP in 3.5 % NaCl. Solid
lines indicate the fitting curve and the magnified portion of PANI-MMT at
high frequency is shown in the inset.
56
Results and discussion
Figure 21 shows Nyquist diagrams of bare passivated and PANI-MMT coated C45 steel
electrodes recorded at OCP in 3.5 % NaCl solution. The values of solution resistance (RS),
charge transfer resistance (RCT), double layer capacitance (CDL), coating capacitor (CC) and
coating resistance (RF) were determined via curve fitting of impedance data using Z-view
software. Two capacitive depressed semi-circles are present in the Nyquist diagrams. One
of them at high frequencies is attributed to the electrical properties of the PANI-MMT film
(RF) and the other to processes occurring underneath the hybrid coating (RCT). The first
loop can be visualized only after magnifying the high frequency range; both loops cannot
be well resolved. Such a behavior can be explained with an equivalent circuit containing a
solution resistance (RS), coating capacitance (CC), double layer capacitance (CDL), coating
resistance (RF) and charge transfer resistance (RCT) as shown in Figure 22 [98, 99, 100].
RS CC
RCT
CDL
RF
Figure 22 Equivalent circuit related to the PANI and PANI−MMT coated C45 steel
electrode
The equivalent circuit used to explain electrochemical processes occurring at bare [27, 98,
101] and passivated C45 steel electrodes is shown in the Figure 23. The number of time
constants and other elements needed to fully describe the impedance data were based on
the condition of a fit with a minimal deviation between the measured and the calculated
results. The corrosion rate is inversely proportional to the value of RCT, high RCT value
corresponds to low corrosion rate. Comparison between RCT value of bare C45 (84.5 Ω)
and passive C45 (95 Ω) shows that the passive layer has no significant protection.
However, passivation prior to the electropolymerization is necessary to form a thin iron
oxalate layer which strongly inhibits metal dissolution without preventing other
electrochemical processes and improves the adherence of the deposited film which
57
Results and discussion
provided good protection to steel in corrosion [102, 103, 104, 105, 106]. PANI−MMT has
been electrodeposited after passivation as described in section 2.6. RCT value of PANI-
MMT coated electrode (446 Ω) shows significant increase compared with the RCT value of
bare C45 steel electrode (Table 3).
RS CDL
RCT
Figure 23 Equivalent circuit for bare C45 electrode and C45 steel electrode after
passivation in 0.5 M oxalic acid solution.
Table 3 RS, CC, RF, CDL and RCT values from impedance data for bare, passivated and
PANI-MMT coated C45 steel electrodes at various exposure time in 3.5 %
NaCl solution.
Coating t/ h RS / Ω CC / μF RF / Ω CDL / mF RCT / Ω
0 3.5 − − 4.2 84.5 Uncoated
0 5.1 − − 6.9 95 Passivated
Electrode
0 4.6 6.3 0.89 7.6 446
24 4.1 4.5 1.36 7.4 391
48 5.7 5.3 1.41 3.9 319
PANI-MMT
72 4.6 4.9 1.38 3.8 268
The thickness of coating is an important factor which greatly affects the corrosion
protection behavior of coating material [5]. In the case of PANI-MMT, when the thickness
of PANI-MMT increases which is controlled by the polymerization time, the value of RCT
increases. However, determination of exact thickness of the composite film deposited on
58
Results and discussion
the electrode surface is a difficult task because not all the PANI-MMT formed during
electropolymerization adheres to the electrode surface. Some composite is always
associated with the anilinium-MMT dispersion due to the mechanical stirring in the WE
compartment. The electropolymerization of PANI-MMT required a dispersed homogeneity
of anilinium-MMT solution around working electrode which is attained by slow magnetic
stirring. For convenience, we kept the polymerization time constant (i.e., coating thickness
is presumably kept constant) for all impedance measurements.
0 40 80 120 160
0
-20
-40
-60
-80
-100
3.9 Hz
0.1 Hz0.1 Hz
0.1 Hz
Z "/Ω
Z ' / Ω
Figure 24 Nyquist plots of PANI-MMT coated C45 electrode recorded at OCP in 3.5 %
NaCl solution (○) 0h, () 24h, (∇) 48h and (∆) 72h immersion time. Solid
lines are fitting curves.
Figure 24 shows Nyquist plots of PANI-MMT coated C45 steel immerged in 3.5 % NaCl
solution recoded after different time intervals. The RCT value decreases with increasing
immersion time. Relative lower RCT values observed in the present case may be due to the
59
Results and discussion
penetration of anions into the electrode surface during electropolymerization which
may interfere the nanocomposite and thereby decreasing the corrosion protection.
However, the R
−24SO
CT value remained much higher than that for bare C45 electrode even after
72 hours of immersion. Y. Zhu [5] has also obtained such a decrease of RCT using chemical
synthesis PANI-MMT nanocomposites coated AA2024-T3 alloy.
The CDL in Table 3 for bare, passivated and PANI-MMT coated C45 steel electrodes lies in
the range of 4 – 7 mF which are much higher than the standard double layer capacitance
values. Higher values of CDL observed in the present study are due to high electrochemical
active surface [107]. In the present work, since the electrode was polished with 13 μm of
alumina slurry, high electrochemical active surface area is expected.
3.9.1.2 Polarization measurements
The corrosion potential (Ecorr) and corrosion current density (icorr) can be obtained by
polarization measurements. The equilibrium open circuit potential (OCP) of an electrode
can be considered as Ecorr. Corresponding to Ecorr is icorr which is proportional to corrosion
rate (CR) as shown in the equation 21 and has inverse relation with polarization resistance
(Rp) of electrode as shown in equation 22 (rearrangement of Stern-Geary equation). The
values of Ecorr, icorr and Tafel slopes (anodic slope ba and cathodic slope bc) can be obtained
by extrapolation from Tafel plots. Using equation 21 and equation 22, the value of Rp and
CR of electrode can be easily determined.
)(129.0R
corr EWdAC
i⋅⋅⋅
= (21)
Where icorr is corrosion current density measured in in μA cm-2, CR is corrosion rate
measured in milliinch per year (MPY), d is density of material measured in g cm-3, EW is
equivalent weight of corroded metal measured in g equivalent-1.
ARbbbb
i⋅
⋅+
⋅=
pca
cacorr
1)(303.2
(22)
60
Results and discussion
where icorr is corrosion current density measured in mA cm-2, ba, bc are anodic and cathodic
Tafel slopes measured in mV decade-1, Rp is polarization resistance measured in ohm.
Figure 25 shows Tafel plots of bare, passivated and PANI-MMT coated C45 steel
electrodes. The values of Ecorr, icorr, ba, Rp and CR of bare, passivated and PANI-MMT
coated C45 steel electrodes are calculated and shown in Table 4. For passivated C45 steel,
Ecorr is negatively shifted by 15 mV compared to bare C45 steel. This shift showed
cathodic protection of passive layer. However, CR did not decrease significantly. The
corrosion potential of PANI-MMT coated C45 steel electrode is positively shifted by 51
mV compared to the bare C45 steel electrode. The shift of corrosion potential indicated
that PANI-MMT coating depressed the anodic current of the corrosion reaction. The
increase of corrosion potential is an indication of anodic protection by PANI-MMT. As
shown in the Table 4, the corrosion rate was significantly reduced for PANI-MMT coated
C45 steel as compared to the bare C45 steel electrode.
-0.65 -0.60 -0.55 -0.50 -0.45
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Passivated C45
PANI-MMT coated C45
Uncoated C45
logi
(mA
.cm
-2)
ESCE/ V
Figure 25 Tafel plot of uncoated, passivated and PANI-MMT coated C45 steel electrode
in 3.5 % NaCl solution.
61
Results and discussion
The polarization resistance Rp calculated from polarization measurements is in good
agreement with RCT value calculated from impedance measurements. The agreement
between polarization resistance Rp (i.e. the slope of the current density versus electrode
potential curve) and the sum of all Ohmic components in the electrode impedance deduced
from impedance measurements at the same electrode potential is expected, because at
frequency zero the sum of all Ohmic components of impedance is equal to said slope
[108]. The relatively small contribution of the film resistance RF and solution resistance RS
results in a fairly good agreement between Rp and RCT in the present case.
Table 4 Ecorr, ba, icorr, Rp and CR values calculated from Tafel plots for bare, passivated
and PANI-MMT coated C45 steel electrode in 3.5 % NaCl.
Coating Ecorr, SCE
(mV)
ba
(mV dec−1)
icorr
(μA cm−2)
Rp
(Ω)
CR
(MPY)
Uncoated −571 40.0 98.80 92 40.12
Passivated -586 39.2 95.50 110 38.80
PANI-MMT −520 35.5 19.90 519 8.10
3.9.2 The anti-corrosion properties of soluble PANI
A soluble PANI which can dissolve completely in common organic solvents has been
synthesized in our laboratory [25]. Chloroform was employed as a solvent to dissolve
soluble PANI for drop-coating onto C45 steel electrode surface. Anti-corrosion
performance of soluble PANI was studied using EIM and polarization measurements.
3.9.2.1 Electrochemical impedance measurements
Figure 26 shows the Nyquist diagrams for the bare C45 steel electrode and PANI coated
(with different feed ratios of DBSA to aniline) electrodes recorded at OCP in 3.5 % NaCl
62
Results and discussion
solution. The charge transfer resistance (RCT), double layer capacitance (CDL) and coating
resistance (RF) values were determined via curve fitting of impedance data using Z-view
software and are given in Table 5. Two capacitive depressed semi-circles are also observed
in the Nyquist diagrams as in the case of PANI-MMT coated C45 steel. This behavior, as
described in section 3.9.1.1, can be interpreted using an equivalent circuit containing RS,
CC, CDL, RF and RCT as shown in Figure 22. A good barrier allows very little current flow
showing high resistance during impedance measurements. The protective effect of PANI-
DBSA is immediately obvious as the RCT value for PANI coated electrodes show
significant increases compared to the bare C45 steel electrode (Table 5).
Table 5 RS, RF, CC, RCT and CDL values from impedance data for bare and PANI-DBSA
coated C45 steel electrodes at various exposure times in 3.5 % NaCl.
Coating t / h RS / Ω CC / μF RF / Ω CDL / mF RCT / Ω
0 3.5 − − 4.2 84.5 Uncoated
0 4.1 16.4 1.58 2.9 696
24 4.5 3.9 1.69 3.2 650
48 4.3 4.9 1.80 2.2 635
72 4.3 4.9 1.76 2.5 378
TIP-5*
0 4.5 11.9 1.12 4.3 880
24 4.6 13.5 1.17 4.3 586
48 4.6 10.6 1.18 4.1 559
72 4.7 10.5 1.12 3.4 356
TIP-6*
0 4.3 8.3 1.44 3.1 505
24 3.4 1.9 0.75 1.6 449
48 3.3 2.2 0.76 1.6 440
TIP-7*
72 4.6 4.7 1.13 2.7 425
* TIP-5, TIP-6 and TIP-7 denote PANI-DBSA samples where the mole ratios of
DBSA/aniline in the feed were 5:1, 7:1 and 10:1, respectively.
63
Results and discussion
Both resistance and capacitance values increase with increasing thickness of the PANI
film, beyond a certain limit only a negligible further increase was observed. All values for
PANIs reported in Table 5 are beyond this threshold. Repeated experiments show that the
RCT and thereby the corrosion efficiency is influenced by the amount of DBSA in the feed.
TIP-6, where DBSA to aniline ratio is 7, shows relatively better corrosion protection. In
case of electrochemically coated PANI (synthesized in the presence of mineral acids), Cl−
ions and water can easily permeate through the film due to the porosity of the film leading
to a lower film resistance [109]. SEM showed morphology of PANI is strongly influenced
by mole ratio DBSA/ aniline in the feed. TIP-5 (DBSA/aniline is 5:1) exhibited filbrillar.
When the ratio of DBSA to aniline is increased to 7:1 (TIP-6) and 10:1 (TIP-7),
morphology changes to porous network type and compact film type, respectively [25].
0 50 100 150 200 250 300
0
-50
-100
-150
-200
-250
1.19 Hz
0.1 Hz
0.1 Hz
0.1 Hz0.1 Hz
4 6 8
Z "/
Ω
Z ' / Ω
Figure 26 Nyquist plots of the bare C45 steel electrode ( ) and electrode coated with
TIP-5 (), (○) TIP-6 and (∆) TIP-7 recorded at OCP in 3.5 % NaCl. Solid
lines indicate the fitting curve and the magnified portion of TIP-6 at high
frequency is shown in the inset.
64
Results and discussion
However, RCT values in our case could not be correlated to the bulk morphology of the
polymer. TIP-5, TIP-6 and TIP-7 exhibit fibre porous network and compact film evidenced
by SEM [25]. One can expect better corrosion protection performance by TIP-7 where
ingress of corrosive ions such as Cl− is unfavorable. However, RCT values in the present
work suggest better anti-corrosion performance for TIP-6 with porous morphology.
Therefore, we assume that morphology of post processed material is different from bulk
morphology which was later confirmed by TEM studies [109]. Correlation of the values of
EIM parameters such as RCT, CDL, etc. with the corrosion protection effect and with already
reported results is a difficult task as the results vary widely and are strongly influenced by
the composition of the steel, corrosion environment, nature of coating (ES or EB) and top
coat (if present).
0 50 100 150 200
0
-50
-100
-150
-200
Z " /
Ω
Z ' / Ω
Figure 27 Nyquist plots of TIP-6 coated C45 steel electrodes recorded at OCP in 3.5 %
NaCl (○) 0 h, () 24h, (∆) 48 h and ( ) 72 h of immersion time. Solid lines
are fitting curves.
65
Results and discussion
Figure 27 shows Nyquist diagrams of PANI 6 coated C45 steel in 3.5 % NaCl recorded
after different time intervals. The shape of the Nyquist diagrams is not much affected up to
72 hours. The RCT values decrease with time but are still higher than with the uncoated
electrodes. Bereket and coworkers [110] have also observed such a decrease in RCT values
for PANI coated 304−stainless steel electrodes. PANI film was generated by
electropolymerization of aniline in acetonitrile containing tetrabutylammonium perchlorate
and perchloric acid. We believe that soluble PANI DBSA protects C45 steel against
corrosion through the formation of a passive layer which could be easily visualized as a
gray oxide film underneath the PANI coating [111].
3.9.1.2 Polarization measurements
The Ecorr, icorr and Tafel slopes were determined from the Tafel plots of potentiodynamic
measurements by extrapolation. The values of Rp and CR were calculated using equation 21
and 22. The Ecorr, icorr, ba, Rp and CR values for uncoated and PANI-coated C45 steel
electrodes are summarized in Table 6. The corresponding Tafel plots for bare C45 steel
and PANI-DBSA (different feed ratios of DBSA to aniline) coated electrodes are shown in
Figure 28. The corrosion potential of the PANI coated electrode was anodically shifted by
66-72 mV compared to the bare electrode whereas the corrosion current and the
corresponding corrosion rate are drastically reduced (Table 6). An anodic shift of 2 mV
was reported for PANI-DBSA coated 08U-steel electrodes in 3.5 % NaCl by Pud and
coworkers [112]. They cast emeraldine base form of PANI dissolved in NMP on the steel
substrate and re-doped it with DBSA in xylene. However, they have found that PANI
redoped with CSA and DBSA increases the corrosion current in 3.5 % NaCl thereby
showing an increase in corrosion rate.
The better performance in our case may be attributed to the stronger complexation of
DBSA with the N-atoms of the polymer backbone. An increase in Ecorr up to 1650 mV was
reported by several investigators [111, 113 ]. The magnitude of potential shift and
corrosion current strongly depends on the processing technology, composition of the steel
and an insulating polymer top-coat.
66
Results and discussion
-0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
dc
ba
logi
(mA
.cm
-2)
ESCE/ V
Figure 28 Tafel plots of bare (a) and PANI-DBSA [TIP-5 (b), TIP-6 (c), and TIP-7 (d)]
coated C45 steel electrode in 3.5 % NaCl.
The polarization resistances (Rp) calculated from Tafel plots (Table 6) are almost in
agreement with the RCT values calculated from impedance data (Table 5). A significant
increase in Rp after PANI coating confirms its protective nature against the corrosion of
C45 steel. The Ecorr and icorr values are influenced by the ratio of DBSA to aniline in the
feed. TIP-6 having a feed ratio of 10 shows better corrosion performance over the others
which was also confirmed by EIM studies. The hydrophobic nature of the long non-polar
chain of DBSA, its strong complexation with PANI backbone and poor wettability of
polymer in aqueous electrolyte [25] hinders the rate of anion exchange which further
reduces the ingress of hydrophilic (and pitting) Cl− ions into the polymer film thereby
enhancing the corrosion performance [114]. The poor ingress of Cl− ions into PANI films
has been confirmed by in situ UV-Vis spectroscopy. UV-Vis spectra of PANI-DBSA drop
coated onto ITO-coated glass as a function of applied potential progressively shifting in
anodic direction have been studied. Spectral responses of PANI, both in acidified (0.5 M
67
Results and discussion
H2SO4) and acid free (0.1 M KCl), are similar except for the fact that LE−EM−PN
transitions occur at lower positive potential in case of 0.1 M KCl. Generally, PANI is
electrochemically inactive in acid free aqueous electrolytes. The above results indicate that
insertion of Cl− ions into PANI is hindered by hydrophobic nature of the film.
Table 6 Ecorr, ba, icorr, Rp and CR values calculated from Tafel plots for bare and PANI-
DBSA coated C45 steel electrode in 3.5 % NaCl.
Coating [DBSA] Ecorr, SCE
(mV)
ba
(mV dec−1)
i corr
(μA cm−2)
Rp
(Ω)
CR
(MPY)
Uncoated − −571 40.0 98.80 92 40.12
TIP-5 5 −506 52.5 15.30 718 6.21
TIP-6 7 −499 47.4 13.79 805 5.60
TIP-7 10 −515 49.5 19.04 505 7.74
68
Summary
4. Summary
The work described in this dissertation clearly demonstrates that novel organic-inorganic
hybrid materials composed of intrinsically conducting polyaniline and montmorillonite
clay minerals could be successfully deposited on a metal surface using a controlled
electropolymerization pathway. Relatively longer polymerization times are inevitably
essential to obtain an adherent composite film as the anilinium ions are truly intercalated
(surface absorbed monomeric species are absent) into the layers of electrochemically
inactive clay. Magnetic stirring in the working electrode compartment has pronounced
effect on the polymerization time.
The larger interlayer spacing observed in X−ray diffraction studies confirms the
intercalation of anilinium ions. It also shows that electropolymerization of aniline inside
the clay tactoids yields highly stereoregular conducting PANI as d−spacing of
PANI−MMT is close to that of anilinium-MMT. Elemental analysis data shows that larger
portion (90 % wt/wt) of PANI−MMT nanocomposites consist of electro−inactive clay
mineral. Electrical conductivity of PANI−MMT nanocomposites is only an order of
magnitude lower than that of PANI. Presence of large amount of clay does not affect the
electrochemical activity of the PANI−MMT nanocomposite. Structural characterization of
the nanocomposites has been performed using FT−IR spectroscopy which reveals the
presence of a physicochemical interaction, most probably hydrogen bonding, between
PANI and montmorillonite. In situ UV−Vis spectroscopy of PANI−MMT nanocomposites
indicates that electrochromic behaviour of PANI in the nanocomposite is retained.
PANI-MMT nanocomposites when electrodeposited on C45 steel surface, exhibit
protection against corrosion. Electrochemical impedance measurements and polarization
studies have been used to study the anticorrosion behaviour of the nanocomposite.
Anticorrosion properties of an organically soluble PANI−DBSA synthesized in our
laboratory have also been studied. Both PANI−MMT and soluble PANI protects steel
against corrosion via the formation of a passive oxide layer. Charge transfer resistance of
the coating material gradually decreases with immersion times, however, the values are
much higher than that of uncoated ones. The two loops observed in the Nyquist plots have
been attributed to the electrical properties of the film and electrochemical processes taking
place at the interface, respectively. Corrosion potential of C45 steel electrode, when coated
with PANI−MMT or soluble PANI is anodically shifted. Significant decrease in the
69
Summary
corrosion current and corrosion rate has also been observed. In the case of soluble PANI,
anticorrosion performance is enhanced by the hydrophobic nature of the dopant ion which
hinders the ingress of anions present in the corrosive environment. The molar feed ratios of
DBSA to aniline influence the anticorrosion performance of PANI. Polyaniline containing
1:7 mole ratio of aniline−to−DBSA exhibit better corrosion protection.
70
References
5. References
[1] V. V. Vasiliev, E. V. Morozov, In Mechanics and analysis of Composite Materials,
1st ed., Elsevier, Netherlands, 2001.
[2] R. M. Jones, In Mechanics of Composite Materials, 2nd ed., Taylor & Frances, USA,
2001.
[3] P. M. Ajayan, L. S. Schadler, P. V. Braun, In Nanocomposite Science and
Technology, Wiley-VCH, Weinheim, 2003.
[4] Q. H. Zeng, D. Z. Wang, A. B. Yu, G. Q. Lu, Nanotechnology 13 (2002) 549.
[5] Y. Zhu, In Synthesis, characterization and corrosion performance of polyaniline
montmorillonite clay nanocomposites, PhD thesis 2003.
[6] C. G. Wu, D. C. DeGoot, H. O. Marcy, J. L. Schindler, C. R. Kannewurf, T. Bakas,
V. Papaefthymiou, W. Hirpo, J. P. Yesinowski, Y. J. Liu, M. G. Kanatzidis, J. Am.
Chem. Soc. 117 (1995) 9229.
[7] G. R. Goward, T. A. Kerr, W. P. Power, L. F. Nazar, Adv. Mater. 10 (1998) 449.
[8] T. A. Kerr, H. Wu, L. F. Nazar, Chem. Mater. 8 (1996) 2005.
[9] M. G. Kanatzidis, C.G. Wu, H. O. Marcy, C. R. Kannewurf, J. Am. Chem. Soc. 111
(1989) 4139.
[10] G. M. do Nascimento, V. R. L. Constantino, R. Landers, M. L. A. Temperini,
Macromolecules 37 (2004) 9373.
[11] W. M. de Azevedo, M. O. E. Schwartz1, G. C. do Nascimento, E. F. J. da Silva, Phys.
Stat. Sol. (C) 1 (2004) S249.
[12] B. H. Kim, J. H. Jung, J. W. Kim, H. J. Choi, J. Joo, Synth. Met. 117 (2001) 115.
[13] T. Lan, P. D. Kaviratna, T. J. Pinnavaia, Chem. Mater. 6 (1994) 573.
[14] H. L. Tyan, Y. C. Liu, K. H. Wei, Chem. Mater. 11 (1999) 1942.
[15] K. H. Chen, S. M. Yang, Synth. Met. 135–136 (2003) 151.
[16] J. W. Kim, S. G. Kim, H. J. Choi, M. S. Jhon, Macromol. Rapid Commun. 20 (1999)
450.
[17] M. Pichowicz, R. Mokaya, Chem. Mater. 16 (2004) 263.
[18] T. Lan, P. D. Kaviratna, T. J. Pinnavaia, Chem. Mater. 7 (1995) 2144.
71
References
[19] G. M. do Nascimento, V. R. L. Constantino, M. L. A. Temperini, Macromolecules 35
(2002) 7535.
[20] H. L. Fisch, B. Xi, Y. Quin, M. Rafailovich, N. L. Yang, X. Yan, High perform.
Polym. 12 (2000) 543.
[21] H. J. Choi, J. W. Kim, M. H. Noh, D. C. Lee, M. S. Suh, M. J. Shin, M. S. Jhon, J.
Mater. Sci. Lett. 18 (1999) 1505.
[22] P. S. Rao, D. N. Sathyanarayana, In Advanced Functional Molecules and Polymers,
H. S. Nalwa (ed.), Gordon & Breach, Tokyo, 2001.
[23] J. C. Chiang, A. G. McDiarmid, Synth. Met. 13 (1986) 193.
[24] E. T. Kang, K. G. Neoha, K. L. Tan, Prog. Polym. Sci. 23 (1998) 277.
[25] S. Shreepathi, R. Holze, Chem. Mater. 17 (2005) 4078.
[26] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada , T. Nakajima, T. Kawagoe,
Macromolecules 21 (1998) 1297.
[27] K. G. Conroy, C. B. Breslin, Electrochim. Acta 48 (2003) 721.
[28] J. E. Mark, In Polymer data Handbook, Oxford University, 1999.
[29] E. M. Genies, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36 (1990) 139. [30] M. M. Popovic, B. N. Grgur, V. B. Miskovic-Stankovic, Prog. Org. Coat. 52 (2005)
359.
[31] A. T. Özyilmaz, Prog. Org. Coat. 54 (2005) 127.
[32] G. Bereket, E. Hür, Y. Sahin, Prog. Org. Coat. 54 (2005) 63.
[33] A. Talo, P. Pasiniemi, O. Forsen, S. Yläsaari, Synth. Met. 85 (1997) 1333.
[34] B. Wessling, Synth. Met. 85 (1997) 1313.
[35] W. K. Lu, R. L. Elsenbaumer, B. Wessling, Synth. Met. 71 (1995) 2163.
[36] A. J. Dominis, G. M. Spinks, G. G. Wallace, Prog. Org. Coat. 48 (2003) 43.
[37] D. E. Tallman, G. Spinks, A. Dominis, G. G. Wallace, J. Solid State Electrochem. 6
(2002) 73.
[38] J. Stejskal, R. G. Gilbert, Pure Appl. Chem. 74 (2002) 857.
[39] B. Feng, Y. Su, J. Song, K. Kong, J. Mater. Sci. Lett. 20 (2001) 293.
[40] M. Kato, A. Usuki, In Polymer-Clay Nanocomposites, John Wiley & Sons, New
York, 2000, p. 98.
[41] S. Yoshimoto, F. Ohashi, Y. Ohnihi, T. Nonami, Synth. Met. 145 (2004) 265.
72
References
[42] S. Yoshimoto, F. Ohashi, T. Kameyama, Macromol. Rapid. Commun. 25 (2004)
1687.
[43] K. David, Jr. Gosser, In Cyclic Voltammetry: Simulation and Analysis of Reaction
Mechanisms, VCH, New York, 1993.
[44] J. Wang, In Analytical electrochemistry, 3rd ed. Wiley-VCH, New Jersey, 2006. p. 84.
[45] In Cyclic Voltammetry using a LabVIEW-based Acquisition System, Academic
Parnership grant from national Instruments, 2004, p. 1.
[46] Q. Zheng, In Fundamental studies of organic and polymer nanocomposites. Ph.D
thesis 2004.
[47] D. Harvey, In Modern Analytical Chemistry, 1st ed. McGraw-Hill, New York, 2001.
p. 397.
[48] G. Natta, G. Mazzanti, P. Corradini, Atti. Acad. Naz. Lincei, Cl. Sci. Fis. Mat. Rend.
25 (1958), 3.
[49] R. Holze, J. Lippe, Synth. Met. 38 (1990) 99.
[50] D. W. DeBerry, J. Electrochem. Soc. 132 (1985) 1022.
[51] K. G. Shah, G. S. Akundy, J. O. Iroh, J. Appl. Polym. Sci. 85 (2002) 1669.
[52] M. C. Bernard, A. H. L. Goff, S. Joiret, N. N. Dinh, N. N. Toan, J. Electrochem. Soc.
146 (1999) 995.
[53] R. Gasparac, C. R. Martin, J. Electrochem. Soc. 148 (1999) B138.
[54] N. A. Ogurtsov, A. A. Pud, P. Kamarchik, G. S. Shapoval, Synth. Met. 143 (2004) 43.
[55] P. J. Kinlen, D. C. Silverman, C. R. Jeffreys, Synth. Met. 85 (1997) 1327.
[56] B. Wessling, S. Schröder, S. Gleeson, H. Merkle, F. Baron, Mater. Corros. 47 (1996)
439.
[57] N. Perez, In Electrochemistry and Corrosion Science, Kluver Academic, Boston,
2004.
[ 58 ] A. J. Bard, L. R. Faulkner, In Electrochemical Methods: Fundamentals and
Applications, 2nd ed., John Wiley & Sons, New York, 2001.
[59] P. R. Roberge, In Handbook of Corrosion Engineering, McGraw-Hill, New York,
2000.
[60] J. Koryta, J. Dvořák, L. Kavan, In Principles of Electrochemistry, 2nd ed., John Wiley
& Sons, Chichester, 1993.
73
References
[61] C. A. A. Brett, A. M. O. Brett, In Electrochemistry: Principles, Methods and
Applications, 2nd ed., Oxford University, England, 1994.
[62] L. Benea, O. Mitoseriu, J. Galland, F. Wenger, P. Ponthiaux, Mater. Corros. 51
(2000) 491.
[63] K. S. Khairou, A. E. Sayed, J. Appl. Polym. Sci. 88 (2003) 866.
[64] A. Popova, S. Raicheva, E. Sokolova, M. Christov, Langmuir 12 (1996) 2083.
[65] D. Talbot, J. Talbot, In Corrosion Science and Technology, CRC, New York, 1998.
[66] J. M. Yeh, C. P Chin, J. Appl. Polym. Sci. 88 (2003) 1072.
[67] J. M. Yeh, C. L. Chen, Y. C. Chen, C. Y. Ma, H. Y. Huang, Y. H. Yu, J. Appl. Polym.
Sci. 92 (2004) 631.
[68] J. M. Yeh, S. J. Liou, C. Y. Lai, P. C. Wu, T. Y. Tsai, Chem. Mater. 13 (2001) 1131.
[69] Y. H. Yu, J. M. Yeh, S. J. Liou, C. L. Chen, D. J. Liaw, H. Y. Lu, J. Appl. Polym. Sci.
92 (2004) 3573.
[70] S. Ito, K. Murata, S. Teshima, R. Aizawa, Y. Asako, K. Takahashi, M. M. Hoffman,
Synth. Met. 96 (1998) 161.
[71] W. Yin, E. Ruckenstein, Synth. Met. 108 (2000) 39.
[72] J. Laska, J. Widlarz, Synth. Met. 135-136 (2003) 261.
[73] P. J. Kinlen, J. Liu, Y. Ding, C. R. Graham, E. E. Remsen, Macromolecules 31 (1998)
1735.
[74] A. A. Athawale, M. V. Kulkarni, V. V. Chabukswar, Mater. Chem. Phys. 73 (2002)
106.
[75] P. S. Rao, S. Subrahmanya, D. N. Sathyanarayana, Synth. Met. 128 (2002) 311.
[76] P. S. Rao, S. Palaniappan, D. N. Sathyanarayana, Macromolecules 35 (2002) 4988.
[77] B. H. Kim, J. H. Jung, S. H. Hong, J. Joo, A. J. Epstein, K. Mizoguchi, J. W. Kim, H.
J. Choi, Macromolecules 35 (2002) 1419.
[78] Q. Wu, Z. Xue, Z. Qui, F. Wang, Polymer 41 (2000) 2029.
[79] D. Lee, K. Char, S. W. Lee, Y. Park, J. Mater. Chem. 13 (2003) 2942.
[80] D. Lee, S. H Lee, K. Char, J. Kim, Macromol. Rapid Commun. 21 (2000) 1136.
[81] H. Inoue, H. Yoneyama, J. Electroanal. Chem. 233 (1987) 291.
[82] E. Rufe, M. F. Hochella, Science 285 (1999) 874.
[83] K. Amram, J. Ganor, J. Geochimica et Cosmochimica Acta 69 (2005) 2535.
[84] L. P. Meier, R. Nüesch, J. Colloid Interface Sci. 217 (1999) 77.
74
References
[85] M. D. Welch, A. K. Kleppe, A. P. Jephcoat, Am. Mineral. 89 (2004) 1337.
[86] Z. Ma, T. Kyotani, A. Tomita, Chem. Commun. (2000), 2365.
[87] G. Chen, S. Liu, S. Chen, Z. Qi, Macromol. Chem. Phys. 202 (2001) 1189.
[88] G. Chen, S. Liu, S. Chen, Z. Qi, Macromol. Rapid Commun. 21 (2000) 746.
[89] I. Harada, Y. Furukawa, F. Ueda, Synth. Met. 29 (1989) E303.
[90] T. Stutzmann, B. Siffert. Clays and Clay Minerals 25 (1977) 392.
[91] A. A. Elwahed, R. Holze, Synth. Met. 131 (2002) 61.
[92] R. Holze, In Handbook of advanced electronic and Photonic Materials H. S. Nalwa
(ed.), Gordon and Breach, Singapore, 2001; Vol 2, p. 171.
[93] S. Mu, Synth. Met. 143 (2004) 259.
[94] A. Hochfeid, R. Kessel, J. W. Schultze, A. Thyssen, Ber. Binsenges. Phys. Chem. 92
(1988) 1406.
[95] D. E. Stilwell, S. M. Park, J. Electrochem. Soc. 136 (1989) 427.
[96] G. D'Aprano, M. Leclerc, G. Zotti, Macromolecules 25 (1992) 2145.
[97] A. Malinauskas, R. Holze, J. Appl. Polym. Sci. 73 (1999) 287.
[98] S. Sathiyanarayanan, S. Muthukrishnan, G. Venkatachari , D.C. Trivedi, Prog. Org.
Coat. 53 (2005) 297.
[99] K. Belmokre, N. Azzouz, F. Kermiche, M. Wery, J. Pagetti, Mater. Corr. 49 (1998)
108.
[100] T. Tüken, A. T. Özyilmaz, B. Yazici, G. Kardas, M. Erbil, Prog. Org. Coat. 51
(2004) 27.
[101] F. Mansfel, M. V. Kendig, Werkstoffe und Korrosion 34 (1983), 397.
[102] J. L. Camalet, J. C. Lacroix, S. Aeiyach, K. C. Ching, P. C. Lacaze, Synth. Met. 93
(1998) 133.
[103] J. L. Camalet, J. C. Lacroix, S. Aeiyach, K. C. Ching, P. C. Lacaze, J. Electroanal.
Chem. 416 (1996) 179.
[104] D. Sazou, C. Georgolios, J. Electroanal. Chem. 429 (1997) 81.
[105] V. Patil, S. R. Sainkar, P. P. Patil, Synth. Met. 140 (2004) 57.
[106] C. K. Tan, D. J. Blackwood, Corros. Sci. 45 (2003) 545.
[107] E. Barsonkov, J. R. Macdonald, In Impedance Spectroscopy: Theory, Experiment
and Applications, 2nd ed., Wiley-Interscience, New Jersy, 2005.
[108] R. Holze, Bull. Electrochem. 10 (1994) 56.
75
References
[109] S. Shreepathi, H. V. Hoang, R. Holze, J. Electrochem. Soc. 2006 in press.
[110] G. Bereket, E. Hür, Y. Sahin, Appl. Surf. Sci. 252 (2005) 1233.
[111] G. M. Spinks, A. J. Dominis, G. G. Wallace, D. E. Tallman, J. Solid State
Electrochem. 6 (2002) 85.
[112] A. A. Pud, G. S. Shapoval, P. Kamarchik, N. A. Ogurtsov, V. F. Gromovaya, I. E.
Myronyuk, Yu. V. Kontsur, Synth. Met. 107 (1999) 111.
[113] A. Mirmohseni, A. Oladegaragoze, Synth. Met. 114 (2000) 105.
[114] S. Shreepathi, R. Holze, Langmuir 22 (2006) 5196.
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Selbständigkeitserklärung
Selbständigkeitserklärung
Hiermit erkläre ich an Eides statt, die vorliegende Arbeit selbständig und ohne unerlaubte
Hilfsmittel durchgeführt zu haben.
Chemnitz, den 01.09.2006 Hung Van Hoang
77
Curriculum Vita
Curriculum Vita
Name Hung Van Hoang
Date of birth 08.12.1973
Place of birth Hanoi, Vietnam
Nationality Vietnamese
Marriage Status Single
School Education
1980-1987 Xuan Thu primary and secondary school
1987-1990 Soc Son High school
University Education
1991-1995 B.Sc, Hanoi University of Education
1995-1997 M.Sc, International Training Institute for Materials Science.
Experience and Skills
1998-2002 Teacher, Hanoi University of Education
Since November 2002 Working as research fellow under the supervision of Prof. Dr.
Rudolf Holze, TU-Chemnitz, Germany.
Publications
1. Electrochemical Synthesis of Polyaniline/Montmorillonite Nanocomposites and Their
Characterization. Hung Van Hoang and Rudolf Holze, Chemistry of Materials 2006, 18,
1976-1980.
2. Corrosion Protection Performance and Spectroscopic Investigations of Soluble Conducting
Polyaniline-Dodecylbenzenesulfonate Synthesized via Inverse Emulsion Procedure.
Subrahmanya Shreepathi, Hung Van Hoang and Rudolf Holze, Journal of electrochemical
society 2007, 154, C67-C73.
78