electrochemical synthesis of novel polyaniline...

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


3.9.2 The anti-corrosion properties of soluble PANI .............................................................. 62

3.9.2.1 Electrochemical impedance measurements ............................................................. 62


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


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


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



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


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


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


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


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


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


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-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-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


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


-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


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


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


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


-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


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


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


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


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


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.


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


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


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


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


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


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


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].


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


-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


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.

76

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

electrochemical synthesis of novel polyaniline...

Documents