Chris DeArmitt PhD Thesis

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NOVEL COLLOIDAL AND SOLUBLE FORMS OF

POLYANILINE AND POLYPYRROLE

UNIVERSITY OF SUSSEX

CHRISTOPHERL.DEARMITT D.PHIL

SEPTEMBER1995


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DECLARATION

I hereby declare that this thesis has not previously been submitted , either

in the same or different form, to this or any other University for a degree

Chris DeArmitt

September 1995


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ACKNOWLEDGEMENTS

This work would not have been possible or as enjoyable without the help and support ofseveral people. I therefore take this opportunity to say thanks to my friends, colleagues

and collaborators.

I would like to thank my supervisor Dr. Steve Armes for his help, friendship and for

giving me the freedom to explore my ideas. The work was funded jointly by the SERC

and Courtaulds Research, so thank you to them and in particular to Dr. David Bott of

Courtaulds Research for sponsoring my studies for the last six years.

The surface characterisation was done with the substantial help of my friends at

Courtaulds. Drs Shen Luk (SERS and XPS), Warren Lineton (TOF-SIMS), Mike

Keane (XPS) and Simon Porter (electrochemical polymerisation). I also had some

collaboration with F.A. Uribe, S. Gottesfeld and C. Mombourquette of the Electronics

Research Group, Los Alamos National Laboratory who helped with the cyclic

voltammetry and SEM in chapter 7. Lastly, I thank Julian Thorpe for teaching me TEM

and SEM, for gold coating and photographic processing.

Finally, I would like to express my appreciation of my special friends Drs Steve

Rannard, Dave Lacey, Mike Gill, Simon Cook and of course Anna Kron.


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to Anna


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SUMMARY

Christopher L. DeArmitt D. Phil

Novel Colloidal and Soluble Forms of Polyaniline and Polypyrrole

This work is concerned with enhancing the poor processability of the conductingpolymers polyaniline and polypyrrole. The majority of the work deals with thepreparation and characterisation of novel aqueous dispersions which are stabilisedagainst aggregation by the common anionic surfactants SDS (sodium dodecyl sulfate)and SDBS (sodium dodecylbenzene sulfonate). We also describe the first dispersion

polymerisation of aniline in non-aqueous media and the synthesis ofpoly(diphenylamine sulfonate) a new water-soluble polyaniline derivative.

We have prepared novel aqueous dispersions of polyaniline and polypyrrole simply byadding anionic surfactant to the chemical synthesis of these polymers. Thesedispersions are stable to aggregation and they allow conductive coatings of thenormally intractable conducting polymer to be made (conductivities in the range 10-3 to2 S cm-1). We have characterised the dispersions using a wide range of techniquesincluding proton NMR and FTIR spectroscopy, TGA, SEM, TEM and DCP (disc-centrifuge photosedimentometry) for bulk analysis and XPS, TOF-SIMS and SERS(surface-enhanced Raman scattering) for surface composition studies. The polypyrrolecolloids were composed of polydisperse particles in the range 200 to 500 nm indiameter. The kinetics of polymerisation were dramatically affected by the presence ofsurfactant, this has been monitored by proton NMR spectroscopy. This techniqueallows polymer formation, proton elimination and surfactant adsorption to bequantified at short time intervals during polymerisation.

We were able to prepare, for the first time, non-aqueous (sterically-stabilised)polyaniline colloids by dispersion polymerisation in acetonitrile, propionitrile orbutyronitrile using Cu(ClO4)26H2O as oxidant. The solid-state conductivities of thesepolymers were however rather low (1x10-4 S cm-1).

We also report the first synthesis of a new water-soluble polyaniline derivative, poly(diphenylamine sulfonate). The resulting self-doped polymer had a solid-stateconductivity of 1x10-3 S cm-1 and exhibited a novel green to red colour change as thesolution pH was raised from pH 4 to pH 6. This polymer was further characterised

using several techniques.


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INSTRUMENTATION

Centrifugation L2-65B Beckman Ultracentrifuge and Beckman J2-21

FTIR Perkin-Elmer 1730, 10 scans at 4 cm-1 resolution on powder

dispersed in a KBr disk

Microanalysis Perkin-Elmer 2400

NMR Bruker AC250SY Fourier Transform 250 MHz Spectrometer.

TEM Jeol 100C

SEM Jeol 100C with ASID ultra-high resolution attachment

TGA Perkin Elmer TGA 7, Scan Rate of 10C min-1

UV/Visible Philips PU 8720 Scanning Spectrophotometer

Voltmeter Keithley 195 System DMM


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TABLE OF CONTENTS

1.1 Background .......................................................................................................................................... 13

1.2 What is a Conducting Polymer ? .......................................................................................................... 14

1.3 Properties, Applications and Limitations ............................................................................................. 15

1.4 Improving the Processability of Conducting Polymers ........................................................................ 161.5 This Work ............................................................................................................................................ 18

1.6 Polyaniline Overview ........................................................................................................................... 19

1.6.1 General Synthesis ......................................................................................................................... 19

1.6.2 Electrochemical Synthesis ............................................................................................................ 19

1.6.3 Chemical Polymerisation .............................................................................................................. 20

1.6.4 Polymerisation Mechanism ........................................................................................................... 21

1.6.5 Redox and pH Behaviour ............................................................................................................. 23

1.6.6 Molecular Weight ......................................................................................................................... 25

1.6.7 Thermal Stability .......................................................................................................................... 26

1.6.8 Chemical and Physical Structure .................................................................................................. 27

1.7 Polypyrrole Overview .......................................................................................................................... 29

1.7.1 General Synthesis ......................................................................................................................... 29

1.7.2 Electrochemical Synthesis ............................................................................................................ 29

1.7.3 Chemical Synthesis ....................................................................................................................... 30

1.7.4 Polymerisation Mechanism ........................................................................................................... 31

1.7.5 Redox and pH Behaviour ............................................................................................................. 32

1.7.6 Molecular Weight ......................................................................................................................... 33

1.7.7 Thermal Stability .......................................................................................................................... 34

1.7.8 Chemical and Physical Structure .................................................................................................. 34

1.8 An Introduction to Colloids .................................................................................................................. 36

1.8.1 Background .................................................................................................................................. 36

1.8.2 Definition of a Colloid .................................................................................................................. 36

1.8.3 Particle Morphology and Size ...................................................................................................... 37

1.8.4 Colloid Stability............................................................................................................................ 38

1.8.5 Charge Stabilisation ...................................................................................................................... 39

1.8.6 The Deryagin-Landau-Verwey-Overbeek (DLVO) Theory ......................................................... 40

1.8.7 Steric Stabilisation ........................................................................................................................ 41

2.1 Background .......................................................................................................................................... 43

2.2 Polypyrrole / Surfactant Dispersions .................................................................................................... 45

2.2.1 Background .................................................................................................................................. 45

2.2.2 Emulsion Polymerisation of Pyrrole ............................................................................................. 46

2.3 Experimental ........................................................................................................................................ 48

2.3.1 Chemicals ..................................................................................................................................... 48

2.3.2 Preparations .................................................................................................................................. 48

2.4 Results and Discussion ......................................................................................................................... 49


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2.4.1 Polypyrrole / SDBS Dispersions .................................................................................................. 49

2.4.2 Polypyrrole / SDS Dispersions ..................................................................................................... 54

2.5 Conclusion ............................................................................................................................................ 58

3.1 Background .......................................................................................................................................... 60

3.1.1 Microemulsion Polymerisation of Aniline .................................................................................... 62

3.2 Experimental ........................................................................................................................................ 63

3.2.1 Chemicals ..................................................................................................................................... 63

3.2.2 Preparations .................................................................................................................................. 63

3.2.3 Determination of the Concentration of DBSA in Reaction Supernatant by UV Spectroscopy ..... 64

3.3 Results and Discussion ......................................................................................................................... 64

3.3.1 Polyaniline / DBSA Dispersions................................................................................................... 64

3.3.2 Polyaniline / SDS Dispersions ...................................................................................................... 70

3.4 Conclusions .......................................................................................................................................... 71

4.1 Introduction .......................................................................................................................................... 72

4.2 Experimental ........................................................................................................................................ 73

4.2.1 The Technique .............................................................................................................................. 73

4.2.2 Chemicals ..................................................................................................................................... 75

4.2.3 Method ......................................................................................................................................... 75

4.3 Polyaniline Results ............................................................................................................................... 77

4.3.1 Precipitation Polymerisation with No Added Surfactant .............................................................. 77

4.3.2 Dispersion Polymerisation with DBSA Surfactant ....................................................................... 80

4.3.3 Kinetic Analysis ........................................................................................................................... 83

4.4 Polypyrrole Results .............................................................................................................................. 85

4.4.1 Background .................................................................................................................................. 85

4.4.2 Precipitation Polymerisation ......................................................................................................... 85

4.4.3 Dispersion Polymerisation ............................................................................................................ 86

4.4.4 Reproducibility ............................................................................................................................. 87

4.4.5 Is 1H NMR a Valid Technique for Studying the Kinetics ? .......................................................... 88

4.4.6 Comparisons of1H Results to Temperature and pH Measurements ............................................. 89

4.4.7 The Effect of D2O as Solvent on the Rate of Pyrrole Polymerisation........................................... 90

4.4.8 The Effect of Surfactant Concentration on Polymerisation Rate .................................................. 94

4.5 Conclusions .......................................................................................................................................... 95

5.1 Background .......................................................................................................................................... 975.2 Experimental ........................................................................................................................................ 98

5.2.1 Preparation of the Polypyrrole / SDBS Dispersion....................................................................... 98

5.2.2 Electrochemical Synthesis of a Dodecylbenzenesulfonate - doped Polypyrrole Film. ................. 99

5.2.3 Scanning Electron Microscopy ..................................................................................................... 99

5.2.4 TOF-SIMS .................................................................................................................................. 100

5.2.5 X-Ray Photoelectron Spectroscopy ............................................................................................ 100

5.2.6 Surface-Enhanced Raman Spectroscopy .................................................................................... 100


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5.3 The Techniques .................................................................................................................................. 101

5.3.1 X-Ray Photoelectron Spectroscopy(XPS) .................................................................................. 101

5.3.2 Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) ........... .............. ............. ......... 103

5.3.3 Surface Enhanced Raman Scattering (SERS) ............................................................................. 105

5.4 Results and Discussion ....................................................................................................................... 106

5.4.1 SEM ............................................................................................................................................ 106

5.4.2 Elemental Analysis ..................................................................................................................... 106

5.4.3 Surface-Enhanced Raman Scattering .......................................................................................... 107

5.4.4 Time-of-Flight Secondary Ion Mass-Spectroscopy .................................................................... 108

5.4.5 X-Ray Photoelectron Spectroscopy ............................................................................................ 111

5.4.6 Stabilisation Mechanism ............................................................................................................. 114

5.4.7 Conductivity ............................................................................................................................... 117

5.5 Conclusions ........................................................................................................................................ 118

6.1 Background ........................................................................................................................................ 119

6.2 Experimental ...................................................................................................................................... 120

6.2.1 Monomer Synthesis .................................................................................................................... 120

6.2.2 Steric Stabiliser Syntheses .......................................................................................................... 121

6.2.3 Attempts to Prepare Polyaniline Colloids ................................................................................... 124

6.2.4 Polyaniline Prepared in Acetonitrile, Propionitrile or Butyronitrile ........................................... 125


6.3.1 Background ................................................................................................................................ 126

6.3.2 Copolymer Characterisation ....................................................................................................... 126

6.3.3 Other Methods to Introduce Aniline Graft Sites ......................................................................... 127

6.3.4 Attempted Dispersion Syntheses ................................................................................................ 130

6.3.5 Polyaniline Prepared in Other Solvents ...................................................................................... 132

6.3.6 Polyaniline Colloids Synthesised in Propionitrile (EtCN) .......................................................... 133

6.4 Conclusion .......................................................................................................................................... 135

7.1 Background ........................................................................................................................................ 136

7.2 Experimental ...................................................................................................................................... 137

7.2.1 Chemicals ................................................................................................................................... 137

7.2.2 Synthesis of Tetrabutylammonium Diphenylamine Sulfonate ............ .............. ............ .............. 137

7.2.3 Polymer Synthesis ...................................................................................................................... 137

7.2.4 Polymer Characterisation............................................................................................................ 1387.2.5 Electrochemical Polymerisation of Tetrabutylammonium Diphenylamine Sulfonate ......... ....... 138


7.3.1 Conductivity ............................................................................................................................... 139

7.3.2 Visible Absorption Spectroscopy ............................................................................................... 139

7.3.3 FTIR Spectroscopy ..................................................................................................................... 140

7.3.4 1H NMR Spectroscopy ............................................................................................................... 141

7.3.5 Scanning Electron Microscopy ................................................................................................... 142


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7.3.6 Thermogravimetric Analysis (TGA) ........................................................................................... 143

7.3.7 Cyclic Voltammetry .................................................................................................................... 144

7.4 Conclusion .......................................................................................................................................... 146

8.1 Non-Aqueous Polyaniline Dispersions ............................................................................................... 147

8.2 Poly(diphenylamine sulfonate) a Novel Water-Soluble Polyaniline Derivative ............ .............. ....... 147

8.3 Aqueous Conducting Polymer Dispersions Prepared using Small-Molecule Surfactants .................. 148

8.4 Summary ............................................................................................................................................ 150

8.5 Future Work ....................................................................................................................................... 150

LIST OF FIGURES

Figure 1.2.1 The Chemical Structure of Polyaniline ............ .............. ............. .............. ............ .............. ..... 15

Figure 1.6.1 Aniline Polymerisation Mechanism, Wei et al.44. .................................................................... 23

Figure 1.6.2 Redox and pH Transitions of Polyaniline .............. ............. ............ .............. ............. .............. 24

Figure 1.6.3 The Chemical Structure of Polyaniline ............ .............. ............. .............. ............ .............. ..... 27

Figure 1.7.1 Charge Movement in Polypyrrole ............ .............. ............. ............ .............. ............. .............. 32

Figure 1.8.1 The Concentration of Ions Near to A Charged Surface....................... .............. ............. .......... 39

Figure 2.1.1 Conventional Preparation of Polypyrrole Dispersions ............ ............ .............. ............. .......... 44

Figure 2.4.1 FTIR Spectrum of Sulfate-Doped Polypyrrole............... ............. .............. ............ ............. ...... 50

Figure 2.4.2 FTIR Spectrum of DBSA Surfactant................... .............. ............. ............ .............. ............. ... 50

Figure 2.4.3 FTIR Spectrum of Dried Polypyrrole / SDBS Dispersion ............. ............ .............. ............. ... 51

Figure 2.4.4 TGA of Polypyrrole / SDBS Colloid .............. ............. .............. ............ .............. ............. ....... 52

Figure 2.4.5 TGA of Sulfate-Doped Polypyrrole ............ ............ .............. ............. .............. ............ ............ 52

Figure 2.4.6 FTIR Spectrum of SDS Surfactant .............. ............ .............. ............. .............. ............ ............ 56

Figure 2.4.7 FTIR Spectrum of Dried Polypyrrole / SDS Dispersion ............. .............. ............ .............. ..... 56

Figure 2.4.8 Thermogravimetry on a Dried Polypyrrole / SDS Dispersion ............ .............. ............ ............ 57

Figure 3.1.1 Conventional Polyaniline Dispersion Polymerisation ............. ............ .............. ............. .......... 61

Figure 3.3.1 FTIR Spectrum of Dried Polyaniline / DBSA Dispersion ..................... .............. ............. ....... 66

Figure 3.3.2 FTIR Spectrum of DBSA Surfactant................... .............. ............. ............ .............. ............. ... 67

Figure 3.3.3 TGA of Dried Polyaniline / DBSA Dispersion .............. ............. .............. ............ .............. ..... 68

Figure 3.3.4 TGA of Polyaniline Prepared Without Surfactant..................... ............. .............. ............. ....... 68

Figure 3.3.5 Determination of the Concentration of DBSA in the Reaction Supernatant by UV

Spectroscopy ........................................................................................................................................ 70

Figure 4.2.1 NMR Kinetics Apparatus ............ ............. .............. ............. ............ .............. ............. .............. 75

Figure 4.2.2 Typical 1H NMR Spectra Before and During Polymerisation.................................................. 77

Figure 4.3.1 Precipitation Polymerisation of Aniline by 1H NMR ............................................................... 79

Figure 4.3.2 Aniline Polymerisation Studied by Temperature Change ........... .............. ............ .............. ..... 80

Figure 4.3.3 Kinetics of the Dispersion Polymerisation of Aniline ............. ............ .............. ............. .......... 81

Figure 4.3.4 Retardation of Polymerisation by Surfactant.................. ............. .............. ............ .............. ..... 81

Figure 4.4.1 Polymerisation of Pyrrole With and Without Surfactant ............. .............. ............ .............. ..... 86

Figure 4.4.2 Dispersion Polymerisation of Pyrrole by NMR Spectroscopy ............ .............. ............. .......... 87


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Figure 4.4.3 Repeated Pyrrole Colloid Preparation ............. ............. .............. ............ .............. ............. ....... 88

Figure 4.4.4 Pyrrole Polymerisation Kinetics by Temperature and H+ Concentration ............. ............. ....... 90

Figure 4.4.5 The Kinetics of Pyrrole Polymerisation in H2O and D2O ........................................................ 91

Figure 4.4.6 Deuteration of Pyrrole Studied by 1H NMR Spectroscopy ...................................................... 93

Figure 4.4.7 The Effect of SDS Concentration on Polymerisation Rate ............. .............. ............. .............. 94

Figure 5.3.1 The Fundamentals of XPS ............ .............. ............ .............. ............. .............. ............ .......... 101

Figure 5.4.1 SEM Micrograph of SDBS-Stabilised Polypyrrole Particles ............. ............ .............. .......... 106

Figure 5.4.2 SERS Spectra for SDBS-Stabilised Colloidal Polypyrrole Particles and for Electrodeposited

Polypyrrole Doped with Dodecylbenzene Sulfonate ......................................................................... 108

Figure 5.4.3 Negative Ion TOF-SIMS Spectrum of SDBS Surfactant ............ .............. ............ .............. ... 109

Figure 5.4.4 Negative Ion TOF-SIMS Spectrum of SDBS-Stabilised Polypyrrole Particles ..................... 109

Figure 5.4.5 Negative Ion TOF-SIMS Spectrum of Polypyrrole Prepared Without SDBS .............. .......... 110

Figure 5.4.6 Negative Ion TOF-SIMS Spectrum of SDBS Surfactant ............ .............. ............ .............. ... 110

Figure 5.4.7 Negative Ion TOF-SIMS Spectrum of SDBS-Stabilised Polypyrrole Particles ..................... 111

Figure 5.4.8 XP Survey Spectrum of Polypyrrole Prepared Without Surfactant ............ .............. ............. . 112

Figure 5.4.9 XP Survey Spectrum of SDBS-Stabilised Polypyrrole Particles............... ............ .............. ... 113

Figure 5.4.10 S 2p Core-Line XPS of SDBS-Stabilised Polypyrrole Particles and of Sulfate-Doped

Polypyrrole Synthesised Without Surfactant ..................................................................................... 114

Figure 5.4.11 Surfactant Adsorption Onto Polypyrrole .............. ............. ............ .............. ............. ............ 116

Figure 6.2.1 GPC Traces for Derivatised PVA-co-VAc Using RI and UV detectors ...................... .......... 124

Figure 6.3.1 Monomers Containing an Aniline Functionality ............ ............. .............. ............ .............. ... 128

Figure 7.1.1 Chemical Preparation of Poly(diphenylamine sulfonate) ............ .............. ............ .............. ... 136

Figure 7.3.1 Visible Absorption Spectra of Poly(DPS) Doped and Dedoped ............ .............. ............. ..... 140

Figure 7.3.2 FTIR Spectrum of DPS Monomer, Poly(DPS) and Polyaniline............. .............. ............. ..... 141

Figure 7.3.3 SEM of a Poly(DPS) Film ............ .............. ............ .............. ............. .............. ............ .......... 143

Figure 7.3.4 TGA of Doped Poly(DPS) Dried from Acidic Solution ........................ .............. ............. ..... 144

Figure 7.3.5 Cyclic Voltammogram of Diphenylaminesulfonate .............. ............. ............ .............. .......... 145

Figure 7.3.6 The First Three Sequential Cyclic Voltammograms of Aniline ............. .............. ............. ..... 145

Figure 7.3.7 Cyclic Voltammogram of Polyaniline Synthesised in Aqueous Solution and Then

Transferred to Acetonitrile................................................................................................................. 146

LIST OF TABLES

Table 1.6.1 The UV-Visible Spectrum of Polyaniline from Cushman et al. ...................... .............. ............ 24

Table 1.6.2 Molecular Weight Distribution by Tang et al.53 ........................................................................ 25

Table 2.1.1 Some Examples of Polypyrrole Colloids................................................................................... 43

Table 2.4.1 Elemental Analysis of Polypyrrole / SDBS Colloid and Dried Supernatant ...................... ....... 53


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Table 2.4.2 Polypyrrole / SDS Dispersions ........... .............. ............. .............. ............ .............. ............. ....... 54

Table 2.4.3 Elemental Analysis of Polypyrrole / SDS................ ............. ............ .............. ............. .............. 57

Table 3.1.1 Examples of Conventional Sterically-Stabilised Polyaniline Dispersions ............. ............. ....... 60

Table 3.3.1 Polyaniline / DBSA Polymerisations ............ .............. ............. ............ .............. ............. .......... 65

Table 3.3.2 Polyaniline / SDS Polymerisations ............ .............. ............. ............ .............. ............. .............. 71

Table 6.2.1 Copolymer Synthesis Conditions ............. ............ .............. ............. ............ .............. ............. . 122

Table 6.3.1 Characterisation of Potential Steric Stabilisers ........................ ............ .............. ............. ........ 127

Table 6.3.2 Attempted Polyaniline Colloid Preparations ............ .............. ............. .............. ............ .......... 130

Table 6.3.3 Polyaniline Prepared in MeCN, EtCN or PrCN ............ .............. ............ .............. ............. ..... 133

Table 6.3.4 Polyaniline Colloids Synthesised in EtCN With Cu(ClO4)26H2O .......................................... 134


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CHAPTER1:INTRODUCTION 13

Chapter OneChapter OneChapter OneChapter OneIntroductionIntroductionIntroductionIntroduction

1.1BACKGROUNDPolymers have evolved substantially in the last fifty years and now represent one of the

most important classes of material. This is due to a number of important factors, which

include:

Versatility

Good mechanical performance

Low cost

Environmental stability

They are often excellent electrical insulators

High processability

A range of commodity polymers has arisen to provide a spectrum of products such as

fibres, elastomers and thermosetting resins. Traditional polymers such as nylon 6,6 and

polyethylene occupy a very competitive market with low profit margins. This is because

these low technology polymers can easily be made by a wide range of companies andso competition is fierce. The result is that manufacturing efficiency is being pushed to

the limit and reasonable profits can only be obtained by selling huge quantities of

polymer.

It is believed by some that no new commodity engineering polymers will emerge. The

future of polymer science would seem to be in tuning existing polymers by learning to


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control molecular weight and distribution, tacticity and microstructure in order to obtain

superior chemical and physical properties. The other main avenue to be explored is in

the development of speciality polymers such as liquid-crystalline polymers and

electrically conductive polymers. These materials fit into a low-volume, niche market

of high value-added polymers with exceptional properties. Conducting polymers suffer

from limited processability and there has been a substantial world-wide research effort

to overcome this drawback. The work described in this thesis is a continuation of the

effort to produce processable conducting polymers.

1.2 WHAT IS A CONDUCTINGPOLYMER ?The common feature of all conducting polymers is an extended system along the

backbone, this confers the possibility of electron movement along the chain, i.e.

conduction.

In order for these polymers to exhibit electrical conductivity it is necessary for them to

be doped (Figure 1.2.11). This doping process corresponds to the oxidation or reduction

of the polymer, with the concomitant incorporation of a counterion in order to preserve

overall electrical neutrality. It is the charge introduced onto the polymer backbone

which imparts electrical conductivity. The change in the delocalised system induced

by doping not only increases the conductivity by many orders of magnitude (typically by

a factor of 1010) but also creates substantial alterations in the visible absorption

spectrum of conducting polymers. It is these characteristics which give conducting

polymers great potential in a number of applications.


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Figure 1.2.1 The Chemical Structure of Polyaniline

1.3PROPERTIES,APPLICATIONS ANDLIMITATIONSIn order to understand the potential of conducting polymers it is important to recognise

some of the principal properties commonly exhibited by them:

They posses stiff, conjugated backbones (in their doped, conductive form)

They can act as electronic switches by suitable changes in their level of oxidation or

pH

They may display changes in their optical spectra with oxidation and reduction or

with changes in pH

The conductivity, solubility and other properties of the polymers can often be 'tuned'

by the synthesis of substituted polymers or by the incorporation of different

counterions

Organic conducting polymers are of much lower density than metals

The use of conducting polymers in actual applications has so far been inhibited by the

very nature of the polymers. Polymers with a conjugated backbone generally have an

inherent susceptibility to aerial oxidation, and, in fact, polyaniline is the only known

conducting polymer to show complete resistance to oxidative degradation under ambient

conditions. Appropriate choice of dopant counter-ion can increase polymer stability,

otherwise it is often necessary to exclude oxygen and water if the conductivity of the

2S(alt) Conductive

1S(alt) Insulating

2A(mine) Insulating

1A(mine) InsulatingAcid

Alkali

Acid

Alkali

Reduction Oxidation OxidationReduction

+

+NH NH NH2 NH

N N NNH


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polymer is to remain constant during use. An even greater obstacle to the use of these

polymers is their lack of processability. Conductive polymers posses stiff backbones,

such polymers tend not to melt or dissolve easily, this behaviour can be attributed to

three main factors. Firstly, these linear or planar polymers can pack efficiently, this

maximises the interchain forces and so they can exhibit significant crystallinity2. These

strong forces must be weakened for melting to take place and this requires high

temperatures, often above the temperature at which the polymer degrades. For

dissolution to occur a very strong interaction between polymer and solvent is essential if

the interchain forces are to be overcome. Secondly, the doped forms of conducting

polymers are charged and this leads to electrostatic interactions between chains which

are much stronger than the ubiquitous van der Waals interactions present between

chains in conventional polymers. Lastly, there is little entropic gain in the melting or

dissolution of macromolecules with stiff chains because these processes do not produce

a large increase in conformational freedom (S). As well as these factors cross-linking

renders some of these polymers intrinsically insoluble (e.g. polypyrrole). The strategies

used to overcome these problems are described in the next section.

Some of the potential and actual uses for conducting polymers are listed below. Actual

uses include rechargeable batteries, lightweight capacitors and anti-static cloth3,4.

Potential uses are for electromagnetic shielding5-7, electrochromic displays8,

gas/chemical sensors1, light emitting diodes1, microwave welding9, and

immunodiagnostics.

1.4IMPROVING THEPROCESSABILITY OFCONDUCTINGPOLYMERSIt has been recognised for some time that it is important to improve the processability of

OCPS. Extensive research and ingenuity have been applied to the problem, resulting inseveral approaches to its solution.

The synthesis and polymerisation of substituted monomers to give soluble,

processable conducting polymer derivatives10,11, e.g. the polymerisation of 4-amino

diphenylamine sulfonate12,13 described in Chapter 7.


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Post-polymerisation derivatisation to impart solubility, e.g. sulfonation of

polyaniline14.

Recently surfactants have been used as dopants for polyaniline, these act as spacers

between chains and impart solubility in common organic solvents15.

The methods above all rely on the disruption of chain packing by spacer groups (usually

alkyl chains), or increasing the polymers interaction with solvent by the introduction of

functional groups, typically SO3-.

The preparation of conducting polymer incorporated within a matrix of a non-

conducting, processable material7,16,17, e.g. polyaniline in nylon18 or NBR19,20.

Polymerisation onto a substrate, such as cloth21,22, printed circuit boards23 or metal.

The thermal or chemical conversion of a non-conductive, processable, precursor

polymer into a conducting polymer, e.g. the Durham route to polyacetylene by

Edwards and Feast24.

The synthesis of sterically- or electrostatically-stabilised colloids by dispersionpolymerisation using a polymeric surfactant, this has been accomplished for

polyacetylene25, polypyrrole26-31and polyaniline32-35.


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1.5 THISWORKTwo of the most studied organic conducting polymers (OCPs) are polyaniline36,37 and

polypyrrole. They can be easily prepared either chemically or electrochemically andexhibit good environmental stability. Polyaniline is the only known completely stable

OCP under ambient conditions but the polypyrrole synthesis can be tuned to provide

reasonable stability to aerial oxidation. Another advantage offered by these two

polymers is that they are amenable to most of the approaches described above to

improve the processability of OCPs. The focus of this work was to create new forms of

processed polyaniline and polypyrrole with the aim of accessing new properties such as

colloid particle sizes and morphologies or new electrochromic responses. In this thesis

we describe for the first time:

The dispersion polymerisation of aniline and pyrrole using anionic, small molecule

surfactants to give a new type of conducting polymer colloid simultaneously.

The use of 1H NMR to study of the polymerisation kinetics of polyaniline and

polypyrrole, with and without added surfactant. NMR reveals the rate of polymer

formation, proton liberation and of surfactant adsorption.

Tailor-made steric-stabilisers containing pendant aniline moieties have been

synthesised and characterised, these were used in the first preparation of polyaniline

colloids in non-aqueous media. We also report the synthesis of good quality (i.e.

high conductivity) polyaniline using acetonitrile, propionitrile and butyronitrile as

solvents and copper (II) perchlorate as oxidant. The latter two solvents have not been

reported previously as being suitable for polyaniline synthesis.

A new water-soluble polyaniline derivative, poly(diphenylamine-4-sulfonate) hasbeen synthesised and characterised12,13.

In the following sections some of the literature on polyaniline and polypyrrole is

reviewed in order to set the context for the research described later in the thesis. The

literature on polypyrrole and polyaniline is vast; it is the aim of this introduction to give

a summary of the papers most pertinent to the studies described herein.


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1.6POLYANILINEOVERVIEWPolyaniline has been known for over 100 years38,39 when it was known as aniline black.

At that time its electrical conductivity was not recognised, but the last 20 years have

seen a large number of publications aimed at characterising and understandingpolyaniline. Its major drawback, as with other organic conducting polymers, is its poor

processability, although it does have the significant advantage of being the only

completely air-stable conducting polymer.

1.6.1 General SynthesisPolyaniline is prepared by the oxidation of aniline in solution to give the polymer

exhibiting a fibrillar microstructure. The oxidation can be achieved either chemically40-

47 or electrochemically48-52 to give similar products. Some of the properties of

polyaniline are summarised below.

15 g cm-3 depending upon the dopant anion

Molecular Weight - 15,000 to 40,000 Daltons49,53,54

Crystallinity 50 % in the base form55

Melting Point - Decomposes without melting at 150-250C (depending on the dopant).

Appearance - dark green film (electrochemical) or powder (chemical synthesis)

1.6.2Electrochemical SynthesisThe electrochemical synthesis of polyaniline is carried out by the anodic oxidation of

aniline in solution, typically using platinum electrodes. Aqueous acids (HCl or H2SO4)

are usually used as the solvent in which case the anion (Cl - or SO42-respectively) is

incorporated into the polyaniline as the dopant counterion. The other common solvent

is acetonitrile containing traces of water plus an added electrolyte to provide BF4-, ClO4-

or tosylate48 as the dopant anions. The applied potential is either held at a constant

value within the range 07-12 V or is cycled between -02 V and 07 to 12 V. The latter

method is reported to afford a more ordered product. The polyaniline is usually formed

as a thin film on the anode. Thicker, self-supporting films can also be produced which

enables determination of the mechanical properties of the polymer.


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The electrochemical method has the advantage that the electrolyte (and hence the dopant

species) can be selected and also that the applied potential can be altered in order to

provide polyanilines with a spectrum of physical and chemical properties.

Polymerisation onto an ITO (indium tin-oxide) glass anode allows in-situ studies on

polyaniline by UV-VIS51,56 and FTIR spectroscopies. Furthermore the oxidation level

of the polymer can easily be altered in these studies to provide clues as to the chemical

structure of the polymer under different redox conditions.

1.6.3 Chemical PolymerisationThe most common chemical synthesis of polyaniline is the oxidation of aniline in 1 M

HCl at 0-25C by Na2S2O841,43. In contrast to free-radical polymerisations, the sodium

persulfate acts as an oxidising agent and not as an initiator. It has been shown that the

persulfate reacts in the ratio of 125 moles : 1 mole of aniline. Other oxidising agents

such as KIO3 or K2Cr2O7 can also be used41. Irrespective of the oxidant employed the

number of electrons transferred per polymerised aniline unit is approximately 25. This

is the same value observed for the electrochemical synthesis and suggests that a similar

polymerisation mechanism is operating in both cases.

It has been suggested that the use of such strong oxidising agents may result in defects

such as cross-links. This is supported by the observation that the dedoped (base-treated)

polymer is never completely soluble irrespective of the chosen solvent. A different

polymerisation system has been employed by Inoue, Navarro and Inoue57, who claim

that their milder oxidative conditions yield a doped, conductive polymer which is mostly

soluble in DMSO and becomes completely soluble in DMSO, ethanol and chloroform

once it is dedoped. Their polymerisation method utilises Cu(ClO4)26H2O as the

oxidant and acetonitrile as the solvent. Copper (II) is usually a relatively weak oxidisingagent due to the instability of the copper (I) oxidation state. However, copper (I) forms

a stable complex with acetonitrile and this enhances the oxidative power of copper (II)

which in turn facilitates polymerisation58.


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During the course of my PhD studies we found that it is also possible to use

propionitrile and butyronitrile as polymerisation solvent with Cu(ClO4)26H2O as the

oxidant to produce polyaniline of comparable conductivity ~5 S cm-1 (see Chapter 6).

1.6.4Polymerisation MechanismThe mechanism of aniline polymerisation has not yet been conclusively established.

This is largely due to the complexity of the reaction, the intractability of the polymeric

products and because the products are chemically altered by changes in pH or oxidation

conditions.

In order to optimise the conditions of a reaction and to control the products

characteristics it is always desirable to understand the reaction mechanism. To this end

several groups have proposed tentative reaction schemes. It is generally accepted that

the mechanism is sufficiently similar for the chemical and the electrochemical syntheses

that observations from both cases can be combined to determine the reaction pathway.

The principal factors which must be considered when postulating a plausible mechanism

include:

1. The anilinium radical cation has been detected by cyclic voltammetry, visible

absorption and IR spectroscopies during the polymerisation44.

2. Polymerisation proceeds only at applied potentials equal to or higher than the

oxidation potential of aniline44 (~07 V).

3. The oxidation potentials of aniline dimer, trimer and subsequent oligomers are

progressively lower than that of aniline monomer. Therefore the polymer is always

formed in the oxidised state59.

4. The addition of a small amount (2-10 weight %) of aniline dimer (or of polyaniline)

to the pre-polymerisation mixture results in an increased polymerisation rate60. It

also enables the polymerisation to proceed even at applied potentials as low as 04 V,

which is well below the oxidation potential of aniline.

5.Neither the dimer61 (4-aminodiphenylamine) nor the tetramer44 couple to give

polyaniline under the conditions used for aniline polymerisation.


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6. Cyclic voltammetry has shown that aniline dimer is formed early in the

polymerisation and a steady-state concentration remains until >95% of the monomer

has been consumed44.

7. The polymerisation is first-order with respect both to aniline concentration and to the

amount of polymer formed44.

8. The polymerisation kinetics often exhibit an induction period followed by an

autocatalytic acceleration in the consumption of monomer62.

The mechanism is not obvious from these observations. However, one widely quoted

mechanism is that of Watanabe et al.. These workers proposed that polymerisation

begins with the formation of anilinium radical cation and then proceeds via successive

additions of the radical cation to the end of the (oxidised) growing chain. It is clear that

this cannot be the major polymerisation route because it is inconsistent with points 4

and 8 above. Formation of anilinium radical cation has been shown to be a very slow

step in the polymerisation (either at or before the rate-determining step), if this were the

adding species the polymerisation would remain slow and would not be expected to

display autocatalysis. Significantly, if dimer is added, then polymerisation can take

place using oxidation potentials lower than that of aniline. Under these conditions no

anilinium radical cation can be formed and so it seems unlikely that the mechanism

proposed by Watanabe is the main polymerisation mechanism.

According to Wei, the principal polymerisation route corresponds to the addition of

neutral aniline to the growing polymer chain. This is the more widely accepted

mechanism of the two. Although attributed to Wei, it should be noted that this

mechanism based on addition of neutral aniline monomer was actually first suggested by

the referee of a paper by Mohilner63et al. in 1962. This mechanism is consistent withall of the experimental data except that it fails to account for the consumption of dimer

in the latter stages of polymerisation (point 6). Wei et al. have therefore suggested that

coupling reactions begin to prevail when the monomer has been consumed. GPC

analysis of polyaniline shows two distinct peaks, which is consistent with the suggestion

that two competing polymerisation mechanisms may be at work. Admittedly, point 5


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states that dimer cannot couple with other dimer molecules but this point is contested in

the above paper by Mohlineret al.63.

Figure 1.6.1 Aniline Polymerisation Mechanism, Wei et al.44.

Very recently, Gospodinova64et al. concluded that polyaniline can be easily oxidised by

traces of oxidant up to pH 11 but can only reduced at low pH. They also noted the fact

that polymerisation, like reduction only occurs at low pH. Combining these

observations they proposed a polymerisation mechanism adapted from that of Wei et al.

It involves the formation of dimer which is first oxidised and then subsequently reduced

by the addition of neutral aniline in the propagation step. This oxidation-reduction

sequence is repeated until either oxidant or monomer is completely consumed. More

work is required to establish the polymerisation mechanism unambiguously.

1.6.5Redox and pH BehaviourPolyaniline can exist in four main forms which can be interconverted depending on pH

and degree of oxidation, these are shown in Figure 1.6.2 as described by MacDiarmids

Oxidized

-H+

-H+

Polyaniline

-2H+

-2e-

Pathway 2Reaction

+

+

ReactionPathway 1

+-H ++

+

-2e-

+-2H

-

-2eNH2 NH NH2

NH NH NH NH2

NH NH

NH2

N H N H

NH

NH2

NH NH N H2

NH2

NH NH NH NH2


24/160


group. During interconversion of these forms the delocalised system in the backbone

is altered leading to the interesting electrochromic and solvatochromic responses of the

polymer.

In order to assign the UV-visible absorption peaks of polyaniline Cushman51,56 et al.

studied aniline oligomers under different conditions of pH and oxidation potential. The

study confirmed the structures proposed by MacDiarmid et al. in Figure 1.6.2.

Figure 1.6.2 Redox and pH Transitions of Polyaniline65

Interestingly, the intensity of the 420 nm peak and the conductivity of the polymer werefound to be proportional to each other for changes in applied potential and pH and so

this peak was assigned to the delocalised radical cations which are known to be

responsible for conductivity. Polyaniline is the only conducting polymer which is an

insulator when fully oxidised or fully reduced, it has been suggested that this unique

two-way switching may prove useful in some applications.

Table 1.6.1 The UV-Visible Spectrum of Polyaniline from Cushman et al.

Applied Potential (V) Redox State of Polymer Absorption maximum(nm)

-034 Fully Reduced, insulating 310

+016 Partially oxidised, conducting 310, 420 and 800

+036 Fully Oxidised, insulating 310 and 580

2S(alt) Conductive

1S(alt) Insulating

2A(mine) Insulating


Alkali

Acid

Alkali


+

+NH NH NH2 NH

N N NNH


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When grown on ITO glass, polyaniline shows some potential for electrochromic display

applications. Initial studies by Lacroix8et al. have shown that reversible colour changes

occur for up to 106 oxidation-reduction cycles and that switching times as low as 100 S

are possible; they claim that this response is faster than for any other conducting

polymer film.

1.6.6Molecular WeightIt is not trivial to determine the molecular weight of polyaniline due to its insolubility in

common organic solvents. One method used involves the measurement of the intrinsic

viscosity of the polymer dissolved in concentrated sulfuric acid66, this produces a value

of 12, 000-40, 000 Da. but the value given by this method is not an absolute molecular

mass. GPC has also been employed to determine the molecular weight distribution of

dedoped polyaniline dissolved in either THF, DMF or NMP. The reported results are

different depending on the solvent as shown below in Table 1.6.2.

Table 1.6.2 Molecular Weight Distribution by Tang et al.53

Peak 1 Peak 2

Solvent Peak Area % Peak Area %

THF 2,800 100 ------- 0

DMF 2,200 76 170,000 24

NMP 4,800 67 200,000 33

(All GPC results are quoted versus polystyrene standards.)

MacDiarmid et al.67 reported that as-synthesised emeraldine base dissolved in NMP

containing 5 wt. % LiCl exhibits a symmetrical, monomodal GPC peak, in contrast to

the results of Wei et al. who observed two separate peaks. MacDiarmid et al. reported

and to be 26,000 and 78,000 respectively relative to polystyrene

standards. However, they stated that the obtained from light-scattering indicates

that the GPC results were too high by a factor of ~2. One likely explanation for the

discrepancy is that the excluded volume of a given molecular weight polyaniline chain


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is much higher than for polystyrene chains which are more flexible. When the synthesis

was performed at low temperatures (-5C) it was found that the resultant polyaniline had

a higher molecular weight ( = 23,000), compared to the product obtained at 25C

( = 14,000). This observation was attributed to a decreased tendency towards

side-reactions at lower temperatures, although these results are also consistent with an

increase in cross-linking at low temperatures.

1.6.7 Thermal StabilityThe thermal stability of polyaniline is strongly dependent on the nature of the dopant

anion40,48,68. Neoh et al.69 have studied the thermal degradation of polyaniline using

TGA, XPS and conductivity measurements; their results are summarised below. The

polyaniline was prepared by the chemical route using (NH4)2S2O8 oxidant in aqueous

HCl and was studied both in the doped and dedoped (base treated) forms.

1.6.7.1 Chloride Doped PolyanilineA 10% reversible weight decrease up to 100C, attributed to water loss, was observed

either in air or nitrogen. The next weight loss occurred between 225 and 350C, this

corresponds to the loss of dopant, XPS was used to show that some chlorine becomes

covalently bound to the aromatic rings during this stage. Above 350C polyaniline

degrades rapidly in air but retains 65% of its original weight under a nitrogen

atmosphere.

1.6.7.2Dedoped PolyanilineDedoped polyaniline is much more stable to thermal degradation that its doped

counterpart. It exhibits little weight loss up to 400C and retains 75% of its weight at

700C.

1.6.7.3 Conductivity Versus TemperatureThe conductivity of polyaniline increases reversibly by ~10 % upon heating to 60C and

then drops rapidly as it is further heated to 100C. The latter change is also reversible if

the polymer is cooled and put into contact with water vapour, it has therefore been


27/160


attributed to loss of water. Above 100C, XPS was used to show that the Cl- dopant

attacks the polymer chain, this causes an irreversible decrease in conductivity.

1.6.8 Chemical and Physical Structure

1.6.8.1 Chemical StructureThe chemical structure of polyaniline has been investigated by several techniques

including FTIR70,71, 13C NMR72, XPS73,74 and UV-visible spectroscopy. After this

intensive study the structure of polyaniline is now generally accepted to be as depicted

in Fig. 1.6.3.

Figure 1.6.3 The Chemical Structure of Polyaniline

2S(alt) Conductive

1S(alt) Insulating

2A(mine) Insulating


Alkali

Acid

Alkali


+

+NH NH NH2 NH

N N NNH

Protonation of emeraldine base (2A) results in the formation of a delocalised

polysemiquinone radical cation. The protonation results in an increase in the

conductivity of the material by approximately ten orders of magnitude to 5 S cm-1.

1.6.8.2Physical StructureElectrochemically grown polyaniline films usually exhibit a fibrillar morphology, the

fibrillar size and shape varies noticeably with polymerisation conditions such as current

density. The morphology is considered to originate from the way in which the polymer


28/160


forms, starting from nucleation sites on the electrode and then continuing outwards from

the electrode. The fibrils are created because aniline polymerises preferentially on

polyaniline rather than on the electrode. A granular morphology has been obtained by

Michaelson et al.75 who added surfactants to the electrochemical polymerisation. The

aniline monomer diffuses preferentially into the surfactant micelles from the aqueous

phase, they suggested that this alters the nucleation process on the electrode, thus

causing a change in polymer morphology.


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1.7POLYPYRROLEOVERVIEWPolypyrrole was first synthesised in 1916 using acidified hydrogen peroxide oxidant to

polymerise pyrrole yielding a product which became known as pyrrole black.Polypyrrole is believed to be a cross-linked polymer because it is insoluble in every

solvent tested to date. As a consequence, its exact chemical structure is still a matter of

debate. The cross-linking also means that the polymer does not melt and therefore has

very poor processability. In spite of these unfavourable properties the polymer has

attracted world-wide attention, much of which has been directed towards overcoming

the processability problem. The interested reader is directed to reviews on the subject of

polypyrrole1,76,77.

1.7.1 General SynthesisPyrrole (like aniline) can be polymerised by oxidation in solution, either chemically78 or

electrochemically79,80. Water is the most commonly used polymerisation solvent but

organic solvents such as aliphatic alcohols, ethers and esters also provide high

conductivity polymer. A summary of some of the physical properties of the product is

given below.

= 148 g cm-3,81

Molecular Weight - due to cross-linking

Crystallinity - low Mpt - none, decomposes without melting

Tg - unknown Conductivity = 1-100 S cm-1

Appearance - Black solid film (electrochemical) or powder (chemical synthesis)

1.7.2Electrochemical SynthesisThe polymerisation of pyrrole can be achieved by the anodic oxidation of the monomer

in solution with an added electrolyte such as Bu4NCl. An electrode potential of 06 V is

typically applied and this results in the deposition of a black, conductive film of polymer

on the anode. The electrolyte anion is incorporated into the conducting polymer (this

would be Cl- in the example given above). Electropolymerisation onto ITO (indium tin


30/160


oxide) glass has been useful for monitoring the properties of polypyrrole in situ at

different oxidation potentials.

The dopant counterion can be chosen in order to affect the properties of the resultant

polypyrrole film82,83, for example, tosylate anions increase both the polymers

conductivity and the retention of conductivity upon exposure of the polypyrrole to air.

Recently, several papers have focused on the use of surfactants such as sodium

dodecylsulfate84-87 as dopants, this imparts improved mechanical properties88, good

conductivity (100 S cm-1) and enhanced retention of conductivity.

1.7.3 Chemical SynthesisPolypyrrole can be prepared easily in various media such as H 2O, Et2O, EtOAc, EtOH

and MeCN using an oxidising agent which is typically FeCl3. Other reported oxidants

include Na2S2O8, H2O2 and I2, the anions from the oxidant usually provide the dopant

for the polymer. However, it is also possible to add an electrolyte as in the

electrochemical synthesis. The chemical polymerisation is a precipitation

polymerisation which forms a black, intractable powder or flakes in the reaction vessel.

This solid product is usually washed with fresh solvent and dried in vacuo. Pelletisation

enables determination of the conductivity which is normally ~10 S cm-1. As with the

electrochemical synthesis, addition of tosylate type89 ions before polymerisation gives

polypyrrole with higher conductivity (~35 S cm-1). It has been suggested that the

advantages attained by the use of tosylate dopant are associated with the increased order

that it induces in the polymer. This order would be expected to produce better

conductivity and increased resistance to aerial oxidation because oxygen diffusion into

crystalline regions of a material should be slower than that into amorphous regions.

The chemical synthesis of polypyrrole has been tuned by Miyata et al.90,91

to producemilder reaction conditions. Their approach was to use FeCl3 as the oxidant and to

change the oxidation potential of the reaction solution either by using solvent mixtures

(of methanol and acetonitrile) or by addition of FeCl2 as a moderator. Both approaches

resulted in highly conductive polypyrrole (>100 S cm-1). The use of low temperatures

(0C), very short reaction times (10 mins) and a solvent mixture (86 vol. % MeCN and


31/160


14 vol. % MeOH) lead to polypyrrole with a relatively high degree of order and

exceptionally high conductivity (>300 S cm-1).

1.7.4 Polymerisation Mechanism

1.7.4.1KineticsThe kinetics of the chemical polymerisation of pyrrole have been monitored by the

quenching of reaction aliquots followed by determination of the amount of

unpolymerised pyrrole by HPLC. This revealed the polymerisation to be first order with

respect to pyrrole and oxidant (FeCl3). Interestingly, the reaction was found to be

sensitive to pH. The polymerisation is autocatalytic if the initial pH value before

monomer addition is above 13. When the initial pH is greater than 13 the protons

eliminated from the pyrrole during the polymerisation lower the pH significantly and

thereby increase the rate of reaction. Whereas for an initial pH above 13 the acidity is

sufficient to lessen the effect of the eliminated pyrrole protons. The dispersion

polymerisation of pyrrole to give a colloid has also been studied. Visible absorption

spectroscopy was performed on the reaction mixture using the polypyrrole peak at 800

nm to determine the amount of polymer formed at a given reaction time. This study

confirmed that the reaction is first order with respect to monomer and oxidant for initialpHs between 00 and 05.

1.7.4.2MechanismThe polymerisation of pyrrole is thought to occurvia a similar mechanism for both the

chemical and the electrochemical syntheses. In both cases 225-233 electrons are

removed per pyrrole molecule polymerised92. This results in 025-033 positive charges

per pyrrole ring and necessitates the incorporation of enough dopant anions to preserve

overall charge neutrality. The radical cation mechanism is very widely quoted,93

although far from proven. The lack of proof for this mechanism is due to the difficulty

of studying such a fast reaction where the product is insoluble and thus difficult to

analyse.


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Polymerisation only occurs at oxidation potentials equal to, or higher than, that of

pyrrole; it is therefore believed that the radical cation is a reaction intermediate. The

rate limiting step is reported to be the oxidation of pyrrole monomer to its radical cation,

which is followed by the 2,5 coupling of two such radicals and the concomitant

elimination of two protons. As the oxidation potentials of pyrrole dimer, trimer and

higher oligomers are progressively lower than that of pyrrole, the polymer is always

formed in the doped oxidised state.

1.7.5Redox and pH BehaviourAs previously stated, polypyrrole is always in the oxidised, conducting state when it is

formed due to the polymerisation conditions. The electrical conduction could, in theory

occurvia the formation of polarons (charge carriers with spin due to unpaired electrons),

bipolarons (spinless due to electron pairing) or a mixture of both. These two species are

illustrated in Fig. 1.7.1. It has been observed by Scott et al.94 using ESR spectroscopy

that at low dopant levels polarons are formed but at higher levels bipolaron formation is

favoured. They also found that polypyrrole can be conductive even when no ESR signal

is detected thus indicating the presence of bipolarons to the exclusion of polarons.

Brdas et al.95 calculated that bipolarons are energetically favoured over polarons and

that the bipolaron length is about four pyrrole rings.

Figure 1.7.1 Charge Movement in Polypyrrole

A Bipolaron

A Polaron

++

+

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H

N

H


33/160


Although polypyrrole is inevitably formed in the oxidised form, it is possible to reduce

the polymer, either chemically or electrochemically, to the neutral form. This addition

of electrons to the delocalised system results in substantial alterations in the UV-

Visible and IR spectra of the material and yields a polymer which is extremely sensitive

to oxidation. The IR spectrum of reduced polypyrrole was originally reported by Street

et al.96. However, it has subsequently been claimed by Martins group97,98 that Streets

spectrum was not in fact representative of the fully reduced polymer due to oxidation

and doping of the polypyrrole by traces of oxygen. These latter workers constructed a

special electrochemical cell which enabled them to prevent the reduced polymer from

undergoing any oxidation and also to record IR spectra of truly neutral polypyrrole in

situ. The spectra obtained gave important information about the chemical structure of

polypyrrole (see section 1.7.8). The potential usefulness of polypyrrole as an

electrochromic display material was evaluated by Diaz and Kanazawa77. It was only

feasible to switch very thin films because the reduced form was too insulating to permit

reoxidation of thick films.

1.7.6Molecular WeightThe molecular weight of polypyrrole is effectively infinite as it is believed to be highly

cross-linked. The cross-link induced insolubility of the polymer has precluded the

determination of molecular weight by the conventional methods, namely GPC, viscosity

measurements or colligative methods.

The molecular weight of poly(3,4-dimethyl pyrrole) has however been estimated

radiochemically99. The monomer was tritiated selectively at the two and five positions

and was then polymerised. Most of the tritium was eliminated during polymerisation

leaving tritium only at the chain ends and possibly at defects, thus the ratio of tritium to

polypyrrole rings was estimated to be 1 : 50-500. If it is assumed that tritium was noteliminated from defects then Nazzal and Street estimated the mean conjugation length to

be at least 100-1000 and the lower limit for the mean degree of polymerisation to be

100-1000.


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The radiochemical method described is ingenious but in principle it is also possible (and

far simpler) to titrate the eliminated 2,5 protons against base in order to more accurately

estimate the end-group plus defect concentration.

1.7.7 Thermal StabilityPolypyrrole exhibits good thermal stability up to 150C in air, as measured by TGA,

however it is thermally stable to 250C if tosylate dopant is employed. This means that

it is feasible to co-extrude polypyrrole with conventional thermoplastics such as

polyethylene in order to produce electrically conductive composites.

1.7.8 Chemical and Physical StructureThe exact chemical structure of polypyrrole has not been determined due to the

insolubility of the polymer. Even so a great deal of investigation has revealed most of

the key features. Several techniques have been used to demonstrate that the pyrrole

moiety is retained in the polymer. For example, the IR spectrum of the neutral polymer

is very similar to that of pyrrole dimer and trimer100 and the XPS spectrum of

polypyrrole is similar to that of pyrrole101,102. It has been proved103 that 2-substituted

pyrroles do not polymerise whereas 3, or 3-4 substituted pyrroles do, this strongly

suggests that polymerisation occurs through the 2 and 5 positions. Solid-state 13C

NMR, IR and XPS all support this but there is also evidence to indicate some reaction

via the 3 position, possibly leading to cross-links. Polymerisation of 3, 4 disubstituted

pyrroles would therefore be expected to result in decreased cross-linking and increased

order in the polymer and this has been shown to be the case for 3, 4-dimethyl pyrrole.

Recent work by Lei and Martin98 using IR spectroscopy has revealed that polypyrrole is

actually a copolymer of pyrrole and hydroxypyrrole. Previously the hydroxyl bands in

the IR spectrum had been obscured by a broad absorption from 1600 cm -1 upwards due

to the conductivity of polypyrrole. Reduction of the polymer had failed to completely

eliminate this broad absorption because of partial oxidation by traces of oxygen. Lei,

Liang and Martin97 reported the use of a sealed electrochemical cell which allowed in

situ IR measurements of oxidised and truly neutral polypyrrole to be recorded under an

argon atmosphere. They demonstrated that traces of water in the polymerisation solvent


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resulted in covalently bound hydroxyl groups in the polypyrrole formed. These bands

were not seen for polypyrrole prepared under meticulously dry conditions.

A variety of possible configurations of the pyrrole rings in neutral polypyrrole are

theoretically possible, these include rings, helices and linear chains. It seems most

probable that the preferred conformation is the linear chain structure shown in Fig. 1.7.1

because x-ray diffraction104 shows it to be the conformation adopted by pyrrole dimer

and trimer. However, polythiophene is an analogue of polypyrrole where the ring

nitrogen is replaced by a sulfur and it has been shown that doped polythiophene adopts a

helical conformation.

Electrochemically grown films of polypyrrole with surfactant counterions exhibit

increased order in their structure compared to polypyrrole formed with other dopant

counterions. Wegneret al. 84,85 used a number of alkylsulfates and alkylsulfonates as

dopant anions resulting in good conductivities (5-160 S cm-1) even though the

polypyrrole was typically composed of 50 wt. % surfactant. X-ray diffraction showed

that the product was formed in a layered structure. The repeat unit was of a layer of

polypyrrole then two molecular layers of surfactant, this sequence was repeated in layers

built up parallel to the electrode. Not surprisingly, such films were reported to be

significantly hydrophobic, due to the incorporation of surfactant.


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1.8ANINTRODUCTION TO COLLOIDS1.8.1BackgroundOne attractive route to processable conducting polymers has been the synthesis of

colloidal dispersions from which conductive films can easily be cast. Several groups

have achieved such syntheses by applying dispersion polymerisation techniques to the

chemical preparation of polyacetylene105, polyaniline32-35 and polypyrrole26-31.

For these polymerisations the polymer produced is insoluble in the reaction mixture and

is therefore usually formed as a precipitate. However, the addition of a polymeric

surfactant can prevent macroscopic precipitation and lead to the formation of a stablecolloid. The polymeric surfactant adsorbs onto the growing polymer particles and this

steric layer prevents the close approach of polypyrrole particles. Thus the van der Waals

attractive forces (which would normally cause particle aggregation and ultimately

destabilisation of the colloidal dispersion) are prevented from becoming dominant over

the average kinetic energy of the particles.

The primary purpose of this thesis is to produce processable conducting polymers and

not to study colloidsper se. However it is clearly important to have an understanding of

colloid science when synthesising and characterising such novel dispersions. The

summary given here is based largely on standard surface and colloid science texts106-109.

1.8.2Definition of a ColloidIt is difficult to precisely define the term colloid, such systems consist of a dispersed

phase (solid, liquid or gas) in a continuous phase (solid, liquid or gas). At least one

dimension of the dispersed units is usually within the range 1 nm to 1 m, i.e. too big to

be considered molecularly dispersed and too small to be classed as a macroscopic

material. The small dimensions of the dispersed phase results in a large surface area to

volume ratio and so a large proportion of the molecules are located at the surface. As a

consequence the properties of colloids tend to be dominated by the characteristics of the

surface and not by the bulk properties which usually dictate the behaviour of a material.


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The colloids described in this work are solid particles dispersed in liquid and therefore

the following information will be biased toward the understanding of such dispersions,

rather that a treatment of the vast subject of colloids in general. To this end a brief

description of the physical aspects and energetics of dispersions is given in order to

place this work in context.

1.8.3Particle Morphology and SizeThe rheological and other properties of a colloid are related to the shape and size of its

constituent particles. These particles can be formed either by condensation of smaller

units (atoms or molecules) or by comminution (breaking apart) of larger particles.

These two different approaches allow for a wide range of particle morphologies to be

accessed. The particles can generally be considered to be in the colloidal domain if one

or more of their dimensions is within the range 1 - 1000 nm (this is not however an

absolute limit and some colloids are not within this size range).

Due to the mechanisms of colloid formation it is usual that the particles are of varying

size and so colloids are best described in terms of a particle size distribution. A colloid

with a very narrow size distribution (Dw/Dn 12) is said to be monodispersed and one

with a wider range of particle sizes (Dw/Dn > 12) is termed polydispersed. There are a

host of techniques available for determination of colloid size distributions, many of

which are also applicable to measurements of polymer size. These techniques include

TEM, SEM, light-scattering and centrifugation although these often measure different

types of size average.


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1.8.4 Colloid StabilityIn order to utilise colloids it is necessary to control the stability of the dispersion, this

section will define what is meant by stability in the colloidal sense and also briefly

discuss the factors which influence it.

1.8.4.1 CollisionsIn a colloidal dispersion, interparticle collisions occur frequently due to Brownian

motion and such collisions provide an opportunity for the colliding particles to

aggregate. If the dispersion is to maintain its integrity there must be no net tendency for

the discrete particles to agglomerate. Colloidal stability results when there is some net

repulsive force between particles which prevents agglomeration.

1.8.4.2Energetics of ApproachThe tendency of particles to associate and thereby destabilise the dispersion will be

determined by two factors:

The balance of the attractive and repulsive forces acting on the particles

The collision frequency

Two uncharged molecules in a vacuum will fall into a deep primary energy minimum

where the attractive London Dispersive forces are in equilibrium with the Born

repulsion force which is very strong at short distances. For particles it is found that the

attractive forces fall off much more slowly with distance than they do in the molecular

case. Another difference is that the attraction between two particles is lessened by the

presence of a dispersion medium. Thus, the dispersion medium itself aids colloidal

stability of the dispersion.

Clearly, uncharged particles have a strong tendency toward agglomeration, the attractive

forces are strong at short distances but fall off quite rapidly with increased separation.

The key to preventing aggregation is to introduce some long range force which can

provide overall interparticle repulsion at intermediate distances and thereby stop

particles from approaching close enough to fall into the deep primary energy minimum.

There are two methods commonly used to provide interparticle repulsion:


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Charge Stabilisation - A charged double layer around like, charged particles can

create a repulsive interparticle force.

Steric Stabilisation - A layer of adsorbed material (usually polymeric) prevents

close interparticle approach

These methods are not mutually exclusive and can be combined to produce electrosteric

stabilisation. By understanding the criteria for effective dispersion stabilisation it

becomes possible to design colloidal systems with good long-term stability.

1.8.5 Charge StabilisationParticles can develop a charged surface by various means including:

Ionisation

Ion adsorption

Ion dissolution

Figure 1.8.1 The Concentration of Ions Near to A Charged Surface

Clearly, a charged particle in an aqueous environment will attract counterions of

opposite charge. There will therefore be an increased concentration of these counterions

near to the particle surface and (to a lesser extent) a decreased concentration of ions with

the same charge as the particle (Fig. 1.8.1). Schulze and Hardy found that

Distance from the Surface

Ion Concentration

Co-ions (same charge as the surface)

Counter-ions (of opposite charge to the surface)

Ion concentration in the bulk


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electrostatically stabilised colloids are very sensitive to added electrolyte110. The reason

is that added electrolyte has the effect of screening the charges on different particles

from each other. The effect is that the Double-Layer repulsion between charged

particles is reduced by increased ionic strength and so there is a lower energy barrier to

particle approach and aggregation.

The stability of a charge stabilised dispersion will depend upon the magnitude of the

repulsion between approaching particles. The surface of shear represents the effective

particle surface and so it is this potential which determines the interparticle repulsions

and dispersion stability. This potential, the electrokinetic or (zeta) potential can be

determined experimentally and is significantly different from the potential o at the

actual particle surface. Adsorption of polyvalent or surface active ions can give a zetapotential of opposite sign to the potential of the particle surface o. For example a

negatively charged surfactant could adsorb at the surface of a positively charged particle

in water. At low surfactant concentrations the negative head group could adsorb on the

surface to neutralise the positive charge and leave the hydrophobic tails facing the water.

Further added surfactant may then adsorb tail first onto the now hydrophobic particle to

form a surfactant bilayer structure. The net charge on the particle has thus changed from

positive to negative.

The zeta potential can be measured experimentally (by electrokinetic methods such as

electrophoresis) and so the likely stability of a dispersion can be predicted for different

conditions of pH, ionic strength etc.. Commonly it is found that particles with an energy

barrier to aggregation about 10 kT will exhibit long term metastability because a

typical particle collision will be insufficiently energetic to overcome the repulsive

energy barrier.

1.8.6

The Deryagin-Landau-Verwey-Overbeek (DLVO) Theory

The overall (kinetic) stability of a dispersion to aggregation is dependent on the balance

of the van der Waals attractive

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Chris DeArmitt PhD Thesis