I: Introduction - CiteSeerX

Electrochemical Synthesis of Novel PolyanilineMontmorillonite 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 Korrosionsgeschwindigkeit 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.

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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, PANIMMT 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 PANIMMT 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 PANIMMT 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.

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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.

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Dedication

Dedication

To my parents To my teachers To my sisters and brothers To whom I love Hung Van Hoang

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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

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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

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Table of contents

3.5 Cyclic voltammetry.............................................................................................................. 50 3.6 In situ UV-Vis spectroscopy............................................................................................... 52 3.7 In situ conductivity measurements..................................................................................... 54 3.9 Corrosion studies ................................................................................................................. 55 3.9.1 The anti-corrosion properties of PANI-MMT ................................................................ 56 3.9.1.1 Electrochemical impedance measurements ............................................................. 56 3.9.1.2 Polarization measurements ...................................................................................... 60 3.9.2 The anti-corrosion properties of soluble PANI .............................................................. 62 3.9.2.1 Electrochemical impedance measurements ............................................................. 62 3.9.1.2 Polarization measurements ...................................................................................... 66 4. SUMMARY............................................................................................................................ 69 5. REFERENCES ...................................................................................................................... 71

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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.

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List of abbreviations

List of abbreviations

A

Surface area

AC

Alternative current

AE

Auxiliary electrode

Aw

Molecular weight

ba

Anodic slope

bc

Cathodic slope

CC

Coating capacitance

CDL

Double layer capacitance

CE

Counter electrode

CEC

Cation exchange capacity

CR

Corrosion rate

CV

Cyclic voltammogram

d

Density of metals or alloys

DBSA

Dedocylbenzene sulfonic acid

DC

Direct current

EB

Emeraldine salt form of polyaniline

EC

Equivalent circuit

Ecorr

Corrosion potentials

Eeq

Equilibrium potential

EIM

Electrochemical impedance measurements

EM

Emeraldine form of polyaniline

EQ

Equivalent weight

ES

Emeraldine base form of polyaniline

ESCE

Potential versus saturated calomel electrode

η

Overpotential

ηa

Anodic overpotential

ηc

Cathodic potential

FT-IR

Fourier transform infrared spectroscopy

i0

Exchange current density

ia

Anodic current density 10

List of abbreviations ic

Cathodic current density

icorr

Corrosion current density

ITO

Indium tin oxide coated glass

LE

Leucoemeraldine form of polyaniline

MMT

Montmorillonite

MPY

Milliinche per year

OCP

Open circuit potential

PANI

Polyaniline

PN

Pernigraniline form of polyaniline

RCT

Charge transfer resistance

RF

Film resistance

Rp

Polarization resistance

RPM

Rotations per minute

RS

Solution resistance

SEM

Scanning electron microscopy

TEM

Transmission electron microscopy

TIP-5

Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 5:1

TIP-6

Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 7:1

TIP-7

Soluble PANI-DBSA with DBSA-to-aniline feed ratio of 10:1

UV-Vis

Ultraviolet visible

WE

Working electrode

XRD

X-ray diffraction

Z

Impedance

Z'

Real part of impedance

Z"

Imaginary part of impedance

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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.

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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].

N H

N H

N H

N H

+

N H2

n

Leucoemeraldine base

N H

N H

N

N

N

N H

n

+

N H

N H

Emeraldine salt N

N n

Pernigraniline Figure 1

N H

N H

n

Leucoemeraldine salt

Emeraldine base N

N H

The different polyaniline forms

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+

N H

n

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].

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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 (104 S cm-1). Protonated PANI converts to a nonconducting emeraldine base when treated with alkali solutions (Figure 2) [38].

+

+

N H A

N H A

N H

-2n H+ A

+2nH+ A

Emeraldine salt

N

N

N H

N H

N H

n

n

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 (