INTERACTIONS OF VIOLOGENS WITH CONDUCTING POLYMERS, METAL SALT SOLUTIONS AND GLUCOSE OXIDASE
ZHAO LUPING
NATIONAL UNIVERSITY OF SINGAPORE
2004
INTERACTIONS OF VIOLOGENS WITH CONDUCTING POLYMERS, METAL SALT SOLUTIONS AND GLUCOSE OXIDASE
ZHAO LUPING (B. Eng, Qingdao Institute of Chemical Technology)
A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004
Acknowledgement
ACKNOWLEDGEMENT
First and foremost, I would like to express my deepest gratitude to my supervisors,
Professor Neoh Koon Gee and Professor Kang En-Tang for their inspired guidance, invaluable advice, and constant supervision throughout the length of my candidature. I gratefully acknowledge the research scholarship offered to me by the National
University of Singapore (NUS), which enabled me to pursue my Ph.D. program.
Special thanks are due to Dr. Zhang Yan and Dr. Li Sheng. They have been most helpful to me, both technically and personally. I enjoyed and benefited tremendously from the discussion on my project and their invaluable suggestions and advices. I would like to thank our group members like Dr. Lin Qidan, Mr. Ying Lei, Miss Cen
Lian for sharing the research experience with me. Particular acknowledgements go to
Madam Chow Pek and other lab technologists of Department of Chemical and
Environmental Engineering for their assistance and help.
Last but not least, I must express my deepest love and gratefulness to my family for their long-term concern and support. In addition, special thanks to my wife, Huang
Yuehong, for her persistent love and encouragement.
Zhao Luping
i Table of Contents
TABLE OF CONTENTS
ACKNOWLEDGEMENT i
TABLE OF CONTENTS ii
SUMMARY vi
NOMENCLATURE ix
LIST OF FIGURES xi
LIST OF TABLES xvi
CHAPTER 1 INTRODUCTION 1
CHAPTER 2 LITERATURE SURVEY 7
2.1 Viologens
2.1.1 Synthesis of Viologens
2.1.2 Physical Properties of Viologens
2.1.2.1 Dimerization of Viologen Radical Cations
2.1.2.2 Association and Charge Transfer
2.1.2.3 Solid State Conductivity
2.1.2.4 Radical Solubility
2.1.3 Photochromism of Viologens
2.1.4 Applications of Viologen Systems
2.1.4.1 Electrochromism and Electrochromic Devices (ECD)
2.1.4.2 Electron Mediation
2.1.4.3 Other Miscellaneous Applications
2.2 Conducting Polymers
2.2.1 Structure and Synthesis
2.2.1.1 Polyaniline
ii
Table of Contents
2.2.1.2 Polypyrrole
2.2.2 Doping of Conducting Polymers
2.2.3 Degradation and Stability
2.2.4 Applications of Conducting Polymers
2.2.4.1 Antistatic Coatings
2.2.4.2 Electromagnetic Shielding
2.2.4.3 Organic Conducting Patterns
2.2.4.4 Optoelectronic Devices
2.2.4.5 Batteries and Solid Electrolytes
2.2.4.6 Sensors
2.2.4.7 ‘Smart’ Structures
2.3 Polymer Surface Modification and Characterization
2.3.1 Surface Grafting
2.3.2 Plasma Modification
2.3.2.1 Plasma Treatment
2.3.2.2 Plasma Polymerization
2.3.3 Surface Characterization
CHAPTER 3 PHOTO-INDUCED REACTION OF POLYANILINE 45 WITH VIOLOGEN IN THE SOLID STATE
3.1 Introduction
3.2 Experimental
3.3 Results and Discussion
3.3.1 LDPE Graft-modified with VBC and Viologen
3.3.2 Photo-induced Doping of PANI Films in EB State
3.3.3 Photo-induced Doping of PANI Films in Other Oxidation States
3.3.4 Stability of Films
iii
Table of Contents
3.3.4.1 Adhesion Tests
3.3.4.2 Stability of Irradiated Films in Air and Water
3.3.4.3 Dedoping Characteristics of EB-viologen Film after Irradiation
3.4 Conclusion
CHAPTER 4 FLUORINATED ETHYLENE PROPYLENE COPOLYMER 87 COATING FOR THE STABILITY ENHANCEMENT OF ELECTROACTIVE AND PHOTOACTIVE SYSTEMS
4.1 Introduction
4.2 Experimental
4.2.1 Preparation of Substrates
4.2.2 Radio Frequency (RF) Sputtering of FEP
4.2.3 Stability Tests
4.2.4 Sample Characterization
4.3 Results and Discussion
4.3.1 PANI-LDPE Film
4.3.2 PANI-viologen Film
4.3.3 Viologen Grafted Film
4.4 Conclusion
CHAPTER 5 NANOSCALED METAL COATINGS AND 110 DISPERSIONS PREPARED USING VIOLOGEN SYSTEMS
5.1 Introduction
5.2 Experimental
5.3 Results and Discussion
5.3.1 Metal Reduction by VBV-LDPE Film
5.3.2 Colloid/Nanosized Metal Particles in PVA-BV Matrix
5.4 Conclusion
iv
Table of Contents
CHAPTER 6 FORMATION OF CONDUCTING PATTERNS 137 USING PANI-VIOLOGEN COMPOSITE FILM AND METALLIZED VIOLOGEN FILM
6.1 Introduction
6.2 Experimental
6.3 Results and Discussion
6.3.1 Photo-irradiated PANI-viologen System
6.3.2 Reaction of VBV-LDPE Patterned Films with Metal Salt Solutions
6.4 Conclusion
CHAPTER 7 CO-IMMOBILIZATION OF ENZYME AND 155 ELECTRON MEDIATOR ON CONDUCTING POLYMER FILM FOR GLUCOSE SENSING
7.1 Introduction
7.2 Experimental
7.3 Results and Discussion
7.3.1 Co-immobilization of GOD and MAV
7.3.2 Effect of Viologen on Enzyme Activity
7.3.3 Electrochemical Characterization of the GOD-MAV-PPY Film
7.4 Conclusion
CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 177
8.1 Conclusions
8.2 Recommendations
REFERENCES 184
APPENDIX PATENT AND PUBLICATIONS 205
v
Summary
SUMMARY
Studies on viologens have led to enormous advances and potential applications which capitalize on the viologens’ reversible redox properties. In this thesis, the interactions of viologens with conducting polymers, metal salt solutions and an enzyme were investigated. The potential applications of viologens resulting from these interactions were also demonstrated.
The conversion of polyaniline (PANI) from the insulating state to the doped and conductive state was accomplished through the photo-induced reaction with viologen in the solid state. Photo-sensitive films consisting of PANI coatings in different oxidative states on viologen-grafted low density polyethylene (LDPE) substrates were employed. The effects of the ultraviolet (UV) irradiation time, grafted vinylbenzyl chloride (VBC) and viologen density, and UV irradiation intensity were discussed. The density of the grafted VBC and viologen does not play an important role in the doping of PANI under UV irradiation since the reactions are confined to the interfacial region between PANI and the grafted moieties. However, the photo-induced doping of PANI shows a strong dependence on the UV intensity. The photo-irradiated films show good electrical stability in air up to 75°C, but undope rapidly in water. The conductivity of the irradiated films decreases sharply after the films were immersed in water due to the loss of the HCl dopants.
To enhance the electrical stability of conducting polymers and prolong the photochromic effect of photoactive materials, a radio frequency sputtering technique to deposit fluorinated ethylene propylene copolymer (FEP) coatings of controllable thickness on these materials was employed. This technique can be applied to both
vi Summary conventional acid protonated PANI film and PANI doped via photo-induced reaction with viologen. Both systems with the FEP coating remain conductive even after 3h in water. With a thicker FEP coating, the stability enhancement can also be achieved in basic solutions of pH up to 12. The photochromic effect of viologen grafted films with the sputtered FEP coating was also prolonged since the sputtered FEP coating retards the diffusion of O2 to the photo-generated viologen radical cations.
Nanoscaled metal coatings on the surface of 1,1’-bis(4-vinylbenzyl)-4,4’-bipyridilium dichloride (VBV) grafted LDPE films were successfully achieved via the photo- induced reactions between the viologen and noble metal salt solutions. The distribution of gold or platinum in the elemental and ionic state on the VBV-LDPE films is dependent on the UV irradiation time and the concentration of the metal salt solutions used. Well-dispersed gold and platinum particles ranging from 10 nm can also be readily obtained via the reduction of the corresponding salt solution in a poly(vinyl alcohol) (PVA) matrix containing benzyl viologen (BV). Conducting patterns can be generated from the UV irradiation of the PANI-viologen composite film through a mask, and via gold deposition on VBV patterns grafted on LDPE film.
In this work, it was also demonstrated that viologen can serve as an effective mediator for electron transfer from the active sites of glucose oxidase (GOD) to the surface of a polypyrrole (PPY) electrode under UV irradiation and in the absence of oxygen. The amounts of GOD and N-methyl-N’-(3-aminopropyl)-4,4’-bipyridilium (MAV) immobilized on the PPY film could be controlled by changing the graft concentration of the linkage group, acrylic acid (AAc), on the PPY film and the ratio of GOD to
MAV in the co-immobilization step. The electrochemical response of the as-
vii Summary functionalized enzyme electrode changes linearly in the range of 0 to 1.0 mM of glucose in solution.
viii Nomenclature
NOMENCLATURE
AAc acrylic acid AAm acrylamide AFM atomic force microscopy BE binding energy BV benzyl viologen CRT cathode ray tubes DMF N-dimethylformamide EB emeraldine base ECD electrochromic devices EDX energy dispersive X-ray spectroscopy EELS electron energy loss spectroscopy EL electroluminescence EM emeraldine ETC electron-transfer catalyst FEP fluorinated ethylene propylene copolymer GOD glucose oxidase HV heptyl viologen LCD liquid crystal displays LDPE low-density polyethylene LED light-emitting devices LM leucoemeraldine MAV N-methyl-N’-(3-aminopropyl)-4,4’-bipyridilium MV methyl viologen NA nigraniline NMP N-methylpyrrolidinone OcV octyl viologen PANI polyaniline PE polyethylene PEEK polyetheretherketone PET poly(ethylene terephthalate) PNA pernigraniline PP polypropylene
ix Nomenclature
PPV poly(p-phenylene vinylene) PPY polypyrrole PTFE polytetrafluoroethylene PTH polythiophene PVA poly(vinyl alcohol) RF radio frequency Rs sheet resistance SEM scanning electron microscopy SIMS secondary ion mass spectroscopy TGA thermogravimetric analysis UV ultraviolet V+• viologen radical cation V2+ viologen dication VBC 4-vinyl benzyl chloride VBV 1,1’-bis(4-vinylbenzyl)-4,4’-bipyridilium dichloride XPS X-ray photoelectron spectroscopy
x List of Figures
LIST OF FIGURES
Figure 2.1 Three common bipyridilium redox states.
Figure 2.2 Dication preparation from quaternized 4,4’-bipyridine.
Figure 2.3 Structure of 1,1’-dialkyl-1,1’4,4’-tetrahydro-4,4’-bipyridilium.
Figure 2.4 An electron-transfer representation from an electrode showing reduction of a biological molecule A by the viologen radical cation generated at the electrode.
Figure 2.5 Chemical structure of viologen with trimethoxysilyl substituents.
Figure 2.6 Functionalized pyrrole with pendant viologen group.
Figure 2.7 Chemical repeat units of several conducting polymers.
Figure 2.8 Octameric structures of polyaniline in various intrinsic redox states.
Figure 2.9 A simple band picture explaining the difference between an insulator, a semiconductor and a metal.
Figure 2.10 The relationship between protonic acid doping and oxidative doping of different forms of polyaniline to the same conducting material.
Figure 3.1 Schematic diagram illustrating the process of Ar plasma treatment, grafting of VBC and viologen, and deposition of the PANI coating on LDPE films.
Figure 3.2 XPS (a) C 1s and (b) Cl 2p core-level spectra of VBC-graft copolymerized LDPE film(Sample VBC-2), (c) N 1s and (d) Cl 2p core-level spectra of viologen-graft modified LDPE film(Sample Vio-2).
Figure 3.3 UV-visible absorption spectra of (a) Sample VBC-2 and (b) Sample Vio-2 before irradiation and after irradiation for 30 min.
Figure 3.4 UV-visible absorption spectra of (a) PANI-VBC film (Film PANI- VBC-2A) and (b) PANI-viologen film (Film PANI-Vio-2A), after irradiation in air for various periods of time.
Figure 3.5 N 1s and Cl 2p core-level spectra of (a, b) PANI-VBC film before irradiation (Film PANI-VBC-2A); (c, d) PANI-VBC film after irradiation in air for 2 hours (Film PANI-VBC-2B); (e, f) PANI- viologen film before irradiation (Film PANI-Vio-2A); (g, h) PANI- viologen film after irradiation in air for 2 hours (Film PANI-Vio-2B).
xi List of Figures
Figure 3.6 Sheet resistance (Rs) of PANI-VBC film (Film PANI-VBC-2A) and PANI-viologen film (Film PANI-Vio-2A) after irradiation in air for various periods of time.
Figure 3.7 Sheet resistance (Rs) of (a) PANI-VBC film (Film PANI-VBC-2A) and (b) PANI-viologen film (Film PANI-Vio-2A) after exposure to irradiation of different intensities in air for various periods of time. A is the degree of attenuation of the irradiation.
Figure 3.8 Comparison of the sheet resistance (Rs) of (a) PANI-VBC film (Films PANI-VBC-1A, 2A) and PANI-viologen film (Films PANI-Vio-1A, 2A), of different VBC graft densities after irradiation in air for various periods of time.
Figure 3.9 XPS N 1s core-level spectrum of NA-viologen film a) before irradiation and b) after 2h of irradiation in air; as well as LM-viologen film c) before irradiation d) after 90 min of irradiation in air.
Figure 3.10 UV-visible absorption spectra of a) NA-viologen film irradiated in air for various periods of time b) LM-viologen film irradiated in air for i) 0 min, ii) 30 min and iii) 90 min; and inset: LM-viologen film irradiated in vacuum.
Figure 3.11 Scotch tape surfaces from peel tests performed on EB-coatings on a) pristine LDPE, b) plasma treated LDPE, c) viologen grafted LDPE with [N]/[C]=0.05 (Film A) and d) viologen grafted LDPE with [N]/[C]=0.08 (Film B).
Figure 3.12 Sheet resistance (Rs) of EB-viologen film exposed to irradiation for 30 min (time = 0) and subsequently left a) under room conditions, b) in the dark, c) in the dark and in a dessicator.
Figure 3.13 UV-visible absorption spectra of a) a conventional HCl-doped PANI coated on LDPE film and b) the photo-irradiated EB-viologen film before and after treatment in water for 5 minutes.
Figure 3.14 Concentration of Cl- ions and the corresponding calculated pH values as a function of treatment time in water. Open symbols denote the data obtained with 3 pieces of the irradiated EB-viologen films and solid ones for 1 piece of the film. Squares, circles, and triangles denote the Cl- ions concentrations, calculated pH values, and measured pH values, respectively.
Figure 4.1 Structure of 1,1’-bis(4-vinylbenzyl)-4,4’-bipyridilium dichloride(VBV).
Figure 4.2 XPS C 1s and N 1s core-level spectra of doped PANI-LDPE film before sputtering with FEP ((a) and (b)), after sputtering with FEP for 10s ((c) and (d)), and for 100s ((e) and (f)), and the FEP sputtered (100s) film after treatment in water for 3h ((g) and (h)).
xii List of Figures
Figure 4.3 SEM and AFM images of (a) and (d) doped PANI-LDPE film, (b) and (e) the film after sputtering with FEP for 100s, and (c) and (f) the FEP sputtered (100s) film after treatment in water for 2h.
Figure 4.4 Sheet resistance of (a) FEP sputtered (100s) PANI-LDPE film after immersion in water and (b) FEP sputtered (600s) PANI-LDPE film after immersion in NaOH solutions of different concentrations for various periods of time.
Figure 4.5 UV-visible absorption spectra of doped PANI-LDPE film before and after water treatment, and the FEP sputtered (100s) film after immersion in water for 3h.
Figure 4.6 Sheet resistance of Sample A and Sample B after immersion in water for various periods of time. Sample A is the PANI-viologen film that was first converted to a conductive state via exposure to UV-irradiation for 1h and then sputtered with FEP for 100s. Sample B is the PANI- viologen film that was first sputtered with FEP for 100s and then exposed to UV-irradiation for 1 h.
Figure 4.7 UV-visible absorption spectra of FEP sputtered (100s) viologen grafted LDPE film after different exposure times in air following UV irradiation for 10 min, compared with that of the viologen grafted LDPE film without FEP coating after an exposure time of 1 min in air following similar UV irradiation.
Figure 5.1 UV-visible absorption spectra of the VBV-LDPE film before and after reaction with a 1000 ppm gold chloride solution under UV irradiation for 15 min.
Figure 5.2 XPS Au 4f core-level spectra of the VBV-LDPE film after reaction with a 1000 ppm gold chloride solution for (a) 15 min without UV irradiation, (b) 5 min, (c) 10 min, (d) 15min under UV irradiation, and with the gold solutions of concentration of (e) 100 ppm, (f) 200 ppm, (g) 500 ppm under UV irradiation for 10 min.
Figure 5.3 XPS N 1s and Cl 2p core-level spectra of the VBV-LDPE film before ((a) and (d)) and after treatment with 1000 ppm gold chloride solutions under UV irradiation for 5 min ((b) and (e)) and 15 min ((c) and (f)).
Figure 5.4 XPS Cl 2p core-level spectra of the VBV-LDPE film after reaction with gold chloride solutions of concentration of (a) 100 ppm, (b) 200 ppm, (c) 500 ppm, and (d) 1000 ppm under UV irradiation for 10 min.
Figure 5.5 XPS Pt 4f core-level spectra of the VBV-LDPE film after reaction with a 1000 ppm platinum chloride solution for (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and with platinum chloride solutions of concentration of (e) 100 ppm, (f) 200 ppm, (g) 500 ppm under UV irradiation for 15 min.
xiii List of Figures
Figure 5.6 XPS Pd 3d core-level spectrum of the VBV-LDPE film after reaction with palladium chloride solutions of concentration of (a) 100 ppm and (b) 1000 ppm under UV irradiation for 15 min.
Figure 5.7 (a) Scanning electron microscopy image and (b) EDX spectrum of the VBV-LDPE film after reaction with 500 ppm gold chloride solution under UV irradiation for 10 min; (c) scanning electron microscopy image and (d) EDX spectrum of the VBV-LDPE film after reaction with 500 ppm platinum chloride solution under UV irradiation for 15 min.
Figure 5.8 UV-visible absorption spectra of the BV-PVA film before and after UV irradiation for 10 min.
Figure 5.9 Transmission electron micrographs of the gold particles formed in the BV-PVA matrix after reaction with gold chloride solutions of concentration of (a) and (b) 1000 ppm, (c) 100 ppm. The BV-PVA matrix was subjected to UV irradiation for 5 min prior to reaction with gold chloride solutions.
Figure 6.1 Image of developed circuit pattern via photo-irradiation of a PANI- viologen LDPE film through a commercial mask and using NMP to dissolve away the unexposed parts.
Figure 6.2 SEM images of a) pristine Al2O3 mask and b) PANI-viologen film after UV irradiation for 1hr without mask; c) 3-D and d) 2-D AFM images of the developed PANI-viologen film after UV irradiation for 1 hr through the Al2O3 mask and subsequent treatment with NMP.
Figure 6.3 Microscopic images of (a) to (c) patterns of the commercial photomask, (d) to (f) developed PANI patterns on the surface of PTFE films.
Figure 6.4 a) SEM image and b) EDX spectrum of the patterned PANI-viologen film after reduction with hydrazine for 10 min followed by reaction with a 100ppm AuCl3 solution for 10 min.
Figure 6.5 XPS Au 4f and Pd 3d core-level spectra of the patterned PANI-viologen film after reduction with hydrazine for 10 min followed by reaction with a) 100 ppm AuCl3 solution, and b) 100 ppm Pd(NO3)2 solution, for 10 min.
Figure 6.6 (a) SEM image of patterned VBV-LDPE film after gold deposition, and corresponding (b) gold, (c) carbon and (d) chlorine EDX mapping images of the patterned VBV-LDPE film after gold deposition.
Figure 7.1 Schematic presentation of the co-immobilization of GOD and MAV on AAc grafted PPY film.
xiv List of Figures
Figure 7.2 XPS N 1s and I 3d core-level spectra of (a) and (b) as-synthesized MAV powder, (c) and (d) GOD-MAV-PPY film. Surface modification was carried out with 10 vol% AAc monomer concentration in the graft- copolymerization step, 4mg/ml GOD and 9mM MAV in the co- immobilization step.
Figure 7.3 Amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the AAc monomer concentration used in the graft copolymerization process (4mg/ml GOD and 9mM MAV were used in the co-immobilization step).
Figure 7.4 Amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the MAV monomer concentration used in the co- immobilization process (GOD concentration was 4mg/ml and the AAc grafted PPY films were prepared with 10 vol% AAc monomer concentration in the graft copolymerization step).
Figure 7.5 Enzymatic activity of the GOD-MAV-PPY film (4mg/ml GOD and 9mM MAV in co-immobilization step) grafted with different AAc concentrations after reaction with glucose solution (a) under UV irradiation for 30min in the absence of O2 (b) with O2 for 30 min and without UV irradiation (c) for 30 min without UV irradiation and O2.
Figure 7.6 (a) Cyclic voltammograms of GOD-MAV-PPY film in glucose solution containing 0, 0.2, 0.4, 0.6, 0.8, 1.0mM glucose. Scan rate =0.1V/s. PBS buffer solution with 0.1M NaCl was used as supporting electrolyte. (b) Peak currents at 0.435V as a function of glucose concentration.
xv List of Tables
LIST OF TABLES
Table 3.1 Surface composition of VBC-graft copolymerized and viologen-graft modified LDPE film . Table 3.2 Surface composition and Rs of PANI-VBC and PANI-viologen films
Table 3.3 Surface composition of NA and LM-coated films before irradiation and after 90 min and 120 min of irradiation, respectively
Table 4.1 Surface compositions from XPS and contact angles (θ) of the PANI- LDPE films with FEP coating (sputtered for 100s) after treatment in water for various periods of time
xvi
CHAPTER 1
INTRODUCTION
1 Chapter 1
INTRODUCTION
Viologens are formally known as 1,1’-disubstituted-4,4’-bipyridilium if the two substituents at nitrogen are the same, and as 1-substituent-1’-substituent’-4,4’- bipyridilium should they differ. It has been seventy years since Michaelis (1933) first reported on the electrochemical behavior of this class of compounds. Since that time there have been successive waves of interest in this class of compounds. The viologens were originally investigated as redox indicators in biological studies (Michaelis and
Hill, 1933). Subsequently, they were the parent compounds of one of the most exciting new types of herbicide discovered for many years, the ‘paraquat’ family. More recently, viologens have been one of the most strongly favoured candidates in constructing electrochromic display devices due to their electrochemically reversible behaviour and the marked color change between the two redox states. Such special reversibility and redox characteristics also resulted in viologens being widely adopted as mediators in a range of biological studies. The applications of viologens in molecular electronics further demonstrate their versatility.
The discovery of highly electrically conductive doped polyacetylene (Chiang et al.,
1977) inspired vast scientific activities in the field of physics and chemistry of conducting polymers during last decade (Skotheim, 1986; Salaneck et al., 1991).
Polyheterocycles such as polyaniline, polythiophene, polypyrrole, poly (para phenylene), and analogs exhibit physical and chemical properties with great technological application potentialities (Salaneck et al., 1991). Among these conjugated polymers, polyaniline (PANI) and polypyrrole (PPY) are the most extensively studied conductive polymers because of their interesting properties, ease of
2 Chapter 1 preparation either by chemical or by electrochemical methods, and potential applications. The highly stable and reversible electrochemical redox activity of members of the conducting polymer family is already exploited commercially in secondary battery systems (Münstedt et al., 1987). During these electrochemical oxidation and reduction processes, the conducting polymers show doping induced insulator-to-metal phase transitions with changes in the electronic structure followed by structural relaxation phenomena due to the large electron-photon couplings in these low-dimensional systems (Feldblum et al., 1982; Crecelius et al., 1983; Bertho and
Jouanin, 1987; Kuzmany et al., 1988). Polarons and bipolarons are proposed to be responsible for the electronic properties of the conducting polymers in the doped state
(Skotheim, 1986).
The objectives of this research are (1) to study the interaction between viologen and conducting polymers such as polyaniline, and the possibility of employing this photo- induced interaction to fabricate the micropatterns of electroactive film, (2) to investigate the stability enhancement of electroactive and photoactive viologen systems, (3) to further explore practical applications of the viologens in the preparation of nanoscaled metal coatings and dispersions, and (4) to study the involvement of the viologens in the electron mediation between the immobilized enzyme and the analyte for glucose sensing. This dissertation comprised eight chapters and one appendix.
Chapter One provides a brief introduction to the dissertation. The research objectives of this dissertation are also given here. This is followed by a more detailed literature survey in Chapter Two. The properties, synthesis and applications of viologens as well as the structures and synthesis, doping mechanism, stability and applications of conducting polymers are presented in Chapter Two.
3 Chapter 1
The conversion of PANI to the doped and conducting state by a new technique of photo-induced doping using viologen moieties is presented in Chapter Three. Photo- sensitive films consisting of PANI coatings on viologen-grafted low density polyethylene (LDPE) substrates can be fabricated using a 3-step process whereby vinylbenzyl chloride (VBC) is first graft copolymerized onto the LDPE substrate, followed by the linking of the viologen moieties to the VBC and finally the deposition of the PANI coating onto the viologen-grafted film. The irradiation of these films results in the conversion of the PANI in the emeraldine (EB) state from the insulating to the conducting state. The effects of the VBC graft density and ultraviolet (UV) irradiation intensity were investigated. PANI with different intrinsic oxidation states from leucoemeraldine (LM) to nigraniline (NA) can be doped by this method as well.
The stability and the dedoping characteristics of the PANI-viologen film were also investigated and presented in this Chapter.
In Chapter Four, a radio frequency sputtering technique to deposit fluorinated ethylene propylene copolymer (FEP) coatings of controllable thickness on electroactive and photoactive polymeric substrates is described. The electrical stability of polyaniline in water is substantially enhanced via the sputtering of a layer of FEP on the order of
10nm thickness on its surface. This technique can be applied to both conventional acid protonated PANI film and the photoinduced doped PANI-viologen film. The technique of FEP sputtering can also be used to prolong the photochromic effect of viologen films by retarding the diffusion of O2 to the viologen radical cations formed under UV irradiation.
4 Chapter 1
The formation of nanoscaled gold and platinum coatings on the surface of 1,1’-bis(4- vinylbenzyl)-4,4’-bipyridilium dichloride (VBV) grafted LDPE films (VBV-LDPE) via the photo-induced reduction of the corresponding metal salt solutions is described in Chapter Five. The distribution of gold or platinum in the elemental and ionic state on the VBV-LDPE films is dependent on the UV irradiation time and the concentration of the metal salt solution used. The existence of these metals primarily in the elemental state on the VBV-LDPE film surface can be achieved with metal salt solutions of a low concentration and long irradiation time. The results indicate that platinum ions are more readily reduced than gold ions by the VBV-LDPE film. The reduction of palladium salt solution is much more difficult with the resultant coating comprising mainly Pd2+ ions rather than Pd metal. For gold and platinum solutions, a smooth and highly homogeneous coating can be achieved on the VBV-LDPE film. Well-dispersed gold and platinum particles can also be readily obtained via the reduction of the corresponding salt solution in a poly(vinyl alcohol) (PVA) matrix containing benzyl viologen (BV).
In Chapter Six, the potential applications of the PANI-viologen system and the reaction between viologen system and metal solutions are demonstrated. Two methods for fabricating conducting patterns on polymeric substrates were successfully employed. In the first method, UV irradiation of the PANI-viologen film on surfaces of
LDPE and polytetrafluoroethylene (PTFE) substrates through a mask resulted in the doping of the exposed areas. Selective areas of conductivity can be developed by dissolving away the soluble undoped parts, using N-methylpyrrolidinone (NMP). The patterns fabricated from the PANI can be treated with metal salt solutions for the incorporation of metal or metal ions. The second patterning approach takes advantage
5 Chapter 1 of the redox property of viologens. VBV patterns were formed on LDPE surfaces via graft copolymerization. Through the reduction of metal salt solutions under UV irradiation, the metal can be successfully deposited on the patterned VBV-LDPE film to form the conducting patterns.
Chapter Seven describes the co-immobilization of glucose oxidase (GOD) and viologen mediator on the surface of conducting PPY film, and the effects of the viologen and enzyme acting in tandem for glucose detection. The as-synthesized N- methyl-N’-(3-aminopropyl)-4,4’-bipyridilium (MAV) serves as an effective mediator for electron transfer from the active sites of GOD to the surface of PPY electrode in the absence of oxygen and under UV irradiation. The amounts of GOD and MAV immobilized on the PPY film could be controlled by changing the graft concentration of the linkage group, acrylic acid (AAc), on the PPY film and the ratio of GOD to
MAV in the co-immobilization step. The electrochemical response of the film modified with GOD and MAV was investigated as well.
Finally, the general conclusions drawn from this research project are summarized in
Chapter Eight. Some recommendations for future research related to this work are also included in this final Chapter.
6
CHAPTER 2
LITERATURE SURVEY
7 Chapter 2
2.1 Viologens
Viologens are commonly known as the 1,1’-disubstituted-4,4’-bipyridilium salts formed by the diquaternization of 4,4’-bipyridine (Monk, 1998). The prototype viologen, 1,1’-dimethyl-4,4’-bipyridilium, is often know as methyl viologen (MV), with other simple symmetrical bipyridilium species being named substitutent viologen.
The viologens exist in three main oxidation states as follows:
+ + 2+ R N N R (bipm )
2X-
+ +• R N . NR(bipm )
X -
0 R N NR(bipm )
Figure 2.1 Three common bipyridilium redox states.
Of the three common viologen redox states as shown in Figure 2.1 (Monk et al., 1995), the dication is the most stable and is colorless when pure unless optical charge transfer with the counter anion occurs. Reductive electron transfer to viologen dications forms radical cations, the stability of which is attributable to the delocalization of the radical
8 Chapter 2 electron throughout the π-framework of the bipyridyl nucleus, together with the N and
N’ substituents bearing some of the charge. In contrast to the bipyridilium dication, the viologen radical cations are intensely colored, with high molar absorption coefficients, owing to optical charge transfer between the (formally) +1 and 0 valent nitrogens
(Monk, 1998). A suitable choice of nitrogen substituents in viologens to attain the appropriate molecular orbital energy levels can, in principle, allow a color choice of the radical cation. In comparison with dications and radical cations, relatively little is known about the third redox state: the di-reduced viologen, which can be formed by either the one-electron reduction of the respective radical cation or the two-electron reduction of the dication. The intensity of the color exhibited by di-reduced viologens is low since no optical charge transfer or internal transition corresponding to visible wavelengths is accessible. A large volume of work has been done on viologens, ranging from the chemical fundamentals to the applications. It includes studies on the structures and preparation of different viologen species, the investigation of the redox states, electrochemistry and electron-transfer reactions, electrochromism, photochemistry, and so on.
2.1.1 Synthesis of viologens
Due to the stability of the dication salt, the majority of viologens synthesized are in this redox state. Two general synthetic routes are available for dication preparation.
The first starts with 4,4’-bipyridine and proceeds with diquaternization. The second general preparative route starts with 1-substituted pyridines which are then coupled.
The first preparation of bipyridilium salts is the reaction of hetero-cyclic amines, which is similar to the better-known Menshutkin reaction (Brown and Cahn, 1955) of
9 Chapter 2 alkyl amines. Bipyridilium dications are the product of the reaction between 4,4’- bipyridine and an excess of alkyl halide (Figure 2.2).
+ + N N + 2R-X RN N R
2X-
Figure 2.2 Dication preparation from quaternized 4,4’-bipyridine
The halogen atom becomes a halide anion. Subsequent ion exchange is necessary if different anions are required. During the reaction, there is no indication in the literature that alkyl substituent undergoes any form of rearrangement. For example, reaction of n-heptyl bromide with 4,4’-bipyridine always yields a viologen with n-heptyl substituents. An asymmetric viologen with different R and R’ substituents can be synthesized through a step by step quaternization reaction. The solvent used in the first step reaction should be non-polar such as acetone or even toluene because a mono- quaternized bipyridine precipitates from solution after incomplete reaction, i.e. as the solubility constant is exceeded. Yields of di-alkylated product by this method are usually poor (<5%). The monoviologen, after collection and purification, is then reacted a second time with a different alkyl halide. A more polar reaction medium such as methanol must be the solvent for the second quaternization step.
Viologen dications also can be formed from coupling of 1-substituted pyridilium species. In this coupling method, reduction of a 1-alkylpyridilium compound with sodium amalgam yields 1,1’-dialkyl-1,1’4,4’-tetrahydro-4,4’-bipyridilium species as shown in Figure 2.3, which is readily oxidized to the corresponding bipyridilium salt.
10 Chapter 2
If oxidation is achieved using a dilute aqueous acid, the anion of the acid becomes the anion of the viologen salt. Oxidizing agents can also be air, SO2, or chlorine (Carey and Colchester, 1971).
H R N NR H
Figure 2.3 Structure of 1,1’-dialkyl-1,1’4,4’-tetrahydro-4,4’-bipyridilium
Di-reduced bipyridilium compounds have also been made using the reductive coupling method, known sometimes as ‘Winters coupling’ in which pyridilium salts are coupled in the presence of cyanide ion (Reuss and Winters, 1973).
Viologen polymers are usually classified into two main types, the ionene-type and the pendant-type (Salamone, 1996), according to the positions of viologen groups in the polymeric backbone. The ionene-type viologen polymers were generally prepared by the condensation of simple bipyridine and dihalides. Alternatively, reactions of bipyridine with alkene halides give rise to viologen monomers bearing two double bonds and polymerization of these monomers again results in ionene-type viologens.
The pendant-type viologen polymers are usually synthesized in two different ways: polymerization of monomers with viologen moiety or copolymerization of the monomers with viologen structure with other monomers and by the polymeric functional reaction to incorporate the viologen structure onto the polymer chain.
11 Chapter 2
2.1.2 Physical properties of viologens
2.1.2.1 Dimerization of viologen radical cations
Aqueous solutions of methyl viologen radical cation vary in color from blue through to purple depending upon the concentration studied. Monomer-dimer equilibration has been proposed (Kosower and Cotter, 1964) to explain this and the related observation that cold solutions of benzyl viologen radical cation in certain concentrations are violet but become blue upon warming. The color change is fully reversible (Kosower and
Cotter, 1964). This phenomenon is common for most types of radical cation as well as for the viologen radicals. The structure of the dimer is thought to be a ’sandwich’ type structure in which the two π clouds overlap. The monomer-dimer equilibrium is not observed in any solvent but water at room temperature (Kosower and Cotter, 1964).
No dimer form of the viologen radical cation is observed in solutions of polar organic solvents such as methanol, acetonitrile, or N-dimethylformamide (DMF), etc., presumably because the radical cation exists as an ion pair rather than as a fully dissociated species, thus precluding formation of dimer. On the other hand, it was shown that the extent of dimerization is enhanced if the radical is adsorbed on a solid surface (Borgarello et al., 1985). The heptyl viologen (HV) radical in aqueous solution
2+ was used in that report and it was found that (HV)2 was formed in greater amounts in the presence of colloidal TiO2 powder.
2.1.2.2 Association and charge transfer
The dicationic redox state of the viologens is well known to form charge transfer complexes in solution with charge donors and undergo charge transfer interactions with inorganic species like sulphide (Kuczynski et al., 1984), halide and thiocyanate
12 Chapter 2
(Bertolotti et al., 1987). Organic donors can include crown ethers (Satoh et al., 1996), and arenes (Yoon and Kochi, 1989), etc. Typically, a charge-transfer complex is detected in UV-visible spectrophotometry. When two compounds in solution are brought together and a new optical absorption band is formed that was not present in the spectrum of either component, then it is likely that a charge-transfer complex has been formed. During complex formation, the orbitals of the approaching molecules overlap, and the resultant distortion causes a movement of charge from the donor to the unoccupied orbitals of the acceptor. The viologens are all excellent electron acceptors.
2.1.2.3 Solid-state conductivity
Bipyridilium dication salts are insulators with low conductivity of typically σ ≈10-10
Scm-1 (Monk, 1998), but if the dication undergoes charge transfer with a donor (neutral or anionic) then the conductivity is enhanced.
2.1.2.4 Radical solubility
The solubility of viologen radical cations is dependent on solvents, counterions, and the substituents. The radical cation is soluble in all organic solvents in which the dication is soluble. However, anions have a different effect on the radical solubility in organic solution and aqueous solution. In organic solvents, the radical is soluble if the
- - - - counter anion is relatively large, such as NO3 , ClO4 , BF4 , PF6 . But the radical is not
- - - - 2- 2- soluble if the counter anion is F , Cl , Br , I , PO4 , or SO4 (Monk, 1998). However, the situation is slightly more complicated in aqueous solutions because some anions impart a high solubility to both radical and dication, while others impart a high solubility to the dication but cause ion-pairing with the radical to a sufficient extent that precipitation is effected on one electron reduction.
13 Chapter 2
2.1.3 Photochromism of viologens
Photochromism is the reversible colour development by photoreduction mechanism.
Organic photochromic compounds usually function via isomerization, heterolytic and hemolytic cleavage, as well as pericyclic reactions. Viologens are one of the few photochromic organic compounds that function by a set of oxidation-reduction reactions (Crano and Guglielmetti, 1998). The viologen dication undergoes photo- induced reduction to form the blue radical cation in the absence of air. Electrons are believed to transfer from the counterions of viologens to the viologen dications due to the activation of both the viologens and the counteranions by near UV-irradiation.
Oxidation of the radical cations by air reverts the blue radicals back to the dication state. While some viologens exhibit photochromism in their crystalline state, the use of matrix polymers or polymers bearing viologen units, as described in the previous section, is usually required to develop colours by light in the film state (McArdle,
1992). When viologens are embedded in appropriate polymer matrices, the viologen cation radicals are stabilized by the surrounding solid matrices through restriction of both air-oxidation and thermal reverse-electron transfer. Photochromic behaviour of viologens is dependent on the matrices in which they are embedded, their substituent –
R groups, as well as anions (Crano and Guglielmetti, 1998).
14 Chapter 2
2.1.4 Applications of viologen system
2.1.4.1 Electrochromism and Electrochromic Devices (ECD)
Electrochromism is defined as the electrochemical generation of color in accompaniment with an electron transfer (or ’Redox’) reaction since an electro-active species often exhibits new optical absorption bands when it undergoes reduction or oxidation (Monk et al., 1995). The species that becomes colored during redox reaction is sometimes called the electrochromophore or electrochrome (Mercier et al., 1983).
Since the color is due to a chemical chromophore, rather than a light-emitting or interference effect, the color persists after the current flow is stopped. A current in the opposite direction reverses the electrochemical process and the display reverts to the colorless or bleached state. This leads to the useful facet of electrochromic devices
(ECD) operation. In contrast with cathode ray tubes (CRT) and liquid crystal displays
(LCD) displays, electrochromic systems possess some advantages. Firstly, ECDs consume little power in producing images which, once formed, persist with little or no additional input of power. Secondly, there is no limit, in principle, to the size an ECD can take: a larger electrode or a greater number of small electrodes may be used (Monk et al., 1995). However, present devices have insufficiently fast response times to be considered for realistic applications and cycle lives are probably also too low. Thus, more efforts are needed in order before ECD can compete with CRT and LCD for commercial viability.
All viologen species are electrochromic since bipm2+ and bipm+• redox states have different electronic spectra. The most thoroughly studied viologens for electrochromic applications is heptyl viologen as the dibromide salt. The first viologen-based ECD
15 Chapter 2 was reported by Schoot et al. (1973). In this ECD, heptyl viologen was the chosen viologen since reduction of the dication formed a durable film on the electrode, whereas shorter alkyl chains yield radical-cation salts which were slightly soluble. The
HV radical cation salt is amorphous as deposited, but soon after electrodeposition slight crystallization occurs and films appear patchy as this ‘aging-process’ takes place.
This problem can be overcome by the addition of a redox mediator such as hexacyanoferrate(II) to aqueous solutions of viologen dication (Monk, 1997). In operation, the electrogenerated hexacyanoferrate (III) chemically oxidizes the solid deposits of the radical-cation salt. An alternative approach of the aging problem is the use of asymmetric viologens (Barna and Fish, 1981). Since crystallization of viologen radical cation salts is an ordering phenomenon, it was rationalized that crystallization could be inhibited by decreasing the extent of molecular symmetry. Benzyl viologen will also form an insoluble film of radical cation salt following one-electron reduction
(Goddard et al., 1983; Scharifker and Wehrmann, 1985; Crouigneau et al., 1987).
However, it has not been investigated for ECD inclusion. Despite the drawbacks, many prototype electrochromic devices have been made. These have not been exploited further owing to competition with LCD, though they may still have a size advantage in large devices.
Besides displays, electrochromic systems find an entirely novel application as optical shutters. Electrochromic sun glasses have been produced which, unlike photochromic lenses, may be darkened at will. In fact, whole windows may be colored electrochromically to cut down the light in a room, office, or through a car windscreen.
Such shutters have been studied extensively by Goldner (1988a, 1988b). These glazing items are frequently called “intelligent” or “smart” windows.
16 Chapter 2
2.1.4.2 Electron Mediation
Many species are able to undergo electron transfer in solution, that is, homogeneous reaction, but not at an electrode (heterogeneous electron transfer). A mediator is an auxiliary redox couple in solution that acts in effect as an electron-transfer ‘catalyst’.
2+ bipm A (red)
- Electrode +e -e- +e-
+ • bipm A (ox)
Figure 2.4 An electron-transfer representation from an electrode showing reduction
of a biological molecule A by the viologen radical cation generated at the electrode.
Many biological systems contain redox-active sites yet are electro-inactive at an electrode. Any redox change at the molecule of interest must therefore be effected chemically. If bulk redox chemistry is undesirable, but redox change is wanted, then a mediator must be included in the same phase as the biological analyte. Electrochemical redox change to the mediator generates either a reducing or oxidizing agent which may then chemically affect electron transfer to the biomolecule. Mediation is represented in
Figure 2.4. The viologens, with their ready reversibility, are among the most widely used mediators. For instance, methyl viologen has been used as an electron mediator in the study of cytochrome-c (Haladjian et al., 1985; Castner and Hawkridge, 1983;
Rauwel and Thévenot, 1977). There are also reports of methyl viologen being used as a mediator for the reduction of sperm-whale myoglobin (Stargardt et al., 1978), spinach ferredoxin (Stargardt et al., 1978) and other proteins (Crawley and Hawkridge,
17 Chapter 2
1981). Recently, Zen and Lo (1996) have contrived a glucose sensor by immobilizing glucose oxidase between two nontronite clay coatings on glassy carbon electrode with methyl viologen as mediator. Similarly, octyl viologen (OcV) has been used as an electron-transfer catalyst in the reduction of various azobenzenes and azoxybenzenes to the corresponding hydrazobenzenes (Park and Han, 1996), the reducing agent OcV+• being formed by sodium dithionite reduction of OcV2+. A similar system has been used to reduce nitroarenes (Park et al., 1993) and nitroalkanes (Park et al., 1995) based on the OcV2+/+• couple. All of these viologens, serving as the mediators, need to be in solution when they undergo the electron-transfer reaction.
The electron mediator can also be immobilized on an electrode, and, in this case, the electron transfer is heterogeneous. Electron mediation is still possible, however, since a mediator rather than a more straightforward electrode is used. Such an electrode coated with a layer or thin-film of mediators is said to be ‘derivatized’ (Monk, 1998).
The derivatized electrodes could be achieved by several ways. The first usual approach is to use an N-substituent that chemically binds to the electrode. Wrighton et al. have derivatized electrodes with bipyridilium species, initially with substituents at the nitrogen consisting of a short alkyl chain terminating in the trimethoxysilyl group
(Figure 2.5), which may lose methoxy groups to bond to the oxide lattice on the surface of an optically transparent electrode (Bookbinder and Wrighton, 1983;
Dominey et al., 1983).
18 Chapter 2
+ + (MeO)3Si(CH2)3 N N (CH2)3Si(OMe)3
- 2X
Figure 2.5 Chemical structure of viologen with trimethoxysilyl substituents
A second method used by Wrighton et al. (1988) was to diquaternize a bipyridilium nucleus with a short alkyl chain terminating in pyrrole (Figure 2.6); anodic polymerization of the pyrrole forms polypyrrole, which adheres to the surface of the electrode, thus immobilizing the viologens (Shu and Wrighton, 1988).
+ + N ( CH ) N NCH 2 6 3 - 2X
Figure 2.6 Functionalized pyrrole with pendant viologen group
Other attempts to achieve the derivatized electrodes were to form a layer on the electrode surface by self-assembly of a viologen with other species, or in which the viologen is occluded within a ‘binder’, ensuring that the viologen remains in the solid layer on the electrode surface. An electroactive polymer formed by reaction of
2+ (bipm -(CH2)4-)n- with glutataldehyde (Chang et al., 1991), and a viologen trapped within a clay layer acting as a mediator within a glucose sensor (Zen and Lo, 1996) fall into this category. Liu et al. also reported selective reduction of nitroarenes to the corresponding anilines with sodium dithionite (Na2S2O4) by using insoluble resins with
19 Chapter 2 a viologen structure as electron-transfer catalyst (ETC) under heterophase condition
(Liu et al., 1997).
2.1.4.3 Other miscellaneous applications
Another well-known ability of viologens is the herbicidal activity, thus they also gain a common name as paraquat, the ICI brand name for the widely used herbicide whose active ingredient is methyl viologen. In addition, viologens also find applications in the production of hydrogen from water (Keller and Moradpour, 1980a; Keller et al.,
1980b), molecular electronics such as viologen-based transistor (Shu and Wrighton,
1988) and photodiode (Park et al., 1998), and so on.
20 Chapter 2
2.2 Conducting Polymers
Polymers as plastic materials have numerous technological applications because of their low cost, lightness, and flexibility. Plastics have been extensively used by the electronics industry because of these very properties and they were utilized as inactive packaging and insulating material. However, this narrow perspective has been rapidly changing since a new class of polymer known as intrinsically conductive polymers or electroactive polymers was discovered in 1977 (Shirakawa et al. 1977). The various groups working under A. J. Heeger, A. G. MacDiarmid, and H. Shirakawa have shown that the electrical conductivity of polyacetylene can be increased by 13 orders of magnitude upon doping with electron acceptors and electron-donors (Chiang et al.,
1978), and this spurred the development of new field of conducting polymers. The achievement of conductivity in polyacetylene as high as that of copper metal by H.
Naarmann and coworkers (1982), was a milestone discovery. Inspired by polyacetylene, remarkable progress has been made in the field of conducting polymers since then with the development of new conductive polymers such as polyaniline, polypyrrole, polythiophene, poly(p-phenylene), poly(p-phenylene sulphide).
Conjugated polymers may be made by a variety of techniques, including cationic, anionic, radical chain growth, co-ordination polymerization, step growth polymerization or electrochemical polymerization. Electrochemical polymerization occurs with suitable monomers which are electrochemically oxidized to create active monomeric and dimeric species which react to form a conjugated polymer backbone.
The main characteristic of a conducting polymer is a conjugated backbone that can be subjected to oxidation or reduction by electron acceptors or donors, resulting in what
21 Chapter 2 are frequently termed p-type or n-type doped materials, respectively. At the same time, the main problem with electrically conductive polymers stems from the very property that gives it its conductivity, namely the conjugated backbone. This causes many such polymers to be intractable, insoluble and non-melting with relatively poor mechanical properties. In addition, the environmental sensitivity of the initial conducting polymer systems proved to be discouraging. Major improvements have since been made in material quality and environmental stability. Soluble conducting polymers either in water or in common organic solvents have been synthesized to enable processing of the conducting polymers into films, fibers or composites (Tokito 1991, Bhattacharya and De 1999). Highly oriented materials, which have excellent mechanical properties together with correlatedly improved electrical properties, have been achieved by post- synthesis tensile drawing (Aldissi 1989; Akagi et al. 1989; Yamaura et al. 1989;
Hagiwara et al., 1990; Cromack et al., 1991; Monkman and Adams 1991). In parallel with these efforts toward materials improvements, a number of potential application areas have been identified. Electrically conductive polymers possess a broad range of applications in batteries, antistatic coatings, protective coatings, electromagnetic shielding, solar cells, printed circuit boards, light-emitting diodes, biosensors, flexible displays, and so on. Therefore, electrically conductive polymers offer good marketing opportunities and have the potentially bright future.
22 Chapter 2
2.2.1 Structure and synthesis
The common molecular feature of pristine conducting polymers is the π-conjugated system, which is formed by the overlap of carbon pz orbitals and alternating carbon- carbon bond lengths (Baeriswyl et al., 1992; Heeger et al., 1988). In some systems, notably polyaniline, nitrogen pz orbitals and C6 rings also are part of the conjugation path (Ginder and Epstein, 1990; Libert et al., 1995). Figure 2.7 shows the chemical repeat units of some most studied conducting polymers - that is, polyacetylene, polyaniline (PANI), polypyrrole (PPY), polythiophene (PTH), poly(p-phenylene vinylene) (PPV).
( )X
polyacetylene
H H NN NN y 1-y X
polyaniline
( ) N x ( ) S x ( ) H x polythiophene poly(p-phenylene-vinylene) polypyrrole
Figure 2.7 Chemical repeat units of several conducting polymers
Among these conjugated polymers, polyaniline and polypyrrole are most extensively studied conductive polymers because of their interesting properties, ease of preparation either by chemical or electrochemical methods, and potential applications.
23 Chapter 2
2.2.1.1 Polyaniline
The base form of polyanilines has the generalized formula as shown in Figure 2.7, where some of the C6H4 rings are in the benzoid form while others in the quinonoid form. Aniline polymers are actually poly(p-phenyleneimineamine)s, in which the neutral intrinsic redox state can vary from the completely reduced material in the leucoemeraldine (LM) oxidation state (y=1), to the completely oxidized material in the pernigraniline (PNA) oxidation state (y=0) (Green and Woodhead, 1910, 1912). The emeraldine (EM) state with y=0.5 is most extensively studied because of its good processibility, stability and application potential. Figure 2.8 gives the octameric representation of the various intrinsic redox states of polyaniline.
H H H H H H H H N N N N N N N N Leucoemeraldine (LM)
H H H H N N N N N N N N Emeraldine (EM)
H H N N N N N N N N Nigraniline (NA)
N N N N N N N N Pernigraniline (PNA)
Figure 2.8 Octameric structures of polyaniline in various intrinsic redox states.
Polyaniline can be prepared by either chemical or electrochemical oxidation of aniline under acidic conditions (Genies et al., 1990). An aqueous medium is preferred. In the classical chemical method, the partly protonated emeraldine salt can be easily synthesized by the oxidative polymerization of aniline monomers, (C6H5)NH2, in aqueous hydrochloric acid, by a variety of oxidizing agents. The most commonly used reagent for synthesis is ammonium peroxydisulfate, (NH4)2S2O8, in aqueous HCl
(Chiang and MacDiarmid, 1986; MacDiarmid et al., 1985). The pernigraniline
24 Chapter 2 oxidation state is believed to form first in this reaction. It then acts as an oxidizing agent by further polymerizing the aniline, while it is itself reduced to the emeraldine state (Asturias et al., 1989). Treatment of this black-green precipitate with aqueous ammonium hydroxide yields the black-blue emeraldine base (EB) powder. In addition, when plastics, textiles, synthetic fibers or glass are immersed in a polymerizing mixture by chemical synthesis method, a thin emeraldine coating can be deposited on the substrate surface (Gregory et al., 1989).
Synthesis of the emeraldine salt can also be accomplished using an electrochemical method. Anodic oxidation of aniline monomers on an inert metallic electrode in aqueous acid is a common method for polyaniline synthesis (Genies et al., 1985a,
1985b; Genies and Tsintavis, 1986). Compared with the chemical synthesis method, the resulting product by this method is clean and does not necessarily need to be extracted from the initial monomer/oxidant/solvent mixture. Similarly, the emeraldine salt formed needs to be further treated with a base to form EB.
The leucoemeraldine base is off-white in powder form and forms thin transparent coatings. It can be conveniently prepared by the reduction of the EB powder or coating, using phenyl hydrazine or hydrazine (Huang et al., 1986). On the other hand, the pernigraniline base (PNA) can be synthesized by the controlled oxidation of EB by m- chloroperbenzoic acid (MacDiarmid et al., 1991). Thin films of PANI, with oxidation states greater than 0.5, have also been formed by dipping thin EB films into ammonium peroxydisulfate solutions for short periods of time (Cao Y., 1990).
25 Chapter 2
2.2.1.2 Polypyrrole
Of all known conducting polymers, polypyrrole is the most frequently used in commercial applications, due to the long-term stability of its conductivity and the possibility of forming homopolymers or composites with optimal mechanical properties. Similarly, there are also two main methods used to synthesize polypyrrole: chemical and electrochemical polymerization. The principal advantage of chemical methods is related to the possibility of mass production at low cost. This is often difficult to achieve with electrochemical methods. On the other hand, electrochemical methods offer materials with better conducting properties and for some applications such as the construction of electronic devices, electrochemical polymerization offers the possibility of in-situ formation. In chemical synthesis, polypyrrole have been prepared in the presence of various oxidizing reagents, including hydrogen peroxide in acetic acid, lead dioxide, ferric chloride, nitrous acid, etc. Initially, chemical methods of preparation with acid or peroxide initiators have resulted mainly in fairly oxidative insulating materials with room-temperature conductivity typically on the order of 10-10 to 10-11 S cm-1 (Salmon et al., 1982; Nalwa, 1992). Those initially insulating polypyrrole has to be doped with halogenic electron acceptors such as bromine and iodine to achieve a stable conductivity in the order of 10-5 S cm-1 (Hsing et al., 1983).
Therefore other chemical methods able to obtain polypyrrole directly in a conducting state were developed. Ferric salts such as FeCl3, Fe(ClO4)3, FeBr3, etc. are the most commonly used oxidants for the chemical synthesis of highly conductive polypyrrole with conductivity values between 10-5 and 200 S cm-1 (Armes, 1987; Rapi et al., 1988;
Machida et al., 1989; Dubitsky et al., 1991; Kang et al., 1991). The polymerization reaction probably follows a mechanism similar to that proposed by Hsing et al. for the oxidative polymerization of benzene to poly(p-phenylene) (Hsing et al., 1983). In
26 Chapter 2 comparison, electrochemical generation of polypyrrole presents some advantages over the chemical way of synthesis. Electrochemical synthesis permits direct grafting of the conducting polymer onto the electrode surface, which can be of special interest for electrochemical applications. As the electrical potential needed for monomeric oxidation is significantly higher than the charging (or doping) of the formed polymer, the polymer is directly obtained in its conducting state. The film thickness can be easily controlled by the electrical charge employed during polymerization. In practice, polypyrrole films can be prepared very easily in a one compartment cell equipped with a counter electrode and a reference electrode. In a typical preparation, the electrochemical bath consists of a 0.1M solution of tetraethylammonium tetrafluoroborate in acetonitrile with 0.02M pyrrole monomer. Prior to electrolysis, it is necessary to purge the oxygen in the system with an inert gas. Electropolymerization is then accomplished either potentiostatically using a potential of 0.7-0.8 V in order to get thin films (0.05 µm thick) or galvanostatically using a current density of 0.22 mA/cm2 in order to get thicker films (10-200 µm) (Diaz and Kanazawa, 1982). The effects of solvent and supporting electrolyte on the quality and electrical conductivity of polypyrrole films prepared by this technique have been discussed by Diaz and
Bargon (1986).
27 Chapter 2
2.2.2 Doping of conducting polymers
For many electrically conductive polymers, “doping” is required to boost conductivity to a point where it is useful. The polymers are transformed into a conductor by doping it with either an electron donor or an electron acceptor. This is reminiscent of doping of silicon based semiconductors where silicon is doped with either arsenic or boron.
The concept of doping is the unique, central, underlying, and unifying theme which distinguished conducting polymers from all other types of polymers (Chiang et al.
1977, 1978). During the doping process, an organic polymer, either an insulator or semiconductor having a low conductivity, typically in the range 10-10 to 10-5 S/cm, is converted to a polymer in the metallic conducting regime (≈1 to ≈104 S/cm). The controlled addition of known, usually small (≤10 percent) and non-stoichiometric quantities of chemical species results in dramatic changes in the electronic, electrical, magnetic, optical and structural properties of the polymer (MacDiarmid, 1993).
A band theory was used for the explanation of conducting polymers. Figure 2.9 shows a simple band picture explaining the difference between an insulator, a semiconductor and a metal. The lowest unoccupied band is called the conduction band and the highest occupied one the valence band. The energy spacing between the highest occupied and the lowest unoccupied band is called the band gap. In a metal, the orbitals of the atoms overlap with the equivalent orbitals of their neighbouring atoms in all directions to form molecular orbitals similar to those of isolated molecules. Therefore the electrons can easily redistribute. However, for a semiconductor, a narrow band gap exists between the valence band and conduction band. In the case of insulators, a wide energy band gap forbids the relocation of electrons. Therefore, for conducting
28 Chapter 2 polymers, a half filled valence band would be formed from a continuous delocalized π
-system, resulting in the electrically conductive materials. Upon doping treatment the atomic or molecular dopant ions are positioned interstitially between chains and donate charges to or accept charges from the polymer backbone. The polymer backbone and dopant ions form new three-dimensional structures. For the degenerate ground-state polymers such as polyacetylene, pernigraniline form for PANI, charges added to the backbone are stored in charged soliton and polaron states (Baeriswyl, et al., 1992;
Heeger et al., 1988). However, for non-degenerate systems, the charges introduced are stored as charged polarons or bipolarons (e.g., for polythiophene, polypyrrole, poly(p- phenylene vinylene), EB form of polyaniline) (Campbell and Bishop, 1981).
Wide band gap Narrow band gap Increasing No gap energy
Insulator Semiconductor Metal
Energy levels in conduction band
Energy levels in valence band
Figure 2.9 A simple band picture explaining the difference between an insulator, a semiconductor and a metal.
29 Chapter 2
Generally, there are four techniques to accomplish the doping process:
(i) Chemical doping. The doping process is accomplished in the vapor phase by exposing the polymer to the vapor of the dopant or in the liquid phase, by immersing the polymer films in the dopant solution. The amount of dopant incorporated in the material (doping level) depends on the vapor pressure of the dopant or its concentration, the doping time, and the temperature of the doping source and of the polymer.
(ii) Electrochemical doping. Conjugated polymers can be doped and undoped by immersing the material as an electrode in an organic electrolyte solution with the application of a current through the electrochemical system (LiClO4 in THF or propylene carbonate, with lithium as a counter electrode (Nigrey et al.,1981)) or in aqueous electrolytes (PbSO4/PbO2 in sulfuric acid solutions, with lead as a counter electrode) (Mammone and MacDiarmid, 1984).
(iii) Ion implantation. When the proper ion beams are used, conjugated polymers can become conducting by implanting ions in the polymer’s lattice, forming covalent bonds with the material. The doping level depends on the energy of the of the ion beam to which the material is exposed (Allen et al., 1980).
(iv) Photochemical doping. This is accomplished by treating the polymer with the dopant species, which are initially inert towards the materials; for example, diphenyliodonium hexafluoride arsenate in methylene chloride or triarlysulfonium salts (Clarke et al., 1981; Pitchuman and Willig, 1983), followed by ultraviolet irradiation.
In the first two types of doping, the induced electrical conductivity is permanent, until the charge carriers are chemically compensated or purposely removed by undoping. In
30 Chapter 2 contrast, a transient effect is observed in the latter two types of doping. In photo- excitation, the photoconductivity lasts only until the excitations are either trapped or decayed back to the ground state, whereas, in the case of charge injection, the carriers in the accumulation or depletion layers remain only as long as the biasing gate voltage is applied.
Polyaniline holds a special position amongst conducting polymers in that its most highly conducting doped form can be reached by two completely different processes- protonic acid doping and oxidative doping. Protonic acid doping of emeraldine base units with, for example, 1 M aqueous HCl results in complete protonation of the imine nitrogen atoms to give the fully doped emeraldine hydrochloride salt (Chiang et al.
1986, MacDiarmid et al. 1987; Ray et al., 1989a, 1989b). A similarly doped polymer can be obtained by chemical oxidation or electrochemically anodical oxidation of leucoemeraldine base (MacDiarmid and Epstein, 1989; Neoh et al., 1991; Zeng et al.,
1997). This process actually involves the oxidation of both σ and π systems rather than just the π system of the polymer as is usually the case in p-type doping. The relationship between protonic acid doping and oxidative doping of different forms of
PANI to give the same conducting material is illustrated diagrammatically in Figure
2.10 (MacDiarmid, 1993).
31 Chapter 2
H H N N N N Emeraldine Base -10 -1 x σ=10 Scm
No change in number H+ protonic acid of electrons on backbone doping
H H H H Emeraldine N N N N Hydrochloride - -1 Cl Cl- x σ=1~5 Scm
-2e 2Cl -
No change in number of H Chemical or electrochemical atoms on backbone oxidative doping
H H H H Leucoemeraldine -10 -1 N N N N σ=10 Scm x
Figure 2.10 The relationship between protonic acid doping and oxidative doping of different forms of polyaniline to the same conducting material.
32 Chapter 2
2.2.3 Degradation and stability
Similar to various other materials such as amorphous polymers, solid electrolytes and semiconductors in one way or the other, the conjugated polymers show a wide range of degradation kinetics in different environments as well as dependence on temperature and the molecular structures. There are two distinct types of stability. Extrinsic stability is related to vulnerability to external environmental agents such as oxygen, water, peroxides. This is determined by the susceptibility of the polymer’s charged sites to attack by nucleophiles, electrophiles and free radicals. If a conducting polymer is extrinsic unstable then it must be protected by a stable coating. Many conducting polymers, however, degrade over time even in dry, oxygen free environment. This intrinsic instability is thermodynamic in origin. It is likely to be caused by irreversible chemical reactions between charged sites of the polymer and either the dopant counter ion or the p-system of an adjacent neutral chain, which may break the conjugation.
Intrinsic instability can also come from a thermally driven mechanism which causes the polymer to lose its dopant. This happens when the charge sites become unstable due to conformational changes in the polymer backbone. This has been observed in alkyl substituted polythiophenes (Tol, 1995). Thermogravimetic analysis (TGA) studies of EM base show two stages of weight loss, namely the first stage due to loss of moisture from hygroscopic polyaniline and the second stage due to the decomposition of the polymer chains, whereas for the EM salt (doped with volatile acid, i.e. HCl, H2SO4) three rather distinct stages of weight loss are observed. It is suggested that the first stage (starting at about 110 °C) is due to the loss of moisture, the second stage (starting at about 230 °C to 275 °C) is due to the loss of acids by volatilization, and the third stage (starting at about 300 °C to 500 °C) is attributed to
33 Chapter 2 the thermal decomposition of the polymer backbone (Kulkarni et al., 1989; Neoh et al.,
1990a, 1990b, 1992, 1994; Traore et al., 1991). The thermal aging causes a decrease in the conductivity of the polyaniline films (Hagiwara et al., 1988; Kulkarni et al., 1989;
Yue et al., 1991; Matveenva et al., 1994; Price and Wallace, 1996; Rannou and
Nechtschein, 1997).
There are several methods that have been investigated to stabilize the conducting polymers including surface graft copolymerization with a hydrophobic monomer
(Zhao et al., 1999), encapsulating or coating the polymer with a hydrophobic polymer via spin coating or by the use of adhesives (Witucki et al., 1987, Tanaka and Doi,
1989), preparation of self-doped polymers (Kim et al, 1994; Ito et al., 1998; Neoh et al.,
1993a, 1994), and use of organic or polymeric anions as dopants (Warrant et al., 1989;
Naoi et al., 1989, Neoh et al., 1990c, 1995, Kang et al., 1995; Pun et al., 1995; Mandal et al., 1996; Jiang et al., 1997; Kaneko et al., 1998; Ma et al., 1998).
34 Chapter 2
2.2.4 Applications of conducting polymers
Conducting polymers offer a unique combination of properties such as relative lightness and ease of processing that make them attractive alternatives to traditional conducting materials. The extended π-systems of these conjugated polymers are highly susceptible to chemical or electrochemical oxidation or reduction. These alter the electrical and optical properties of the polymer. Since these oxidation and reduction reactions are often reversible, it is possible to systematically control the electrical and optical properties, which provides exciting possibilities for various applications. The possible uses of electrically conductive polymers are many and varied, ranging from antistatic devices through batteries and solar cells to transistors.
2.2.4.1 Antistatic coatings
By coating an insulator with a very thin layer of conducting polymer it is possible to prevent the buildup of static electricity. This is particularly important where such a discharge is undesirable. Such a discharge can be dangerous in an environment with flammable gasses and liquids and also in the explosives industry. In the computer industry the sudden discharge of static electricity can damage microcircuits. This has become particularly acute in recent years with the development of modern integrated circuits. To increase speed and reduce power consumption, junctions and connecting lines are finer and closer together. The resulting integrated circuits are more sensitive and can be easily damaged by static discharge at a very low voltage. Modifying the thermoplastic used via adding a conducting plastic into the resin results in a plastic that can be used for the protection against electrostatic discharge (Dhawan and Trivedi,
1993; Omastova et al., 1996).
35 Chapter 2
2.2.4.2 Electromagnetic shielding
Many electrical devices, particularly computers, generate electromagnetic radiation, often at radio and microwave frequencies. This can cause malfunctions in nearby electrical devices. The plastic casings used in many of these devices are transparent to such radiation. By coating the inside of the plastic casing with a conductive surface this radiation can be absorbed. This can best be achieved by using conducting polymers. This is cheap, easy to apply and can be used with a wide range of resins
(Kim et al., 2003; Dhawan et al., 2001; Koul et al., 2000).
2.2.4.3 Organic conducting patterns
The doped, conductive forms of electroactive polymers are being evaluated as possible alternatives to metals as connecting wires and conductive channels, since the conductivity of these materials can be tuned over a wide range by changing the dopant and/or doping level. By controlling the doping of a conductive polymer, interconnects, passivation layers, and even the semiconductor layers could be the same material. That would be a great advantage for the production of integrated circuit. Flexible, all-plastic, micro-electronic devices based on conjugated organic polymers are now appearing in prototype forms (Gustafsson et al., 1992; Garnier et al., 1994).
2.2.4.4 Optoelectronic devices
The mechanical flexibility and tunable optical properties of some of the conjugated polymers make them attractive materials for optical and electronic devices. The discovery of electroluminescence (EL), which is the emission of light in conjugated polymers (Burroughes et al., 1990) when excited by flow of an electric current, has provided a new impetus to the development of light-emitting devices (LEDs) for
36 Chapter 2 display and other purposes (May, 1995). Inorganic electroluminescent materials have been known for many years. Inorganic semiconducting and organic dye materials have to be deposited as thin films by the relatively expensive techniques of sublimation or vapor deposition, which are not well suited to fabrication of large-area devices. For these reasons, the possibility of using fluorescent conjugated polymers, which can be readily deposited from solution as thin films over large areas, is most attractive.
2.2.4.5 Batteries and solid electrolytes
Probably the most publicized and promising of the current applications are light weight rechargeable batteries. Considerable interest has been focused since the mid-1980s on the marketing of lithium-polymer batteries, especially for prototypes using PANI as the cathode material. The lithium-PANI system was exploited by Japan’s Bridgestone
Company in the large-scale production of a high-cycle-life, button-type three-volt battery. The battery features an electrochemically prepared PANI cathode and a lithium-aluminium alloy anode (Nakajima and Kawagoe, 1989a). In addition, the
German Varta and BASF companies jointly attempted the development of lithium-
PPY, cylindrical and sandwich types, three-volt batteries. It was reported that lithium-
PPY cells are generally characterized by high charge-discharge efficiency (Münstedt et al., 1987). Some prototype cells are comparable to, or better than nickel-cadmium cells now on the market.
2.2.4.6 Sensors
The development of biological and chemical sensors based on conducting polymers has been investigated extensively during the past ten years. There are several advantages in using conducting polymers as sensing materials over the conventionally
37 Chapter 2 used materials (Partridge et al., 1996). These advantages include the availability of a diverse range of monomer types and synthetic monomer analogues; electrochemical preparation allowing the ready mass production and miniaturization of the sensors; the ease with which biomaterials, such as enzymes, antibodies and whole cells, may be incorporated in the polymer; the ability to change the oxidation state of the polymer after deposition and thereby tailor the sensing characteristics of the film; and finally, the ability to obtain reversible responses at ambient temperature. Due to these advantages, the conducting polymer sensors can be applied in a number of different modes, such as pH-based (change in pH), conductometric mode (change is conductivity), amperometric mode (monitoring the variation in current), potentiometric mode (change of open circuit potential) and so on (Chandrasekhar, 1999).
2.2.4.7 'Smart' structures
One of the most futuristic applications for conducting polymers is 'smart' structures.
Conducting polymer coated fabrics show piezo and thermoresistivity and have remarkable transducing properties. For example, integrated fiber optic sensors using conducting polymer as the functional materials can be used to sense various battlefield hazards in real-time, such as chemical and biological warfare threats, above-normal field temperatures, and other hazards. They enable the realization of truly wearable instrumented garments capable of sensing various environmental conditions (El-Sherif et al., 2000). Other applications of smart structures may include active actuators, and smart windows. Electrochromic smart windows are able to vary their throughput of radiant energy by low-voltage electrical pulses (Granqvist et al., 1998).
38 Chapter 2
Other possible applications in anticorrosion coatings (Rammelt et al., 2003; Tan and
Blackwood, 2003; Racicot et al., 1997), catalyst (Qi and Pickup, 1998), gas-selective membrane (Anderson et al., 1991), and solar cells (Cong et al., 2000) have also been targeted and are being developed.
39 Chapter 2
2.3 Polymer surface modification and characterization
Polymers usually have very good bulk properties and are inexpensive, but many industrial applications, such as adhesion, biomaterials, protective coatings, friction and wear, microelectronic devices, thin-film technology, and composites, require these polymers to have special surface properties. Because polymers are inert materials and generally have a low surface energy, they often do not possess the surface properties needed to meet the demands of various applications. Therefore, surface modification of polymers has become an important research area in the plastic industry. Many improvements have been made in developing surface treatments to alter the chemical and physical properties of polymer surfaces without affecting bulk properties. Various innovative techniques including chemical and physical processes have been developed.
2.3.1 Surface grafting
Surface grafting is one of the most commonly performed means to change the surface properties of polymers for such applications as biomaterials, membranes, and adhesives. Grafting has advantages over other methods in several points, including easy and controllable introduction of graft chains with a high density and exact localization of graft chains to the surface with the bulk properties unchanged.
Furthermore, the surface of the same polymer can be modified to have very distinctive properties through the choice of different monomers. New functionalities can be incorporated into the original polymers like surface hydrophilicity (Suzuki et al., 1986), hydrophobicity (Hoebergen et al., 1993), biocompatibility (Ikada, 1994) and adhesive properties (Ikada and Uyama, 1993). Surface grafting can be generally achieved by two methods- UV irradiation method, and ozone method.
40 Chapter 2
Ultraviolet (UV) energy has been extensively applied for surface graft polymerization of polymers with the aid of a photoinitiator or a photosensitizer, such as benzophenone.
During UV irradiation, the photoinitiators are excited to a high-energy state which promotes abstraction of hydrogen from the polymer substrate. Thus, radicals are generated on the polymer surface for the subsequent initiation of graft copolymerization. Earlier reports dealt with UV irradiation of the vapor phase of monomer and sensitizer under a reduced pressure (Wright, 1967) or in the presence of inert gas (Ogiwara et al., 1981; Tazuke and Kimura, 1978). A similar method was performed for graft polymerization of acrylamide (AAm) and acrylic acid (AAc) onto polypropylene (PP) film (Zhang and Ranby, 1991), poly(ethylene terephthalate) (PET) fiber surface (Zhang and Ranby, 1990), and 4-vinyl pyridine onto PET fiber (Feng and
Ranby, 1992). Prior to graft copolymerization, polymer surfaces are pretreated with high-energy radiation, or plasma treatment to introduce peroxides or hydroxyl peroxides, which are capable of initiating the subsequent graft copolymerization.
On the other hand, Mathieson and Bradley (1996) modified the surface of polyethylene
(PE) and a polyetheretherketone (PEEK), oxidizing the surface with ozone in addition to UV radiation, namely an ultraviolet–ozone oxidation process. The pretreated polymer surfaces exhibited significant improvement in adhesion when tested using an epoxy adhesive. The combination of UV and ozone treatment was also utilized by
Walzak et al. (1995) to improve surface properties of PP and PET.
41 Chapter 2
2.3.2 Plasma modification
Plasma modification of polymer surfaces may be categorized into two major types of reaction: plasma treatment and plasma polymerization. Advantages of plasma processes include the following: uniform modification over the whole surface, selective chemical modification for the polymer surface by choice of the gas used, and confined modification to the surface layer without changing the bulk properties of the polymer.
2.3.2.1 Plasma treatment
Plasma treatment is by far the most practical pretreatment method (Mittal, 1995; Chan et al., 1996). The plasma may contain charged and neutral species, including some of the following: electrons, positive ions, negative ions, radicals, atoms and molecules, which can react at the substrate surface. Inert gas plasmas, such as helium, neon and argon, have been used for cleaning and etching purposes and the surface is activated during the subsequent exposure to the atmosphere. The inert gas plasma also removes low-molecular-weight materials or converts them to higher-molecular-weight by cross- linking reactions. Oxygen and oxygen containing plasmas are most commonly employed to modify polymer surfaces. It is well known that an oxygen plasma can react with a wide range of polymers to produce a variety of oxygen functional groups, including C-O, C=O, O-C=O, C-O-O, and CO3 at the surface. Two processes occur simultaneously during the oxygen plasma treatment: etching of the polymer surface through the reactions of atomic oxygen with the surface carbon atoms, giving volatile reaction products, and the formation of oxygen functional groups at the polymer surface through the reactions between the active species from the plasma and the
42 Chapter 2 surface atoms (Morra et al., 1989). Nitrogen-containing plasma is widely used to incorporate nitrogen containing functional groups on polymer surfaces, improving wettability, printability, bondability and biocompatibility of the treated surfaces
(Nakayama et al., 1988; Lub et al., 1989; Petrat et al., 1994). In addition, fluorine and fluorine-containing plasmas are used to decrease the surface energy and to increase the hydrophobicity of polymer surfaces, resulting in surface fluorination of polymers
(Brinen et al., 1991; Mark et al., 1985).
2.3.2.2 Plasma polymerization
Plasma polymerization is a unique technique for modifying polymer and other materials by depositing a thin polymer film, which may have various applications in adhesion (Wertheimer and Schreibe, 1981; Moshonov and Avny, 1980), surface hardening (Inagaki et al., 1983), contact lens coating (Ho and Yasuda, 1988), and blood compatibility (Ertel et al., 1990). Plasma deposit films have many special advantages: (i) a thin conformal film of thickness of few hundred angstroms to one micrometer can be easily prepared; (ii) highly cross-linked and pinhole-free films can be prepared with unique physical and chemical properties; (iii) films can be formed on practically any substrate, including polymer, metals, glass, ceramics, and semiconductors. However, plasma polymerization is a very complex process that is not well understood. The structure of plasma-deposited films is highly complex and depends on many factors such as reactor design, power level, monomer structure, monomer pressure, etc.
43 Chapter 2
2.3.3 Surface characterization
Improvement in surface modification techniques cannot be made without an in-depth understanding of the chemical and physical properties of polymers’ surface and interface. The nature of the functional groups introduced by different surface modification methods must be determined in order to understand the various surface modification mechanisms. The level of surface treatment also must be well controlled to obtain the optimum results. Hence, it is obvious that polymer surfaces and modified surfaces, as well as the interfacial region must be characterized at the molecular level.
Techniques normally used for characterization of bulk properties are not suitable if only the chemical and physical properties within the first few nanometers of the surface are of interest (Bergbreiter et al., 1997). During recent years, some surface sensitive techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), electron energy loss spectroscopy (EELS) and contact angle measurements have been developed to analyze polymer surface. Among these techniques, XPS and SIMS, which can provide surface chemical and structural information, have been most widely used by scientists and engineers in both academia and industry. Each technique has distinct advantages, but only their complementary use can produce a definitive characterization of complex systems.
44
CHAPTER 3
PHOTO-INDUCED REACTION OF POLYANILINE
WITH VIOLOGEN IN THE SOLID STATE
45 Chapter 3
3.1 Introduction
As mentioned in Section 2.2.2, highly electrically conducting PANI can be achieved by protonic acid doping and oxidative doping. In addition to these more conventional doping methods, several other novel methods have been proposed recently. Concurrent doping of PANI during the N-alkylation process was reported by Zhao et al (2000).
The conversion of PANI from insulating to conducting state in aqueous viologen solutions, which represent a milder environment was recently reported (Ng et al.,
2001). These previous studies on the doping of PANI were carried out in a solution or
‘wet’ environment. Recently, there is a growing interest in radiation-induced doping of PANI in the dry solid state due to the potential for a wide range of applications, and photo-induced doping of PANI involving the generation of acid upon UV irradiation has been reported. The use of onium salt-PANI blends (Angelopoulos, 1990) was the first recorded attempt at radiation-induced doping of PANI. In this method, onium salts, which decompose upon UV-irradiation, generate protonic acids, which in turn dope the
PANI. In 1995, Venugopal et al. used blends of nonionic photo acid generators with methyl-substituted PANI to the same effect. Later, poly(vinyl chloride) (PVC)/PANI composites (Sevil et al., 1998; Sertova, 1998) were investigated where the HCl produced through the dehydrochlorination of PVC under UV-irradiation resulted in the doping of PANI.
46 Chapter 3
The current study also utilizes near-UV irradiation for the conversion of PANI from the insulating to the conducting state, but it is different from previous studies in that
PANI and the doping agent are not in the form of blends. Through proper surface functionalization of the substrate, surface conductivity can be achieved. This technique offers the advantage of providing conductivity to selective surface regions of substrates and may have particular pertinence to pattern fabrication and electronic design. In the present work, the surface functionalization of LDPE substrates via the grafting of viologen moieties followed by the deposition of a thin PANI coating was first carried out. The PANI coating was deposited in the EB state and could be converted to either LM or NA state as well. The UV-induced reaction of the PANI in
EB state and viologens were studied in detail and a comparison of the reaction of
PANI with vinyl benzyl chloride (VBC) grafted film was also carried out. In addition, the photoinduced reaction of viologens and PANI in different intrinsic oxidation states as well as the stability and the dedoping characteristics of the PANI-viologen assembly were investigated. The films were characterized by UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and sheet resistance measurements. The use of thin films of PANI on the transparent LDPE substrate allows the use of UV-visible absorption spectroscopy for easy and rapid monitoring of the changes occurring in the PANI during the course of the reaction.
47 Chapter 3
3.2 Experimental
Materials
4-vinyl benzyl chloride (VBC) (97%), dichloro-para-xylene, 4,4’-bipyridine, and anhydrous 98% hydrazine were obtained from Aldrich Chemical Co. The aniline monomer (99.5%), dimethylformamide (DMF) was obtained from Merck Chemical Co.
Low density polyethylene film (LDPE, 0.125mm in thickness) was from Goodfellow
Inc. (U.K.)
Sample preparation
The LDPE films were first washed in acetone in an ultrasonic bath for 1 hour to get rid of surface impurities and then dried under dynamic vacuum in a desiccator. The cleaned films, cut into strips of 2cm×4cm, were treated by argon plasma for 60s on both sides, using an Anatech SP100 plasma system. The plasma power applied was kept at 36 W. These plasma treated films were exposed to air for 5~10 minutes before proceeding with the graft copolymerization experiments. This results in the formation of the surface oxide and peroxide groups, which were utilized for initiating the graft copolymerization of VBC onto the plasma treated LDPE films (Suzuki et al., 1986;
Fujimoto et al., 1990).
The graft copolymerization of VBC with LDPE was carried out as shown in part (a) of
Figure 3.1. A small amount of VBC monomer was placed on both surfaces of the argon plasma pretreated LDPE film, and then the films were sandwiched between two
48 Chapter 3
LDPE Substrate
Ar plasma pretreatment followed by exposure to air
H OH
Formation of surface oxide (a) and peroxide species
VBC monomer UV irradiation VBC units
VBC grafted LDPE
DMF solution of dichloro-p-xylene Viologen units and bipyridine, 60°C, 20 hr (b)
Viologen grafted moieties
HClO4 solution of aniline
and (NH4)2S2O8, 0°C, 2 hr, followed by undoping with NaOH solution PANI (c) coating
PANI-viologen on LDPE PANI-VBC on LDPE
Figure 3.1 Schematic diagram illustrating the process of Ar plasma treatment, grafting of VBC and viologen, and deposition of the PANI coating on LDPE films.
49 Chapter 3 pieces of quartz plate. This assembly was inserted into a Pyrex tube and then exposed to UV-irradiation from a 1 kW high-pressure Hg lamp in a Riko rotary photochemical reactor (RH400-10W) at 24~28°C for either 30 minutes or 40 minutes to achieve different VBC graft concentrations. The graft-copolymerized films were extracted from the quartz plates after soaking in DMF in an ultrasonic bath for 1 hour and then washed thoroughly with DMF to remove the VBC homopolymer.
The viologen moieties were introduced via the reaction of the VBC-graft copolymerized films with an equimolar mixture of dichloro-para-xylene and
4,4’-bipyridine (0.06M of each) in DMF at 60°C for 20 h (part (b) of Figure 3.1), which is similar to the preparation of ionene-type viologen polymers (Factor and
Heinsohn, 1971). The reacted films were washed with DMF followed by deionized water to remove unreacted reactants and homopolymers. The viologen-grafted films were finally dried under reduced pressure.
The coating of PANI onto the viologen-grafted and VBC-grafted LDPE films was accomplished by immersing the films into a mixture of 0.1M aniline and 0.025M
(NH4)2S2O8 in 1M HClO4 for 2 hours at 0°C (part (c) of Figure 3.1). The films were then washed with 1M HClO4 solution for 1 hour followed by dipping into 0.5M NaOH for 1 hour to obtain the EB-coated films (MacDiarmid, 1993). Finally, these films were washed with doubly-distilled water and dried under reduced pressure to obtain the
PANI-VBC and PANI-viologen films. The PANI-coated films can also be undoped by directly washing the films in doubly-distilled water for 4 hours. The EB-coated films
50 Chapter 3
were further treated with 0.01M (NH4)2S2O8 for 10 minutes to obtain PANI coatings of intrinsic oxidation state greater than 50% (denoted as NA for nigraniline), or with 50 vol% hydrazine in doubly-distilled water for 15 h to obtain the fully-reduced leucoemeraldine (LM) coatings. These films were finally dried under reduced pressure.
The oxidant of (NH4)2S2O8 is not expected to have an effect on the viologen, while the viologen is reduced to the radical cation state upon treatment with hydrazine. However, upon exposure to air the radical cations very rapidly reoxidized to the dications.
Testing and Characterization
The irradiation of the PANI-VBC and PANI-viologen films was performed at a temperature between 24°C and 28°C, using a 1 kW Hg lamp in the Riko rotary reactor.
The samples were placed in a Pyrex tube and positioned 5cm from the light source. For investigations on the effect of light intensity on the samples, optical density filters
(from Ealing Corp.) were used to reduce the transmission by 50% and 90% respectively.
The UV-visible absorption spectra of the films were obtained using a Shimadzu
UV-3101 PC scanning spectrometer, with pristine LDPE films as reference. XPS analysis of the films was made on an AXIS HSi spectrometer (Kratos Analytical Ltd.) using the monochromatized Al Kα X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The anode voltage was 15 kV and the anode current was 10 mA. The pressure in the analysis chamber was maintained at
51 Chapter 3
5.0×10-8 Torr or lower during each measurement. The films were mounted on the standard sample studs by means of double-sided adhesive tape. The core-level signals were obtained at a photoelectron take-off angle of 90° (with respect to the sample surface). All binding energies (BE’s) were referenced to the C 1s hydrocarbon peak at
284.6 eV. In the peak synthesis, the linewidth (full width at half maximum or FWHM) of the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak-area ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to ±5%. The elemental sensitivity factors were determined using stable binary compounds of well-established stoichiometries. Sheet resistances of the films were measured using the standard two-probe method (conductivity=1/(Rs×thickness of the film)) (Jaeger, 1993). The reported value is the average of 2 or more measurements, and for the more conductive samples (Rs < 105 Ω/sq), the relative error ranges from
10% to 50%.
To get a qualitative indication of how well the PANI coatings adhere to the viologen-grafted LDPE surface, peel tests were conducted by means of pressing a piece of Scotch tape firmly onto the films surface and peeling it off with a perpendicular motion. In the dedoping experiments where irradiated EB-viologen films were immersed in water, ion chromatography (Metrohm AG) was used to determine the amount of the chlorine ions present in water as a function of treatment time. The analytical column (Metrosep Anion Dual 2 (6.1006.100)) enabled the measurement of
52 Chapter 3 inorganic anions such as chloride, nitrite, nitrate, and sulfate based on their different retention times in the column.
53 Chapter 3
3.3 Results and Discussion
3.3.1 LDPE graft-modified with VBC and viologen
After the pristine LDPE film was graft copolymerized with VBC, it becomes less transparent. After the introduction of the viologen moieties onto the VBC-grafted film, the film becomes pale yellow. The success of surface graft copolymerization of
VBC and viologen on the LDPE substrates is ascertained by XPS analysis as shown in
Figure 3.2. The C 1s core level spectra of the VBC-grafted film (Figure 3.2(a)) show a dominant peak at 284.6 eV representing the C-H bonds and a smaller peak at a BE of
286.2 eV attributed to C-O (Moulder et al., 1992). For the VBC graft-copolymerized
LDPE film, the Cl 2p core-level spectrum (Figure 3.2(b)) shows a spin-orbit split doublet (Cl 2p3/2 and Cl 2p1/2) with peaks at 200.2 and 201.7 eV attributable to covalent Cl (Neoh et al., 1993b; Moulder et al., 1992; Kang et al., 1992). Based on the
XPS analysis, the VBC graft copolymer density defined as moles of VBC per mole of
LDPE repeat unit can be calculated as follows:
Peak area of Cl 2p MVBC = MEthylene (Peak area of total C 1s - 9×Peak area of Cl 2p)/2
where the peak areas have been corrected with the appropriate sensitivity factors and the stoichiometric factors of 9 and 2 are introduced to account for the nine C atoms per
VBC unit and two C atoms per repeating units of the LDPE substrate. The graft
54 Chapter 3 concentration as calculated above is used as a basis for comparing the results obtained under different conditions.
(a) C 1s (b) Cl 2p
C-H,C-C -Cl
C-O
282 284 286 288 290 (d) -Cl Cl 2p (c) N 1s + N Cl- N• -N=
Intensity (arb. units)
396 398 400 402 404 194 196 198 200 202 204
Binding Energy (eV)
Figure 3.2 XPS (a) C 1s and (b) Cl 2p core-level spectra of VBC-graft copolymerized LDPE film (Sample VBC-2), (c) N 1s and (d) Cl 2p core-level
spectra of viologen-graft modified LDPE film (Sample Vio-2).
For viologen-grafted films, a N 1s signal was observed, indicating the existence of the pyridine rings. The N 1s core-level spectra of the viologen-grafted films can be deconvoluted into three peaks with the binding energies at 398.6 eV, 399.5 eV and
401.7 eV (Figure 3.2(c)). The first peak at 398.6 eV suggests the presence of unreacted imine nitrogen of the pyridine rings (Camalli et al., 1990). The peak at 399.5 eV is attributed to the viologen radical cation formed during X-ray excitation in the XPS
55 Chapter 3 analysis chamber, and the peak at the highest binding energy is attributed to the positively charged nitrogen of the diquaternized bipyridine molecules (Alvaro et al.,
1997). The corresponding Cl 2p core-level spectra of the viologen-grafted films are resolved into two-spin-orbit split doublets with binding energies for the Cl 2p3/2 peaks at 197.1 and 200.1 eV (Figure 3.2(d)) attributable to ionic (Cl-) and covalent (-Cl) chlorine species, respectively (Neoh et al., 1993b; Moulder et al., 1992; Kang et al.,
1992). The amount of viologen grafted on the LDPE surface can be qualitatively inferred from the [N]/[C] ratio and the ratio of the covalent Cl ([-Cl] /[ClT]) of the films. An idealized situation would be for all the VBC units to react with bipyridine and dichloro-para-xylene to form the viologen moieties. A high [N]/[C] ratio would indicate that a large number of the bipyridine units was successfully introduced, whereas a high [-Cl] /[ClT] ratio would imply that a number of VBC graft copolymer units did not react with the bipyridine units although some dichloro-para-xylene serving as end groups would also contribute to the covalent chlorine species.
The surface composition of the VBC-graft copolymerized and viologen-graft modified
LDPE film as determined from the XPS analysis are summarized in Table 3.1. It can be seen from this Table that an increase in the UV-induced graft copolymerization time of VBC from 30 to 40 min results in a very substantial increase in the VBC graft density. The higher VBC graft density in turn results in more viologen moieties being grafted in step (b) of Figure 3.1. This is evidenced by the higher [N]/[C] ratio and lower [–Cl]/[ClT] ratio in Sample Vio-2 compared to Sample Vio-1 (Table 3.1).
56 Chapter 3
0.03 0.21 0.03 0.21 VBC density grafting /N T ------1.02 1.01 Cl bonded Cl. y N/C ------0.017 0.046 t modified LDPE film T 1.0 1.0 0.68 0.52 -Cl/Cl Surface Composition* /C T 0.013 0.056 0.018 0.047 Cl merized and viologen-graf ecies while –Cl denotes covalentl p e at e at DMF DMF with with pyridin pyridin i i ondition quimolar quimolar C C e e ° ° f f 60 60 denotes total Cl s for 20 hr for 20 hr T
ylene and b ylene and b BC-2 treated BC-1 treated for 40 min for 30 min Synthesis C solution o solution o ro-p-x ro-p-x sis. Cl y Ar plasma pretreatment for 60s Ar plasma pretreatment for 60s dichlo dichlo anal UV-induced graft copolymerization UV-induced graft copolymerization Sample V Sample V XPS Surface composition of VBC-graft copoly
from Vio-1 Vio-2 VBC-1 VBC-2
mined PE Table 3.1 Sample deter D film * As VBC-graft LDPE film viologen-graft copolymerized modified L
57 Chapter 3
Figure 3.3 shows the UV-visible spectrum of VBC-graft copolymerized LDPE film
(Sample VBC-2) and viologen-graft modified LDPE film (Sample Vio-2) before and after irradiation for 30 minutes. From Figure 3.3(a), it can be seen that there is no detectable difference between the spectra of the VBC-graft copolymerized LDPE film before and after irradiation. However, for viologen-grafted film (Sample Vio-2), the
4 (a) VBC-LDPE
Before UV irradiation 2 After UV irradiation
. units)
b 0 r a
ce ( 4 (b) Viologen-LDPE an rb
2
Abso
0
400 600 800 1000 Wavelength (nm)
Figure 3.3 UV-visible absorption spectra of (a) Sample VBC-2 and (b) Sample Vio-2 before irradiation and after irradiation for 30 min.
58 Chapter 3 film turns blue after UV-irradiation in an evacuated quartz cell and a peak is observed at around 610 nm as shown in Figure 3.3(b). This peak is due to the highly colored radical cation produced by the transfer of one electron from the counter anion (Cl-) to viologen dication during photo-irradiation (Kamogawa and Nanasawa, 1993).
However, bleaching of the color is very fast once the film is exposed to air, and the film will return to the dication state. The sheet resistances of the viologen graft copolymerized film before and after irradiation and bleaching were similar at about
2.7×107 Ω/sq, while the VBC graft copolymerized film remains insulating (>1010 Ω/sq) before and after irradiation. These results will be compared to those of the PANI-VBC film and PANI-viologen film below.
3.3.2 Photo-induced doping of PANI films in EB state
The PANI-VBC film and PANI-viologen films were blue in color, as expected of polyaniline in the insulating EB state which was coated on the film surface. Upon exposure of these films to the irradiation from the 1kW Hg lamp, the color of these films changed to green and this gives the first indication of the conversion of the PANI from the insulating base state to the conducting salt state. In the following discussion, the PANI-VBC films prepared from film VBC-2 without UV irradiation and after irradiation for 2 hrs were denoted as PANI-VBC-2A and PANI-VBC-2B, respectively.
Similarly, the PANI-Viologen films prepared from film Vio-2 without UV irradiation and after irradiation for 2 hrs were denoted as PANI-Vio-2A and PANI-Vio-2B, respectively. The UV-visible absorption spectra of these films before and after
59 Chapter 3 irradiation are illustrated in Figure 3.4(a) and 3.4(b). The absorption spectra of EB films before irradiation is expected to show two absorption peaks at around 325 nm and 620nm, which are due to π-π* transition of the benzenoid rings and exciton absorption of the quinoid rings, respectively (Chen and Lin, 1995). For the PANI-VBC
0 min (a) 4 10 min 20 min 3 30 min
60 min
2 120 min
1
. units) 0 b r a
(b) ce ( 4 an rb 3
Abso
2
1
0 400 600 800 1000
Wavelength (nm)
Figure 3.4 UV-visible absorption spectra of (a) PANI-VBC film (Film PANI-VBC-2A) and (b) PANI-viologen film (Film PANI-Vio-2A), after irradiation in air for various periods of time.
60 Chapter 3 and PANI-viologen films, the band in the 325nm region would be associated with EB
(McCall et al., 1990; Cao et al., 1989) and either VBC or viologen. As can be seen from Figure 3.4 (t=0 curves), the absorbance in the 325 nm region is higher for the
PANI-viologen film than that of PANI-VBC film, consistent with the larger number of benzene rings in the former. The exciton peak is clearly seen in both spectra. Upon irradiation, there is a decrease in the exciton absorption peak intensity and the appearance of new bands at 430 nm and beyond 800 nm. These two new bands are due to the formation of the polaron/bipolaron structure and is characteristic of doped and conductive PANI (Furukawa et al., 1988). Comparing Figure 3.4(a) and 3.4(b), it is evident that the photo-induced doping of PANI-viologen film is more rapid than that observable with PANI-VBC film.
The XPS N 1s and Cl 2p core-level spectra of the PANI-VBC and PANI-viologen films before and after irradiation are shown in Figure 3.5. The N 1s spectra of the
PANI-VBC and PANI-viologen films before irradiation (Film PANI-VBC-2A and
PANI-Vio-2A as shown in Figures 3.5(a) and Figure 3.5(e), respectively) have similar peak shape, which can be curve fitted with two major components with binding energies at about 398.2 eV and 399.4 eV. These peaks are assigned to the imine (-N=) and amine (-NH-) nitrogen of PANI, respectively (Tan et al., 1989; Kang et al., 1990).
The almost equal amounts of imine and amine nitrogen in the N 1s core-level spectrum of PANI-coated films before irradiation are consistent with the structure of EB. The smaller peaks at BEs greater than 400 eV can be attributed to the positively charged
61 Chapter 3
(a) N 1s (b) Cl 2p
-N= -NH-
(c) (d) Cl* -Cl + N Cl-
(e) (f)
Intensity ( arb.units)
(g) (h)
396 398 400 402 404 194 196 198 200 202 204
Binding Energy (eV)
Figure 3.5 N 1s and Cl 2p core-level spectra of (a, b) PANI-VBC film before irradiation (Film PANI-VBC-2A); (c, d) PANI-VBC film after irradiation in air for 2 hours (Film PANI-VBC-2B); (e, f) PANI-viologen film before irradiation (Film PANI-Vio-2A); (g, h) PANI-viologen film after irradiation in air for 2 hours (Film PANI-Vio-2B).
62 Chapter 3 nitrogen on the PANI backbone possibly due to some surface oxidation. From Figures
3.5(b) and 3.5(f), the chlorine signal of the PANI-VBC and PANI-viologen films was found to be insignificant before irradiation. It implies that the PANI coating is thicker than about 7.5nm, which is regarded as the probing depth of the XPS technique in an organic matrix (Tan et al., 1993). This corroborates the similarity of the spectra in
Figures 3.5(a) and 3.5(e) with that of EB, i.e. the contribution from the viologen N is not significant in Figure 3.5(e).
After irradiation of the PANI-VBC and PANI-viologen films for 2 hours, there is an increase in the intensity of the Cl 2p signals as shown in Figures 3.5(d) and 3.5(h). The
Cl 2p core-level spectra of these irradiated films, Film PANI-VBC-2B and
PANI-Vio-2B, can be deconvoluted into three spin-orbit split doublets (Cl 2p3/2 and Cl
2p1/2), with the BE for the Cl (2p3/2) peaks at about 197.1 eV, 198.6 eV, and 200.2 eV.
The first component can be assigned to the ionic chlorine (Cl-), the third to the covalent chlorine (-Cl), while the second is assigned to the intermediate chloride species, Cl*, which has been widely observed (Mirrezaei, et al., 1988; Tan et al., 1991;
Dannetun et al., 1991; Kang et al., 1991). This intermediate chloride species is associated with anionic chloride species resulting from the charge-transfer interactions between the halogen and the metal-like conducting state of the polymer chain (Zhao et al., 2000b). In both cases, more than 50% of the chlorine exists as the (Cl-+Cl*) species. Zhao et al. (2000a) reported earlier that PANI reacts with alkyl halide in organic solvents resulting in N-alkylation of the imine units. This reaction can be
63 Chapter 3 classified as an aliphatic nucleophilic substitution, whereby the imine N is converted to
N+ and halide ions are formed in the process. It is postulated that a similar reaction takes place in the PANI-VBC film under irradiation. Since the imine nitrogen is preferentially attacked by alkyl halides, the chloride ions formed during the alkylation then serve as the counterions to the N+ components of PANI, resulting in the doping of
PANI (Zhao et al., 2000b). A comparison of the N 1s core-level spectra of Film
PANI-VBC-2A (Figure 3.5(a)) and Film PANI-VBC-2B (Figure 3.5(c)) shows a decrease in the proportion of imine N and a simultaneous increase in the proportion of positively charged nitrogen after irradiation, thus supporting this postulate. The increase in Cl 2p signal observed after irradiation may be due to the greater electrostatic interaction between the Cl- anions and the positively charged PANI resulting in the PANI coating “sinking in”, and/or the migration of the Cl- anions from the inner VBC-graft copolymer layer towards the outer PANI layer. It should be noted that the ratio of (Cl-+Cl*)/N+ is substantially less than 1 (Table 3.2). Since charge neutrality is expected to be maintained, it is postulated that a portion of the N+ is balanced by some negative oxygen species.
In the case of the PANI-viologen film, a comparison of the N 1s core-level spectra before and after irradiation also shows a decrease in the proportion of imine N and a simultaneous increase in the proportion of positively charged nitrogen after irradiation as shown in Figures 3.5(e), 3.5(g) and Table 3.2, again confirming the formation of charge carriers in the PANI coating of the irradiated film. It should be noted from
64 Chapter 3
5 4 10 10 10 10 × × /sq.) 10 10 Ω Sheet ( 2.4 3.0 Resistance +
0.54 1.16 ------+Cl*)/N - (Cl /N T 0.18 0.42 ------Cl
T 0.66 0.84 ------+Cl*)/Cl - (Cl T coated with PANI) 0.34 0.16 ------2 -Cl/Cl PANI-VBC and PANI-viologen films. /N e Vio-2 coated with PANI) + 0.10 0.22 0.12 0.30 N ple VBC- m (Sampl
(Sa 0.43 0.59 0.44 0.58
-NH-/N
0.47 0.19 0.44 0.12 -N=/N
Surface composition and Rs of PANI-VBC Film
PANI-Viologen Film Nil Nil ation for 2 hrs ation for 2 hrs Table 3.2 Treatment Conditions After irradi After irradi
A B Film PANI-Vio-2 PANI-Vio-2 PANI-VBC-2A PANI-VBC-2B
65 Chapter 3
Table 3.2 that there is also an increase in the proportion of amine groups after irradiation of the PANI-VBC and PANI-viologen films. This may be due to the reduction of some imine groups of the PANI to amine groups. In the case of the
PANI-viologen film, a greater amount of viologen radical cations may also be detected due to the change in the arrangement of the PANI and viologen chains after irradiation as a result of electrostatic interactions as mentioned above. The XPS data is consistent with the UV-visible absorption spectra (Figures 3.4(a) and 3.4(b)), which show a decrease in the exciton peak due to the quinoid rings after irradiation.
The results in Figure 3.5(e) and 3.5(g) were those from PANI coated films which were undoped by NaOH. As mentioned in Section 3.2, some PANI coated films were also undoped using distilled water. The XPS spectra of these films before and after UV irradiation are similar to Figure 3.5(e) and 3.5(g), respectively. This indicates that the methods of undoping did not affect the photoinduced PANI-viologen reactions.
The conversion of Film PANI-VBC-2A and Film PANI-Vio-2A from the base state to the doped state results in a decrease in their sheet resistance, Rs, as shown in Figure
3.6. Since the Rs values of the VBC-graft copolymerized LDPE film (Sample VBC-2) and the viologen-graft modified LDPE film (Sample Vio-2) do not change significantly upon irradiation as mentioned above, the decrease in Rs of Film
PANI-VBC-2A and PANI-Vio-2A after irradiation confirms that the doping of PANI resulted from the reaction of PANI with alkyl chloride and viologen, respectively.
66 Chapter 3
10 PANI-VBC PANI-viologen 9
8
7
6 Log (Rs, in ohms/sq.) 5
4 0 20 40 60 80 100 120 Treatment Time (min)
Figure 3.6 Sheet resistance (Rs) of PANI-VBC film (Film PANI-VBC-2A) and PANI-viologen film (Film PANI-Vio-2A) after irradiation in air for various periods of time.
After 30 minutes of irradiation, the sheet resistance of Film PANI-Vio-2A decreases sharply from ~ 1010 to 5×104 Ω/sq, compared with Film PANI-VBC-2A in which a Rs of 107 Ω/sq is achieved during the same period of irradiation. The slower rate of conversion of the PANI from the insulating to conducting state in the PANI-VBC film as compared to the PANI-viologen film is consistent with the UV-visible absorption spectroscopy results in Figures 3.4(a) and 3.4(b) and the difference in the N+/N ratios
(N+/N =0.30 for PANI-viologen film (Film PANI-Vio-2B) versus N+/N =0.22 for the
PANI-VBC film (Film PANI-VBC-2B)). In Figure 3.4, the long absorption tail
67 Chapter 3 extending beyond 800 nm into the infrared region is attributed to the polaron band structure of the PANI as it undergoes doping (Cao et al., 1989). It can be seen from
Figure 3.4(b), the highest degree of change in this region is during the first 20 min of irradiation, which also corresponds to the sharpest drop in Rs in Film PANI-Vio-2A
(Figure 3.6). After this initial period, the spectral changes in the region >800 nm for
Film PANI-Vio-2A is minimal, and the changes are essentially in the region <800 nm, which may reflect some changes in the electronic structure of the PANI that do not have a strong effect on Rs. It should also be recognized that the UV absorption spectroscopy technique is highly sensitive and the spectrum is representative of the structure of the entire film (surface and bulk) whereas the Rs measurement is more limited to the surface region. In the case of Film PANI-VBC-2A, the polaron band structure and Rs continue to change substantially even after 30 min (Figures 3.4(a) and
3.6). The results presented above clearly show that the photo-induced doping of PANI is more effectively achieved with viologen than alkyl halide. The Rs value of 5×104
Ω/sq is about 1 order of magnitude higher than that obtained with a PANI (EB state) film protonated with 1M HClO4 (Zhao et al., 2000b).
(i) Effect of UV intensity.
The rate of decrease in Rs of the PANI films provides a convenient indication of the conversion of the PANI from the insulating EB state to the doped state. The effect of the UV intensity on this process is shown in Figure 3.7. When the light intensity was reduced by 90%, neither Film PANI-Vio-2A nor Film PANI-VBC-2A shows any sign
68 Chapter 3 of doping after 2 hours of irradiation and the Rs of these films remains at about
1010Ω/sq. With 50% attenuation of the light intensity, the Rs of Film PANI-VBC-2A decreases to only 3×108Ω/sq after 20 minutes of irradiation and no further reduction was observed. In contrast, the Rs of Film PANI-Vio-2A shows a steady decrease and after 2 hrs of irradiation the Rs is similar to that achievable within 30 minutes without attenuation of the light intensity. Thus, Figure 3.7 shows that the intensity of the UV irradiation is an important factor in determining the extent of conversion of PANI from the insulating to the conducting state, especially in the reaction with VBC.
69 Chapter 3
10 (a) 8
6 A=0% A=50% 4 A=90%
10 (b) 8 Log (Rs, in ohms/sq)
6
4
0 20406080100120
Treatment Time (min)
Figure 3.7 Sheet resistance (Rs) of (a) PANI-VBC film (Film PANI-VBC-2A) and (b) PANI-viologen film (Film PANI-Vio-2A) after exposure to irradiation of different intensities in air for various periods of time. A is the degree of attenuation of the irradiation.
70 Chapter 3
(ii) Effect of VBC graft density
The VBC graft density can be varied by changing the UV irradiation time in the Riko
UV reactor in part (a) of Figure 3.1. The VBC graft density increases substantially when the time of UV irradiation is increased from 30 min to 40 min (Table 3.1). A higher VBC graft density also allows for more viologen moieties to be grafted as indicated by the higher [N]/[C] ratio in Table 3.1. A comparison of the Rs of these 2 sets of films (Films PANI-VBC-1A, 2A and Films PANI-Vio-1A, 2A) upon irradiation is given in Figure 3.8(a). As can be seen from this figure, the VBC density has a larger effect on the Rs of PANI-VBC film as compared to the PANI-viologen film, especially in the initial stages of irradiation. After 2h of irradiation, the Rs values obtained are very similar even though the VBC graft density differs by a factor of 7.
This indicates that even at a graft density of 0.03 a sufficient number of VBC groups are available to achieve alkylation of the thin PANI layer. Similarly, the number of viologen moieties grafted at a VBC graft density of 0.03 is also sufficient to achieve the doping of PANI. This relative insensitivity of the PANI doping process to the number of VBC and viologen groups is probably due to the interaction of the PANI with the VBC and viologen groups being confined to the interfacial region only. The results presented were obtained with thin PANI films. If the PANI layer is significantly thicker, the effectiveness of the photo-induced doping method would be diminished for two reasons. Firstly, the confinement of the VBC and viologen groups to the interfacial region may restrict the doping process to this region and the surface of the PANI film may not achieve a similar conductivity level. Secondly, a thicker PANI layer may
71 Chapter 3 significantly attenuate the intensity of the irradiation, and as shown in Figure 3.7, the intensity of the irradiation is an important factor in determining the level of conductivity achieved.
10 (a) VBC-PANI
VBC Density =0.03 8 VBC Density =0.21
6
4
10 (b) Viologen-PANI
Log (Rs, in ohms/sq) 8
6
4 0 20 40 60 80 100 120
Treatment Time (min)
Figure 3.8 Comparison of the sheet resistance (Rs) of (a) PANI-VBC film (Films PANI-VBC-1A, 2A) and PANI-viologen film (Films PANI-Vio-1A, 2A), of different VBC graft densities after irradiation in air for various periods of time.
72 Chapter 3
3.3.3 Photo-induced doping of PANI films in other oxidation states
The XPS N 1s core-level spectrum of the NA-viologen film shows an imine proportion of 0.69 (Figure 3.9(a)), indicating that the PANI coating is of an oxidation state substantially higher than emeraldine base. The UV-visible absorption spectrum of the film consists of an absorption peak at approximately 560nm (Figure 3.10(a)), further confirming the high oxidation state of the coating (Albuquerque et al., 2000) since this peak has earlier been reported to be due to transitions from the valence to the conduction band of pernigraniline (Cao, 1990). The absorption band around the 300nm region is again attributed to both viologen and PANI (Cao, 1990) (Figure 3.10(a)). The irradiation of the NA-viologen film results in a decrease in absorption of the 560nm peak and an increase in absorption of the 430nm peak and in the near-IR region. The two new peaks, as previously seen in the case of doping of the EB-viologen films, are characteristic of the metallic behavior of PANI. Also similar to the XPS analysis of the
EB-viologen film, the N 1s core-level spectrum of the NA-viologen film shows a significant increase in the proportion of N+ with a simultaneous decrease in the proportion of imine N, as well as an increase in the proportion of amine N after irradiation (Figure 3.9(b), Table 3.3). The doping of the NA-viologen film proceeds at a slower rate than that of EB-viologen film and the sheet resistance (Rs) of the former decreases from > 1010 Ω/sq. to 105Ω/sq. in 2 h, whereas a similar decrease in Rs of the latter can be achieved in less than 30 min.
73 Chapter 3
(a) N 1s (c) N 1s
-N=
) -NH N+ arb. units
( (b) N 1s (d) N 1s
y Intensit
398 400 402 398 400 402
Binding Energy (eV)
Figure 3.9 XPS N 1s core-level spectrum of NA-viologen film a) before irradiation and b) after 2h of irradiation in air; as well as LM-viologen film c) before irradiation d) after 90 min of irradiation in air.
Table 3.3 Surface composition of NA and LM-coated films before irradiation and after 90 min and 120 min of irradiation, respectively.
+ -N=/N -NH-/N N /N Before irradiation:
NA 0.69 0.18 0.13 LM 0.06 0.82 0.12
After irradiation: NA 0.19 0.35 0.46
LM 0.10 0.63 0.28
74 Chapter 3
4 a) 0 min 15 min 3 30 min 60 min 120 min 240 min 2
1 . units) b r a 3 b) ce ( Vacuum an released rb 15 min 30 min 2 90 min Abso
0 min
1 420 620 820 iii) ii) i) 0 200 400 600 800 1000
Wavelength (nm)
Figure 3.10 UV-visible absorption spectra of a) NA-viologen film irradiated in air for various periods of time b) LM-viologen film irradiated in air for i) 0 min, ii) 30 min and iii) 90 min; and inset: LM-viologen film irradiated in vacuum.
75 Chapter 3
The UV-visible absorption spectrum of a LM-viologen film, on the other hand, shows only one absorption band around the 300nm region again attributed to both PANI
(Neoh et al., 1993b; Albuquerque et al., 2000) and viologen (Figure 3.10(b)). The absence of peaks around the 600nm region is consistent with what is expected for a fully reduced PANI (Neoh et al., 1993b; Albuquerque et al., 2000). The XPS N 1s core-level spectrum of the film confirms that the predominant N species is the amine group (Figure 3.9(c)). When the LM-viologen film is exposed to UV irradiation in air, a small peak at approximately 620nm first emerges. This peak is characteristic of the exciton-like transitions from the benzenoid to quinoid rings and indicates the formation of imine groups. Upon further irradiation, spectral changes similar to those of the irradiated EB-viologen film are observed, i.e. the 620nm peak disappears while the 430nm peak and the IR-tail emerge. The N 1s core-level spectrum of the irradiated film shows an increase in the proportion of imine N and positively charged nitrogens, with a decrease in the proportion of amine N (Figure 3.9(d), Table 3.3). The Rs of the
LM-coated viologen-grafted film decreases from >1010 to 5 x 105Ω/sq. after 2 h of irradiation. The Rs of the film after 2h of irradiation is approximately 1 order of magnitude higher than that of the irradiated EB-viologen film after the same irradiation period. When a LM-viologen film was placed in a quartz cell under vacuum and subjected to irradiation, neither the 620nm peak due to imine groups nor the 430nm peak and IR-tail were observed. Instead, a spectrum characteristic of the viologen radical cation (with a peak at 610nm (Kamogawa et al., 1984)) is observed (inset of
Figure 3.10(b)). The irradiated film is also observed to turn slightly bluish in the
76 Chapter 3 vacuum cell. Prolonged exposure of the film to irradiation resulted in a decrease in absorption of this radical peak, but no indication of the doping of the LM-viologen film can be observed. The Rs of the film remained at >1010Ω/sq. even after 90 min of irradiation in vacuum.
These results suggest that the viologen reacts more favorably with emeraldine than with nigraniline or leucoemeraldine. A two-step mechanism is proposed for these reactions. The first step may involve a ring-opening reaction (of the viologen) resulting from the attack of the viologen with the amine nitrogen of PANI, similar to the mechanism whereby 2,4-dinitrophenyl-pyridilium chloride undergoes a ring-opening reaction with aniline, as reported by Marvell et al. (1970). The hydrochloric acid formed in the process serves as the dopant to react with the imine nitrogen, resulting in the formation of polaron/bipolarons. As will be shown in Section 3.3.4.3 on the treatment of the doped polyaniline (after reaction with viologen) with water, HCl is like to be the dopant of the polyaniline. In the case of NA, since the proportion of amine is less than that of EB, the extent of reaction will be less than that with EB, which may contribute to the slower rate of doping and lower conductivity of the
NA-viologen films. On the other hand, when LM-viologen films are irradiated in air, imine N may have formed through the auto oxidation of LM in air (Neoh et al.,1993b;
Kang et al., 1990), and is observed as the initial emergence of the 620nm peak in
Figure 5b. These imine nitrogens then could react with the hydrochloric acid formed in the interaction of viologen with PANI, resulting in the doping of LM-viologen film.
77 Chapter 3
Similarly, the conductivity is not as high as that achievable with EB-viologen films due to the smaller amounts of imine nitrogens. In the absence of oxygen, the formation of imine groups in the LM layer is retarded. Without the imine nitrogens, there would be no acceptor for the hydrogen ion formed in the ring-opening reaction. According to the mechanism proposed by Marvell et al. (1970), the ring-opening reaction is reversible and with no acceptor for the hydrogen ion, the ring-opening reaction may not occur to a significant extent in the LM-viologen film. Thus the predominant reaction upon irradiation of the LM-viologen film is the conversion of the viologen dications to the radical cations as shown by the blue colour and the 610nm peak. This would explain why the LM-coated films remains insulating even after 90 min of irradiation in vacuum. The subsequent decrease in absorbance of the 610nm peak upon prolonged irradiation of the film in vacuum may be due to the conversion of the radicals to other viologen species. An increase in absorbance around the 400nm region is observed after 90 min of irradiation (see inset of Figure 3.10(b)). This increased absorbance of the irradiated film in the 400nm to 500nm region, as compared to the film before irradiation, persists even after the release of vacuum. The film was also visually observed to turn faintly orange. Observations of the irreversible formation of the yellowish-brown direduced viologen (Monk et al., 1992; Monk, 1997; Belinko,
1976) in electrochromic display devices, and the appreciable formation of the orangey red viologen dimers in solid state reactions (Monk, 1997) support the possibility that the decrease in the viologen radicals is caused by these side reactions during irradiation.
78 Chapter 3
3.3.4 Stability of films
3.3.4.1 Adhesion tests
Peel tests performed on the EB-viologen films prior to irradiation indicate fairly good adhesion of PANI to the viologen-grafted films. A comparison of the peel test results from coatings of PANI on pristine LDPE, 90s argon plasma pretreated LDPE and two viologen-grafted LDPE films with different extents of viologen grafting (these films were prepared according to the method in the literature (Ng et.al, 2001b)) is shown in
Figure 3.11. The PANI coating on pristine LDPE is easily removed when a piece of
Scotch tape is applied and then peeled off.
Figure 3.11 Scotch tape surfaces from peel tests performed on EB-coatings on a) pristine LDPE, b) plasma treated LDPE, c) viologen grafted LDPE with [N]/[C]=0.05 (Film A) and d) viologen grafted LDPE with [N]/[C]=0.08 (Film B).
79 Chapter 3
On the other hand, adhesion of the PANI coating to the plasma pretreated LDPE is very much better, with minimum peeling. The adhesion of the PANI coating on viologen-grafted films is not as strong as that achievable with the plasma pretreated substrate and the adhesion is also dependent on the viologen graft density. The peel test results obtained with two EB-viologen films prepared with different viologen densities, Film A and Film B, are compared in Figure 3.11. The [N]/[C] ratio of Film
A before coating with PANI is 0.05, and the corresponding value for Film B is 0.08.
The adhesion of the PANI coating on the film with a higher viologen graft density
(Film B) is observed to be better possibly due to the greater interaction between the
PANI and the viologen polymers. Entanglement between the viologen chains and
PANI polymer provides sufficient adhesion of the PANI coating onto the film, thus ensuring that the PANI coating will not be easily removed by physical means.
3.3.4.2 Stability of irradiated films in air and water.
The monitoring of photo-irradiated EB-viologen films left at room conditions (28°C, relative humidity of 85% and under normal room lighting) over a period of 4 months shows that the conductivity of the films remains high even after the cessation of irradiation (Figure 3.12(a)). An initial increase in Rs of approximately 1 order of magnitude is observed within the first 2 weeks after the removal of the irradiation source. Subsequently, however, the Rs of the films was observed to stabilize. Similar photo-doped films kept in the dark show a similar trend (Figure 3.12(b)). However, films which were kept in the dark and away from moisture in a dessicator show the
80 Chapter 3 greatest increase in Rs over the 4 month period (Figure 3.12(c)). An irradiated
EB-viologen film heated to 75°C for 24 h in air also shows little change in Rs. The Rs of the film before heating was 2 x 105Ω/sq., and the Rs of the thermally-treated film cooled to room temperature was 3 x 105Ω/sq, indicating that the photo-doped films are also heat stable. While the irradiated films are stable in air up to 75°C, they rapidly undope when placed in water (as shown in the next section).
7
c)
6
q.)) b) /s Ω a)
5 Log (Rs(
4 0 2 4 6 8 10 12 14 16 18 Time (weeks)
Figure 3.12 Sheet resistance (Rs) of EB-viologen film exposed to irradiation for 30 min (time = 0) and subsequently left a) under room conditions, b) in the dark, c) in the dark and in a dessicator.
81 Chapter 3
3.3.4.3 Dedoping characteristics of EB-viologen film after irradiation
When the irradiated EB-viologen film is immersed into the water, the original green color of the film, corresponding to the protonated form of PANI, changes to blue rapidly, indicating the conversion of the PANI from the conducting state to the insulating state. The sheet resistance of the irradiated EB-viologen film increases to
~1010Ω/sq after 5 min immersion in water. The UV-visible absorption spectra of the conventional HCl-doped PANI and the photo-irradiated EB-viologen films before and after immersion in water for 5 minutes are very similar as shown in Figure 3.13. It is proposed that when the irradiated EB-viologen film is dipped into the water, the HCl dopants diffuse away from the PANI layer similar to the process observed with conventional HCl doped PANI.
The assumption that HCl was released into the water from the irradiated EB-coated film can be tested using ion chromatography and a pH meter. Figure 3.14 shows the changes of the concentration of chloride anions and the pH of the water calculated from the concentration of the chloride anions, assuming that HCl is released into the water after the irradiated EB-viologen film (1 or 3 pieces) has been immersed in water for different periods of time. In the first 10 min, there was a sharp increase in the Cl- concentration and a corresponding decrease in the pH value. The rapid loss of the Cl- anions is consistent with the significant increase in the Rs in the first 5 min. The pH of the water was measured with a pH meter after 20 min and the measured values are close to the values calculated from the Cl- concentration (4.72 versus 4.44 for 1 piece
82 Chapter 3
3 a) HCl-doped PANI film
0 min
2 5 min
1 . units) b r
a 0
3 b) Photo-irradiated EB-viologen film ce ( an rb 2 Abso
1
0
400 600 800 1000 Wavelength (nm)
Figure 3.13 UV-visible absorption spectra of a) a conventional HCl-doped PANI coated on LDPE film and b) the photo-irradiated EB-viologen film before and after treatment in water for 5 minutes.
83 Chapter 3 of the film, and 3.98 versus 3.89 for 3 pieces of the film). The small difference between the measured and the calculated pH values supports the assumption that HCl is involved in the photo-induced doping of PANI via reaction with the viologen.
5 7.5
7.0 4 (ppm) 6.5 n o i t 6.0 a 3 r t n 5.5
e c n 2 5.0 pH Value o C
4.5 - 1 Cl 4.0
0 3.5 0 5 10 15 20 Treatment Time (min)
Figure 3.14 Concentration of Cl- ions and the corresponding calculated pH values as a function of treatment time in water. Open symbols denote the data obtained with 3 pieces of the irradiated EB-viologen films and solid ones for 1 piece of the film. Squares, circles, and triangles denote the Cl- ions concentrations, calculated pH values, and measured pH values, respectively.
84 Chapter 3
3.4 Conclusion
The conversion of PANI to the doped and conducting state was achieved by a new technique of photo-induced doping using viologen moieties. Photo-sensitive thin films consisting of PANI coatings on viologen-grafted LDPE substrates can be fabricated using a 3-step process whereby VBC is first graft copolymerized onto the
LDPE substrate, followed by the linking of the viologen moieties to the VBC and finally the deposition of the PANI coating onto the viologen-grafted film. The irradiation of the PANI-VBC and PANI-viologen films results in the conversion of the
PANI in the EB state from the insulating to the conducting state. But the photo-induced doping of PANI is more effectively achieved with the viologen grafted film than the VBC grafted film. The density of the grafted VBC and viologen does not play an important role in the doping of PANI under UV irradiation since the reactions are confined mainly to the interfacial region between PANI and the VBC or viologen.
But the photo-induced doping of PANI shows a strong dependence on the UV intensity. PANI with different intrinsic oxidation states ranging from LM to NA can be doped by this method as well. However, the presence of O2 is essential for the photo-induced doping of LM but not for PANI in the higher oxidation states. The mechanism is proposed to involve the ring-opening of the bipyridine rings in the viologens upon reaction with the amine group of PANI and the subsequent formation of hydrogen ion, resulting in the protonation of the imine groups of PANI with the Cl- anions from the viologen moieties serving as the counterions. Viologen-grafted LDPE
85 Chapter 3 film shows marked improvement in the adhesion with PANI as compared to the pristine LDPE. The photo-irradiated films show good electrical stability in air up to
75°C, but undope rapidly in water. The conductivity of the irradiated films decreases sharply after the films were immersed in water due to the loss of the HCl dopants, similar to the process observed in conventional protonic acid doped PANI.
86
CHAPTER 4
FLUORINATED ETHYLENE PROPYLENE
COPOLYMER COATING FOR THE STABILITY
ENHANCEMENT OF ELECTROACTIVE AND
PHOTOACTIVE SYSTEMS
87 Chapter 4
4.1 Introduction
Although electrically conductive polymers have many attractive potential applications as summarized in Section 2.2.4, the problems faced by these electroactive materials, for example, instability to air and aqueous environments limit their full scope of possible applications. PANI doped by mobile inorganic anions readily undergoes undoping when immersed in water. A similar undoping process occurs with the PANI film doped via the photo-induced reaction with viologen as shown in Section 3.3.4.3.
In basic solutions, the undoping process is even more rapid. Therefore, some investigations have been devoted to improve the electrical stability of PANI in aqueous media through either physical or chemical methods. Surface graft copolymerization of
PANI with a hydrophobic monomer is a practical way to retard deprotonation (Zhao et al., 1999). Other methods for improving the electrochemical properties of conducting polymer chemical sensors include the application of a hydrophobic polymer via spin coating or the use of adhesives (Tanaka and Doi, 1990). However, the surface conductivity of the film will decrease substantially after the coating or grafting process, and the thickness and distribution of the coating or graft layer is difficult to control.
Hence, these methods are useful only for special products. An alternate way of retarding the deprotonation of the conducting polymer is to minimize the loss of counterions from the polymer by using organic anions (e.g. sulfosalicylic acid) rather
- - than smaller and more mobile inorganic acid anions (e.g. Cl , ClO4 ) (Neoh et al.,
1994). Unfortunately the films doped with organic or polymeric anions have substantially weaker mechanical properties as well as a lower doping level and conductivity.
88 Chapter 4
In the present chapter, the use of a magnetron sputtering technique to deposit a very thin and transparent fluorinated ethylene propylene copolymer (FEP) coating on polyaniline for enhancing its electrical stability in aqueous media is illustrated. The deposition of fluorocarbon plasma polymers via radio frequency (RF) sputtering has been shown to result in films with special optical and electrical properties, and coatings for friction reduction and corrosion resistance (Rossnagel, 1995). The technique of sputtering FEP on electroactive polymers offers the advantage of requiring no solvents or pretreatment of the surfaces of the active materials, unlike the methods described earlier. In Chapter 3, we have shown that polyaniline can undergo doping via photoinduced reaction with viologens. The application of the FEP sputtering technique for such polyaniline-viologen system was also investigated and presented in this chapter. Of particular interest would be how the sequencing of the photoinduced doping and FEP sputtering steps might affect the stability enhancement.
In the third part of the present chapter, the use of sputtered FEP to prolong the photochromic effect of viologens is described. The photochromic effect of viologens is achieved when the dications are reduced to radical cations. This effect has received considerable attention for application to electrochromic displays (Schoot et al., 1973), and dosimeters (Ohtani et al., 1992). However, the radical cations of viologens readily undergo reaction with oxygen and the color rapidly bleaches in air. Hence, the FEP coating was employed as a barrier against oxygen diffusion to the viologen radical cations.
89 Chapter 4
4.2 Experimental
4.2.1 Preparation of substrates
PANI-LDPE film. LDPE films (0.125mm in thickness) obtained from Goodfellow
Inc. were washed in acetone for 30 min using an ultrasonic bath to remove surface impurities. The films were then dried under reduced pressure and cut into strips of 2cm x 4cm. The washed films were pretreated with argon plasma for 30s on both sides at an argon pressure of approximately 0.6 Torr, using an Anatech SP100 plasma system. The pretreated films were then exposed to air for approximately 5 min to facilitate the formation of surface oxide and peroxide groups. A coating of PANI on the LDPE film was obtained by immersing the film into a mixture of 0.1M aniline and 0.025M
(NH4)2S2O8 in 1M HClO4 for 2 hr at 0°C. The film was then washed with 1M HClO4 solution for 1 hr to remove the unreacted oxidant and monomer, followed by immersion in 0.5M NaOH for 1 hr. The film was then washed with doubly-distilled water and dried under reduced pressure to obtain PANI-LDPE base films.
PANI-viologen film. A polyaniline-viologen thin film assembly on LDPE (PANI- viologen film) was synthesized according to the method described in section 3.2.
Vinylbenzyl chloride (VBC) was first grafted onto the LDPE surface, followed by the introduction of the viologen moieties via the reaction of the VBC-graft copolymerized films with an equimolar mixture of dichloro-para-xylene and 4,4’-bipyridine. The grafted film was then immersed in a mixture of 0.1M aniline and 0.025M (NH4)2S2O8 in 1M HClO4 for 2 h at 0°C followed by washing with HClO4 and NaOH solution to achieve the undoped PANI-viologen film, similar to the procedures for the PANI-
LDPE film as mentioned above.
90 Chapter 4
Viologen-grafted film. Viologen grafted LDPE film was prepared by UV-induced surface graft copolymerization of viologen monomer with the LDPE film. 1,1’-bis(4- vinyl benzyl-4,4’-bipyridinium dichloride (Figure 4.1) was synthesized first via the reaction of 4,4’-bipyridine and vinyl benzyl chloride in acetonitrile according to the method reported in the literature (Liu et al., 2002). This vinyl containing viologen will be denoted as VBV in the following discussion. A 30 wt% aqueous solution of the
VBV monomer was placed on the surface of argon plasma pretreated LDPE film, followed by exposure to near UV-irradiation in a Riko rotary photochemical reactor
(RH400-10W) at 24~28°C for 20 minutes. The viologen grafted LDPE film was then washed extensively with doubly-distilled water and then dried under reduced pressure.
+ + H2CHC H2C N N CH2 CH CH2 Cl- Cl-
Figure 4.1 Structure of 1,1’-bis(4-vinylbenzyl)-4,4’-bipyridinium dichloride (VBV)
4.2.2 Radio frequency (RF) sputtering of FEP
The RF plasma sputtering process was carried out in a triple-cathode S-2000 sputtering system, assembled by Korea Vacuum Technology of Seoul, South Korea. The FEP target (film of 0.1 mm thickness, obtained from Goodfellow Cambridge Limited) was installed in the RF magnetron sputter gun powered by a Comdel CX-600S RF generator (RF=13.56 MHz). The sputtering process was pre-programmed and microprocessor-controlled. The electroactive polymer (PANI-LDPE or PANI- viologen) or photoactive (viologen-grafted LDPE) substrates were fixed on the rotary sample stage placed 13cm away from the target. The system pressure was maintained
91 Chapter 4 at 12 mTorr during the sputter deposition process by a MKS 600 automatic pressure controller. The sputter deposition process was carried out under an argon atmosphere at a RF power of 150 W for a specific period of time. For a fixed RF power and target and sample separation, the thickness of the FEP layer would be determined by the sputtering time. In order to measure the thickness of the FEP coating, the sputtering process was carried out with a glass plate substrate and the thickness was measured using an Alpha-STEP 500 Surface Profiler (KLA-Tencor Co., San Jose, CA, USA).
For a target and sample separation of 13 cm and an RF power of 150 W, the thickness of the FEP deposited on the substrate can be varied from < 10 nm for a 100s sputtering time to 16 nm for 300s, and 43 nm for 600s.
4.2.3 Stability tests
The electrical stability of the electroactive assemblies was evaluated in either doubly- distilled water or dilute NaOH solution. To assess the effectiveness of the FEP coating in enhancing the electrical stability of the electroactive systems in water, the PANI-
LDPE and PANI-viologen films (of size 2cm x 4cm) were first converted to the doped conducting state via treatment with 0.5M H2SO4 and UV irradiation, respectively.
These films with and without FEP coating were immersed in 100 mL of doubly- distilled water for varying periods of time under ambient conditions. The films were then dried before being subjected to surface and bulk characterization. The same immersion procedure was also used with NaOH solutions of different pH values. The films used were small enough relative to the volume of the solution used so that the pH of the solution can be considered as essentially constant (pH changed by < 0.01) during the course of the experiment.
92 Chapter 4
In the case of the PANI-viologen assembly, two kinds of samples were investigated—
Sample A and Sample B. Sample A is the PANI-viologen film, which was first converted to a conductive state via exposure to UV-irradiation from a 1kW mercury lamp for 1 h (as described in Chapter 3) and then sputtered with FEP for 100s. For
Sample B, the PANI-viologen film was first sputtered with FEP for 100s and then exposed to UV-irradiation for 1 h.
In the case of the FEP sputtered (100s) viologen-grafted film, UV-irradiation for 10 min was first carried out to convert the dications to the radical cations. Upon termination of the irradiation, the bleaching of the film’s coloration in air was monitored using UV-visible absorption spectroscopy.
4.2.4 Sample characterization
The surface compositions were measured using X-ray photoelectron spectroscopy
(XPS) on an AXIS HSi spectrometer (Kratos Analytical Ltd.) as described in Section
3.2. The UV-visible absorption spectra of the films were obtained using a Shimadzu
UV-3101 PC scanning spectrometer, with pristine LDPE films as reference. Sheet resistances (Rs) of the films were measured using the standard two-probe method
(conductivity=1/(Rs×thickness of the film)) (Jaeger, 1993). The reported value is the average of 2 or more measurements, and for the more conductive samples (Rs < 105
Ω/sq), the error ranges from 10% to 50%. The surface morphology of the films was investigated using scanning electron microscopy (SEM) (Jeol, JSM 5600LV) and atomic force microscopy (AFM). The AFM studies were carried out using a
Nanoscope III scanning atomic force microscope. All images were collected in air under constant force mode (scan size 10 µm, scan rate 1.0 Hz). The root-mean-square
93 Chapter 4
(RMS) roughness of the film surface was directly calculated from the whole sample images automatically. Static water contact angles measurement were made using the sessile drop method at 25°C and 65% relative humidity on a contact angle goniometer
(Model 100-00-(230)), manufactured by the Rame-Hart, Inc. of Mountain Lakes, NJ,
USA. The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each contact angle reported, six readings from different parts of the film surface were averaged. Each angle reported was reliable to ±3°.
94 Chapter 4
4.3 Results and Discussion
4.3.1 PANI-LDPE film
Figure 4.2 shows the XPS C 1s and N 1s core-level spectra of the doped PANI-LDPE film before sputtering (Figure 4.2 (a) and (b)), after sputtering with FEP for 10s
(Figure 4.2 (c) and (d)), and for 100s (Figure 4.2(e) and (f)). Before the FEP sputtering process, the C 1s core-level spectrum of the doped PANI-LDPE film consists of three peak components at BEs of 284.6ev, 286.0ev and 287.7ev, attributable to the C-C and
C-N species, C-O species, and C=O species, respectively (Moulder et al., 1992). As expected, no CF species was observed. After sputtering for 10s, a COO peak (at
288.5eV) (Moulder et al., 1992) appeared, possibly due to the reaction of the C in
- PANI with oxygen from the dopant (HSO4 ) under the plasma environment. In addition, the presence of CF species can be seen from the C 1s spectrum. An increase in the proportion of the different CF species, such as C-CFn species at 287.2ev, CF species at
289.6ev, CF2 species at 291.6ev and CF3 species at 293.6ev (Zhang et al., 2002), with sputtering time is shown in Figure 4.2.
The N 1s core-level spectrum of the PANI-LDPE film shown in Figure 4.2(b) is characteristic of that of doped PANI, with an amine peak at 399.4eV and a high-BE tail (>401ev) attributed to positively charged nitrogen (N+) (Tan et al., 1989; Kang et al., 1990). Although the high BE tail has been resolved into two peaks separated by about 1.4 and 2.9ev from the amine peak (at 399.4ev), respectively (Nakajima et al.,
1989b), it is more appropriately ascribed to positively charged nitrogen atoms with a continuous distribution of BEs, or positively charged nitrogen atoms in a large number
95 Chapter 4
(a) C 1s (b) N 1s
C-C,C-N -NH- N+ C-O
C=O
(c) (d) C-CFn COO CF CF2
CF3
(e) (f)
Intensity (arb. units)
(g) (h)
-N=
280 284 288 292 296 396 398 400 402 404 Binding Energy (eV)
Figure 4.2 XPS C 1s and N 1s core-level spectra of doped PANI-LDPE film before sputtering with FEP ((a) and (b)), after sputtering with FEP for 10s ((c) and (d)), and for 100s ((e) and (f)), and the FEP sputtered (100s) film after treatment in water for 3h ((g) and (h)).
96 Chapter 4 of different environments arising from inter- and intra-chain charge distributions
(Kang et al., 1997). The [N+]/[N] ratio of 0.51 is consistent with that reported earlier
(Kang et al., 1997). Since the PANI is doped with H2SO4, a peak was observed at
- + 168.5eV, attributable to the HSO4 anions (Barbero et al., 1994). The [N ]/[N] ratio is
- very close to the [HSO4 ]/[N] ratio of 0.52, which is expected for charge neutrality to be maintained. After the PANI-LDPE film was sputtered with FEP for 10s, the N 1s core-level spectrum (Figure 4.2(d)) shows a decrease in the proportion of amine nitrogen from 0.51 to 0.45 and a simultaneous increase in the proportion of positively charged nitrogen from 0.49 to 0.55. An increase in the sputtering time to 100s results in a further decrease in the amine component (to 0.16) and the N+ peaks become the dominant peaks ([N+]/[N]=0.84). This increase in N+ may be attributed to the generation of nitrogen radical cation as a result of the interactions between PANI film and plasma. The radicals may be generated by the abstraction of hydrogen from PANI chains (Inagaki, 1996). It can be seen that the distribution of BEs of the N+ species in
Figure 4.2(f) is rather different from that in the PANI before sputtering (Figure 4.2(b)).
Hence the environment of the N+ after the plasma sputtering process is likely to be different. At the same time, there is a substantial decrease in the intensity of S species from [S]/[N]=0.49 to [S]/[N]=0.26 after plasma sputtering for 10s, and a further decrease to [S]/[N]=0.14 after 100s. This decrease in S species may be due to the loss of dopant anions resulting from plasma bombardment on the surface of the PANI film during the sputtering process. Beyond a sputtering time of 100s, the N1s signal is no longer detectable since the thickness of the FEP layer is more than 7.5 nm, which is regarded as the probing depth of the XPS techniques in an organic matrix (Tan et al.,
1993; Briggs, 1998).
97 Chapter 4
The SEM images of the surfaces of the PANI-LDPE films before and after sputtering with FEP for 100s are shown in Figure 4.3(a) and 4.3(b), respectively. From Figure
4.3(a), it can be seen that the PANI coating on the LDPE comprises a tightly packed assembly of granules which are fairly homogeneously distributed. After the sputtering process, the granules appear larger due to the covering layer of FEP but the surface appears rough (Figure 4.3(b)), which may be attributed to the poor step coverage of the conventional sputtering technique (Shacham-Diamand and Lopatin, 1997). The AFM images of the corresponding films are given in Figure 4.3(d) and 4.3(e), respectively.
The increase in surface roughness (RMS) from 23.6 nm to 47.0 nm after the PANI-
LDPE film was sputtered with FEP for 100s confirms the SEM results. The pristine
PANI-LDPE film is fairly hydrophilic with a static water contact angle of about 50°.
After the sputtering (for 100s) with a FEP layer, the water contact angle increases substantially to about 117° due to the highly hydrophobic nature of the FEP layer.
The PANI-LDPE film (after redoping with H2SO4) before sputtering with FEP has a sheet resistance (Rs) value of 4×103 Ω/sq. After the sputtering with FEP for 100s, Rs increases to 6×103 Ω/sq. This increase in Rs is less than that observed in an earlier work where PANI was surface graft copolymerized with hydrophobic monomers
(Zhao et al., 1999). In the earlier work, in order to achieve significant stability enhancement, a substantial amount of hydrophobic monomer has to be grafted, and this results in Rs increasing by an order of magnitude or more. The efficiency of the
FEP coating in enhancing the electrical stability of PANI in aqueous solutions is assessed by measuring the change in Rs after various periods of immersion in the aqueous solutions. Figure 4.4(a) shows the changes in Rs of the PANI-LDPE film sputtered with FEP for 100s after immersion in water for various periods of time. The
98 Chapter 4
(a) (d)
(b) (e)
(c) (f)
Figure 4.3 SEM and AFM images of (a) and (d) doped PANI-LDPE film, (b) and (e) the film after sputtering with FEP for 100s, and (c) and (f) the FEP sputtered (100s) film after treatment in water for 2h.
99 Chapter 4
Rs of the films increases by about 1 order of magnitude within the first 30 min of treatment in water. Further increase in immersion time results in only a very gradual increase in Rs. After 3h, the Rs is ~ 1×106 Ω/sq. In contrast, for the PANI-LDPE film without an FEP coating, the Rs increases by 6 orders of magnitude within 5 min. The decrease in Rs is attributed to the dedoping of the PANI and this process is also evident from the UV-visible absorption spectra.
10 (a) Water Uncoated film
8 FEP sputtered film
6
4
10 (b) NaOH solution
pH=12 8 pH=13 Log (Rs, in ohms/sq)
6
4
0 40 80 120 160 200
Treatment Time (min)
Figure 4.4 Sheet resistance of (a) FEP sputtered (100s) PANI-LDPE film after immersion in water and (b) FEP sputtered (600s) PANI-LDPE film after immersion in NaOH solutions of different concentrations for various periods of time.
100 Chapter 4
The UV-visible absorption spectra of the doped PANI-LDPE film without and with the
FEP coating (sputtered for 100s) after treatment in water are compared with that of doped polyaniline before treatment in water in Figure 4.5. In the spectrum of the doped
PANI before water treatment, the band at 430nm and the absorption tail extending into the near IR region have been assigned to excitations from the highest and second highest occupied energy bands to the partially filled polaron in PANI, and are indicative of the conductive state of the polyaniline (Cao et al., 1989; Furukawa, 1988).
The spectrum of the PANI-LDPE film without the FEP coating after 5 min immersion in water shows two absorption peaks at 325 and 625nm, which are characteristic of undoped polyaniline (emeraldine base) (Chen and Lin, 1995). This implies that polyaniline has been undoped and converted to the insulating state within 5 min. In the case of the film with the FEP coating, the spectrum of the film after 3h immersion in water remains similar to that of the doped polyaniline prior to treatment with water, as shown in Figure 4.5. This indicates that the electrical stability of the doped PANI-
LDPE film in aqueous solutions is substantially enhanced after the sputtering with FEP.
The XPS C 1s and N 1s core-level spectra of the FEP sputtered (for 100s) PANI-LDPE film after 3h in water are shown in Figure 4.2(g) and 4.2(h). The C 1s core-level spectrum of this film is similar to that of the film before water treatment (Figure
4.2(e)). In both cases, the peaks of the CF species are the predominant peaks and the relative intensities remains similar, which imply that the surface composition of the sputtered FEP layer is unchanged after the treatment in water. In Figure 4.2(h), the most obvious changes are the appearance of the –N= peak at 398.2eV, the increase in the proportion of –NH-, and the corresponding decrease in the proportion of N+ species.
Nevertheless, the N+ remains as the predominate species. A more detailed analysis of
101 Chapter 4
3.0
2.5 before water treatment 5 min without FEP 3 hr with FEP 2.0 . units) b r a 1.5
ce ( an
rb 1.0
Abso 0.5
0.0 200 400 600 800 1000 Wavelength (nm)
Figure 4.5 UV-visible absorption spectra of doped PANI-LDPE film before and after water treatment, and the FEP sputtered (100s) film after immersion in water for 3h.
the surface compositions of the films before and after water treatment for various periods of time are given in Table 4.1. The results show that the increase in the [–
N=]/[N] ratio with treatment time is also accompanied by a decrease in the [S]/[N] ratio and the largest change occurs within the first 15 min. This observation is consistent with the loss of the dopant anions out of the film resulting in the dedoping of the PANI. The rapid initial loss of dopant anions is also consistent with the increase in Rs as shown in Figure 4.4.
102 Chapter 4
Table 4.1 Surface compositions from XPS and contact angles (θ) of the PANI- LDPE films with FEP coating (sputtered for 100s) after treatment in water for various periods of time
Treatment time + in H2O -N=/N -NH-/N N /N S/N θ 0 min 0 0.133 0.867 0.142 117° 15 min 0.023 0.230 0.747 0.072 110° 30 min 0.034 0.251 0.715 0.066 110° 1 hr 0.053 0.252 0.695 0.056 108° 2 hr 0.068 0.274 0.658 0.055 106°
3 hr 0.102 0.289 0.609 0.053 102°
The surface morphology of the FEP sputtered PANI-LDPE film also shows some changes after the film has been immersed in water. The SEM image in Figure 4.3(c) shows the surface of the FEP sputtered (for 100s) film after treatment in water for 2h.
Compared to Figure 4.3(b), the granules appear to have coalesced resulting in a surface which is smoother than that before treatment with water. The corresponding AFM image of the FEP sputtered PANI-LDPE film after treatment with water is shown in
Figure 4.3(f). The decrease in surface roughness (RMS decreased from 47.0 nm to 29.1 nm) supports the postulate of morphology changes occurring in the FEP coating after water treatment. With the increase in the treatment time in water, the water contact angle also decreases from 117° to 102° as shown in Table 4.1. The surface roughness and the static contact angle can be related by the equation proposed by Good (1952):
cos θr = r cos θtrue where θr, θtrue and r are the apparent static contact angle with a solid surface, the true static angle with a geometrically “smooth’ surface, and the roughness factor for the
103 Chapter 4 surface, respectively. The roughness factor is equal to 1 for a ‘smooth’ surface and is always greater than 1 for a real surface. The hydrophobicity of a surface will therefore be diminished (i.e. θr decrease) if the roughness of the surface decreases. Thus, the decrease in contact angle as shown in Table 4.1 is consistent with the morphological changes seen in Figure 4.3. It is not clear what constitutes the changes in morphology although it is possible that some rearrangement of plasma-deposited films and plasma- treated polymers may occur upon contact with water, as reported in an earlier publication (Johnston and Ratner, 1996).
The results presented so far were obtained using water as the test medium. When basic solutions were used, a FEP coating of less than 10 nm (100s sputtering time) did not offer adequate stability enhancement. In order to achieve a higher level of electrical stability enhancement of the PANI-LDPE films in basic solutions, the samples were sputtered with FEP for 600s (FEP layer thickness of 43 nm) under an argon atmosphere. The effects of the concentration of the basic solutions and treatment time on these films are shown in Figure 4.4(b). Good stability can be achieved in 0.01M
NaOH solution (pH=12). The sheet resistance increases in the first 30 min to
~1.0×106Ω/sq and remained at this level after 3 h treatment. On the other hand, when the film was immersed into 0.1M NaOH solution (pH=13), the color of the films changed from green to blue after 90 min and the Rs of the film increased steadily to
>1010 Ω/sq, indicating the conversion of the PANI from the doped conducting state to the undoped insulating state. In contrast, the uncoated PANI-LDPE film instantaneously turned blue upon immersion in 0.1M NaOH solution. The results presented above show that FEP sputtered PANI film is more effective than PANI film grafted with hydrophobic pentafluorostyrene monomer. In the latter case, the
104 Chapter 4 protective effect of the graft layer is diminished when pH exceeds 10 and the deprotonation is accelerated beyond the pH of 11(Zhao et al., 1999).
4.3.2 PANI-viologen film
The photoinduced doping of PANI via interaction with viologen has been demonstrated in Chapter 3. The thin and transparent nature of the FEP coating formed via the sputtering deposition technique gives the flexibility for the photoinduced doping process to be carried out either before or after the coating is deposited. The Rs of the PANI-viologen film which was first converted to a conductive state via photoinduced doping and then sputtered with FEP for 100s (Sample A) is 4.4×104 Ω/sq, whereas the corresponding value for the sample which was first sputtered with FEP and then converted to the conductive state (Sample B) is 3.6×104 Ω/sq. Since FEP polymers absorb UV irradiation only at wavelength < 180 nm (Scheirs, 1997), the FEP coating does not affect the photoinduced doping process significantly.
In Figure 4.6 the Rs values of Sample A and Sample B after treatment in water for various periods of time are compared. It can be seen that the change in Rs of Samples
A and B with treatment time shows a similar trend as that of the PANI-LDPE film with
FEP coating (Figure 4.4(a)). Without the FEP coating, the undoping of the irradiated
PANI-viologen film in water is very rapid and its Rs increases to > 1010 Ω/sq after 5 min immersion in water. The FEP coating appears to be more effective in preserving the electrical conductivity of Sample A compared to Sample B. The Rs of Sample B after 3h of water treatment is about 1 order of magnitude higher than that of Sample A, although the two samples have almost the same Rs before immersion in water. Since the FEP coating of Sample B was subjected to UV irradiation from a 1kW mercury
105 Chapter 4 lamp during the photoinduced doping process, it is possible that some degree of degradation of the FEP layer might have occurred (Scheirs, 1997). Hence, even though the FEP layer does not affect the photoinduced doping process, in terms of achieving maximum stability enhancement for the PANI-viologen film, it would be better for the doping process to be carried out before the deposition of the FEP layer.
7
6
5 Sample B Sample A
Log (Rs, in ohms/sq) 4
0 30 60 90 120 150 180 Treatment Time (min)
Figure 4.6 Sheet resistance of Sample A and Sample B after immersion in water for various periods of time. Sample A is the PANI-viologen film that was first converted to a conductive state via exposure to UV-irradiation for 1h and then sputtered with FEP for 100s. Sample B is the PANI-viologen film that was first sputtered with FEP for 100s and then exposed to UV- irradiation for 1 h.
106 Chapter 4
4.3.3 Viologen-grafted film
In the previous sections, it has been demonstrated that the sputtered FEP coating can be employed to enhance the electrical stability of electroactive polymers in aqueous media. In this section, the effectiveness of the FEP coating in retarding the oxidation of viologen to achieve a longer lasting photochromic effect is presented. It has been established that under UV-irradiation viologens readily undergo reduction to the radical cation state which is highly colored (blue) (Kamogawa and Nanasawa, 1993)
Upon termination of the irradiation, the blue coloration rapidly bleaches as the radical cations react with oxygen and revert to the dication state (Levey and Ebbesen, 1983).
The UV-visible absorption spectrum of the FEP sputtered (100s) viologen-grafted film immediately after UV irradiation for 10 min is shown in Figure 4.7 as the “0 min” curve. This spectrum shows a peak at around 610nm attributed to the radical cations.
Without FEP coating the bleaching process is essentially completed in less than 1 min after the termination of irradiation, as shown by the solid curve in Figure 4.7. In comparison, with the FEP coating the blue coloration is maintained for a longer period of time. However, the gradual bleaching indicates that the FEP coating is not completely impervious to oxygen. An increase in the FEP coating thickness to 43 nm resulted in only 10% improvement in the bleaching behavior. This is in contrast to the significant stability enhancement observed with thicker FEP coatings on PANI-LDPE film exposed to basic solutions. This difference may be due to the relatively higher permeability of the FEP polymer to oxygen as compared to water vapor (Massey,
2003).
107 Chapter 4
1.0
0 m i n 2 m i n 4 m i n 6 m i n w i t h o u t F E P . units) b r a 0.5 ce ( an rb Abso
0.0 400 600 800 1000 Wavelength (nm)
Figure 4.7 UV-visible absorption spectra of FEP sputtered (100s) viologen grafted LDPE film after different exposure times in air following UV irradiation for 10 min, compared with that of the viologen grafted LDPE film without FEP coating after an exposure time of 1 min in air following similar UV irradiation.
108 Chapter 4
4.4 Conclusion
A radio frequency sputtering technique to deposit fluorinated ethylene propylene copolymer (FEP) coatings of controllable thickness on electroactive and photoactive polymeric substrates has been demonstrated. The electrical stability of the electroactive polymer, polyaniline, in water is substantially enhanced via the sputtering of a layer of
FEP on the order of 10nm thickness on its surface. This technique can be applied to both conventional acid protonated PANI-LDPE film and photoinduced doped PANI- viologen film. Both assemblies remain conductive even after 3h in water, although some changes in the surface morphology of the FEP coating are observed. With a thicker FEP coating, the stability enhancement can also be achieved in basic solutions of pH up to 12. This technique of FEP sputtering can also be used to prolong the photochromic effect of viologen films by retarding the diffusion of O2 to the viologen radical cations formed under UV irradiation.
109
CHAPTER 5
NANOSCALED METAL COATINGS AND
DISPERSIONS PREPARED USING VIOLOGEN
SYSTEMS
110 Chapter 5
5.1 Introduction
Metallized polymers are widely used today in a large variety of technological applications such as appliance, automobile, and electronics industries. In addition to the conventional physical vapor deposition (e.g. via thermal evaporation or sputtering process) techniques, metallization of polymeric substrates can also be carried out by electroless deposition. An advantage of the latter lies in the simplicity of the operation which requires no external source of current and no elaborate equipment. Generally, electroless metal deposition on a polymer surface involves the surface pretreatment of the polymer substrate, followed sequentially by surface sensitization, surface activation, and finally metal deposition (Romand et al.,1998; Zhang et al., 2001). In the case of electroless gold deposition, although a surface activation process is not required, the plating bath with a reducing agent and highly toxic potassium cyanide is necessary. As a result, this process may have adverse environmental effects. Furthermore, electroless gold deposition usually takes place on noble metals rather than polymeric substrates due to the incompatibility of certain polymers with the bath (Yutaka, 1990).
Recently, different ways of achieving metal deposition on polymer surfaces and preparing metal clusters or nanoparticles through the interaction with electroactive polymers have been reported. Thompson’s group (Dokoutchaev et al., 1999) has demonstrated that metal colloids (Au, Pt, and Pd) could be uniformly anchored onto the surface of functionalized polystyrene microspheres. Their approach employed physical methods such as the electrostatic deposition and adsorption of the preformed metal colloids, or the chemical reduction of the metal oxide or hydroxide precursors deposited on the substrate surface. Another approach involves the reduction of
111 Chapter 5 chloroaurate and the electrochemical formation of Au clusters in polyaniline (Hatchett et al., 1999). Our group has also carried out earlier investigations on electroless metal deposition and the preparation of nanostructured metal dispersions using electroactive polymers such as polyaniline (Huang et al., 1998; Wang et al., 2001). In the studies involving electroactive polymers, in order to achieve effective metal reduction, the electroactive polymers were either cast on electrodes and then electrochemically reduced or first chemically converted to its lowest oxidation state (Huang et al., 1998;
Wang et al., 2001). A controlled growth of gold nanoparticles by the application of ligand exchange has been reported as well (Brown and Hutchison, 1999).
This chapter describes a process formulated for coating metal on the surface of polymeric substrates using viologen. By employing the redox reaction of viologen under UV-irradiation, metal ions can be reduced directly from the metal salt solutions without a plating bath to result in a thin metal coating on the surface of the viologen grafted polymeric substrate. Colloidal/nanosized metal particles can also be prepared using viologen in a water-soluble polymer matrix. In this work, the deposition of gold, platinum and palladium from the respective salt solutions was investigated. The metal- viologen films and matrices were characterized using different techniques, including
UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Of particular interest would be the chemical state of the metal species after interaction with the viologen functionalized substrate, and the nature of the dispersion of the metal on/within the substrate.
112 Chapter 5
5.2 Experimental
Materials
Polyethylene film (LDPE, 0.125mm in thickness) was from Goodfellow Inc. (U.K.)
Gold(III) chloride standard solution in 0.5M HCl (1000ppm Au) and platinum(IV) chloride standard solution in 0.5M HCl (1000ppm Pt) were obtained from BDH
Laboratory Supplies (England). Palladium (II)-chloride powder was purchased from
Merck (Germany). Benzyl viologen dichloride (BV) was from Aldrich Chemical Co. of Milwaukee, USA. Poly(vinyl alcohol) (PVA) (MW ~ 25,000) was obtained from
Polyscience Inc.(USA). 1,1’-bis(4-vinylbenzyl)-4,4’-bipyridinium dichloride (VBV) was synthesized according to the method reported in the literature (Liu et al. 2002).
Sample preparation
LDPE films were cut into strips of 2cm×4cm, washed in acetone for 30 min using an ultrasonic bath to remove surface impurities, and then dried under dynamic vacuum in a desiccator. The cleaned films were treated by argon plasma for 30s on both sides, using an Anatech SP100 plasma system. The plasma power applied was kept at 36 W.
These plasma treated films were exposed to air for 5~10 min before proceeding with the graft copolymerization experiments.
Viologen grafted LDPE (VBV-LDPE) film was prepared according to the method described in Section 4.2.
To prepare the BV-PVA film, BV was added into an aqueous solution of PVA (10% wt) to achieved a 0.05M concentration. The resulting colorless transparent solution
113 Chapter 5 was dropped uniformly onto a glass plate and then dried in a desiccator for 24 h at room temperature.
Metal Reduction
Gold chloride standard solution (1000ppm Au) and platinum chloride standard solution
(1000ppm Pt) were used as received. Palladium chloride salt solution (1000ppm Pd) was prepared by first dissolving the palladium chloride powder into an appropriate amount of 37 wt% HCl solution and then adding doubly-distilled water to obtain a solution containing 1.67mg/mL PdCl2 in 0.5M HCl. Metal salt solutions of lower concentrations were obtained by diluting the corresponding 1000ppm metal salt solutions with doubly-distilled water. The VBV-LDPE films was cut into pieces of
0.5cm×4cm and put into a quartz cell. The metal solution was injected into the quartz cell to displace the air and the cell was then sealed with a stopper. This assembly was then inserted into a Pyrex tube and exposed to UV-irradiation in a Riko rotary photochemical reactor (RH400-10W) at 24~28°C for various periods of time. The
Pyrex tube cuts off wavelengths below 290nm completely, with approximately 50% transmission at 305nm. Different concentrations and/or UV irradiation times were tested for each type of metal salt solution. In the case of the BV-PVA matrix, the film coated on the glass plate (2.5cm×4cm) was first irradiated for 10 min in the above- mentioned photochemical reactor and then put into 75ml of metal salt solution for 2 minutes.
114 Chapter 5
Testing and Characterization
Surface analysis of the films was carried out using X-ray photoelectron spectroscopy
(XPS) on a Kratos AXIS HSi spectrometer as described in Section 3.2.
The UV-visible absorption spectra of the films were obtained using a Shimadzu UV-
3101 PC scanning spectrometer, with pristine LDPE films as reference.
Scanning electron microscopy images and energy dispersive X-ray (EDX) spectrum of the coatings on the surface of the VBV grafted film was obtained using the Jeol JSM
5600LV scanning electron microscope. The surface morphology of the pristine and metal coated VBV-LDPE films was compared using a Nanoscope IIIa scanning atomic force microscope (AFM). All AFM images were obtained in the air using the tapping mode under a constant force at a scan rate of 1.0 Hz.
For the transmission electron microscopy (TEM) studies of the metal dispersions in the
BV-PVA matrix (on a Jeol TEM 1220 microscope), the samples were prepared by gently dipping the copper grid into the aqueous solution containing 0.05M BV and
10%wt PVA, and then removed. The copper grid was then left in desiccator to dry under room conditions for 24 h in order to form a thin layer of BV-PVA matrix on the grid. To obtain a colloidal gold or platinum dispersion, the copper grid with the BV-
PVA matrix was irradiated in Riko rotary photochemical reactor for 5 min and then immersed in the metal solution for 3 seconds. After this treatment, the copper grid was removed and dried in a vacuum desiccator for 12 hr before the TEM characterization.
115 Chapter 5
5.3 Results and Discussion
5.3.1 Metal reduction by VBV-LDPE film
After the pristine LDPE film was graft-copolymerized with VBV, it was yellow in color. After the VBV-LDPE film was immersed in gold salt solution and subjected to
UV-irradiation for over 10 min, the color of the film changed from yellow to red. The color change of the VBV-LDPE film gives the first indication that colloidal gold was formed on the surface of the film since colloids of gold have been known to be red, blue or violet (Richard, 1978). The UV-visible absorption spectra of the VBV-LDPE film before and after reaction with the gold standard solution (1000ppm) under UV irradiation for 15 min are shown in Figure 5.1. No distinct absorption peaks can be observed in the spectrum of pristine VBV-LDPE film. However, after it has reacted with the gold salt solution under UV irradiation for 15 min, a new band at 548 nm appears in the absorption spectrum (and the VBV-LDPE film has turned red). This absorption is a manifestation of the electronic structures of the metallic gold particles.
It has been reported that for spherical gold particles of about 20 nm in diameter, the absorption maximum in the UV-visible absorption spectrum occurs near 520 nm
(Creighton, 1982). However, the wavelength of the absorption maximum is dependent on the size and shape of the particles, and their proximity to each other. This result confirms the formation of a gold coating on the VBV-LDPE film. The state of the gold on the VBV-LDPE film was further investigated using the XPS technique.
116 Chapter 5
2.0
Pristine VBV-LDPE After Au reduction 1.5 . units) b r a 1.0
ce ( an rb 0.5 Abso
0.0 400 600 800 1000
Wavelength (nm)
Figure 5.1 UV-visible absorption spectra of the VBV-LDPE film before and after reaction with a 1000 ppm gold chloride solution under UV irradiation for 15 min.
In the absence of UV-irradiation, the VBV-LDPE film shows no color change after immersion in 1000ppm gold standard solution for 15 min. Figure 5.2a shows the Au 4f core-level spectrum of the VBV-LDPE film after immersion in 1000ppm gold standard solution for 15 min without UV irradiation. The spectrum can be curve-fitted with a dominant spin-orbit-split doublet, having BEs at about 85.4 eV (4f7/2) and 89.1eV (4f5/2)
+ for the Au species, and a minor doublet at about 87.3 eV (4f7/2) and 91 eV (4f5/2) for
117 Chapter 5
(a) Au 4f (e) Au 4f 0 Au+ Au Au3+ Au+
(b) (f)
Au0
(c) (g) Intensity (arb. units)
(d) 80 82 84 86 88 90 92
82 84 86 88 90 92 94
Binding Energy (eV)
Figure 5.2 XPS Au 4f core-level spectra of the VBV-LDPE film after reaction with a 1000 ppm gold chloride solution for (a) 15 min without UV irradiation, (b) 5 min, (c) 10 min, (d) 15min under UV irradiation, and with the gold solutions of concentration of (e) 100 ppm, (f) 200 ppm, (g) 500 ppm under UV irradiation for 10 min.
118 Chapter 5
Au3+ species (Moulder et al., 1992). This data indicates that reduction of the Au3+ ions from the solution (the predominant gold species in the acidic solution is expected to be
- + AuCl4 ) (Cotton and Wilkinson, 1980) to the Au complex has occurred through the interactions with viologens. This interaction may be facilitated by the X-ray excitation in the XPS analysis chamber since there is evidence of viologen radical cation formation during XPS measurement (please see below). It has been reported that gold exhibits affinity for bond formation with nitrogen atoms and the great majority of the gold complexes with nitrogen-donor ligands occur with gold in its Au+ and Au3+ oxidation states (Richard, 1978). In the present work, the Au3+ ions from the solution which are adsorbed on the surface of the VBV-LDPE film are likely to have formed
Au+ complexes with the nitrogen atoms of the bipyridine ring of VBV.
Figure 5.2b-5.2d show the XPS Au 4f core-level spectra of the VBV-LDPE film after reaction with gold standard solutions for different periods of time under UV irradiation.
After UV irradiation of the VBV-LDPE film in 1000ppm gold salt solution for 5 min, the Au3+ peaks are no longer discernible in the spectrum (Figure 5.2b) and new peaks
0 appear at about 84.0 eV (4f7/2) and 87.7 eV (4f5/2) assigned to the Au species
(Moulder et al., 1992). Upon increasing the period of UV irradiation, there is a substantial decrease in the proportion of Au+ component peaks and a simultaneous increase in the proportion of Au0 component peaks. After 10 min of UV irradiation the peaks associated with Au0 species become the dominant components (fraction of total adsorbed gold species which is Au0, defined as Au0/Au, is 0.71) (Figure 5.2c) and further increase in irradiation time does not result in further significant changes. This suggests that under the present experimental conditions, a UV-irradiation time of 10 min is adequate to achieve the maximum reduction of gold from the salt solution to the
119 Chapter 5 elemental form using the as-synthesized VBV-LDPE films. Therefore, 10 min of UV irradiation was used for investigating the effect of the concentration of the gold solution on the formation of the gold coating on the VBV-LDPE film.
Figure 5.2e-5.2g show the XPS Au 4f core-level spectra of the VBV-LDPE film after reaction with gold standard solutions of different concentrations after 10 min UV irradiation. When the concentration of the gold salt solution is as low as 100ppm
(Figure 5.2e) and 200ppm (Figure 5.2f), almost all of the gold ions absorbed on the surface of the VBV-LDPE film were reduced to Au0 species after 10 min UV irradiation. But the Au 4f signal is weak, which implies that the amount of gold coating on the surface of the VBV-LDPE film is not significant (Au/N = 0.02 for films after reaction with gold salt solutions of concentration of 200ppm or lower). As the concentration of the gold salt solution increases, the Au/N ratio on the VBV-LDPE film surface also increases (Au/N=0.09 and Au/N=0.12 when the gold concentration is
500ppm and 1000ppm, respectively). At the same time, the Au+ component peaks becomes more prominent as shown in Figure 5.2g and Figure 5.2c for the 500ppm and
1000ppm gold salt solution, respectively. The inability to achieve complete conversion of the gold ions to the elemental state with increasing gold ion concentration in solution may be due to the diminishing accessibility of the active sites as a result of the coverage of the deposited gold and/or the limitation in the photoinduced redox reactions of the VBV (see discussion below).
Figure 5.3 shows the XPS N 1s and Cl 2p core-level spectra of the as-synthesized
VBV-LDPE film (Figure 5.3a and 5.3d) and the films after treatment with 1000ppm gold solutions under UV irradiation for 5min (Figure 5.3b and 5.3e) and 15 min
120 Chapter 5
(a) (d) N+ N 1s Cl 2p N• Cl- -N=
(b) (e)
Cl* -Cl
Intensity (arb. units)
(c) (f)
396 398 400 402 404 194 196 198 200 202 204
Binding Energy (eV)
Figure 5.3 XPS N 1s and Cl 2p core-level spectra of the VBV-LDPE film before ((a) and (d)) and after treatment with 1000 ppm gold chloride solutions under UV irradiation for 5 min ((b) and (e)) and 15 min ((c) and (f)).
121 Chapter 5
(Figure 5.3c and 5.3f). For the pristine VBV-LDPE film, the N 1s core-level spectra of the viologen-grafted films can be deconvoluted into three peaks with the binding energies at 398.6 eV, 399.5 eV, and 401.7 eV (Figure 5.3a). The first peak at 398.6 eV suggests the presence of imine nitrogen from the pyridine rings in the graft copolymerized VBV (Camalli et al., 1990). The peak at 399.5 eV is attributed to the viologen radical cation formed during X-ray excitation in the analysis chamber, and the peak at the highest binding energy is attributed to the positively charged nitrogen of the diquaternized bipyridine molecules (Alvaro et al., 1997). The intensities of both the nitrogen and chlorine signals in the XPS wide scan spectra of the VBV-LDPE films after reaction with the gold salt solutions under UV irradiation for 5 and 15 min have not been substantially reduced. This implies that the thickness of the gold coating is less than 7.5 nm, which is regarded as the probing depth of the XPS technique
(Clark and Dilks, 1979; Tan et al., 1993; Briggs, 1998). The N 1s core-level spectra of the VBV-LDPE film after treatment with gold solutions under UV irradiation can be curve-fitted in a similar fashion as in Figure 5.3a and there is not much difference between the spectra before and after the reaction with the gold salt solution.
The corresponding Cl 2p core-level spectrum of the pristine VBV-LDPE film is deconvoluted into a spin-orbit-split doublet with the BEs for the Cl 2p3/2 and Cl 2p1/2 peaks at 197.1 and 198.6 eV (Figure 5.3d) attributable to ionic (Cl-) chlorine species
(Moulder et al., 1992; Kang et al., 1992). After the VBV-LDPE film has reacted with gold solution under UV irradiation, two new spin-orbit-split doublets (Cl 2p3/2 and Cl
2p1/2) in the Cl 2p core-level spectra were observed with BEs for the Cl 2p3/2 peaks at about 198.6 and 200.2 eV. The former component can be assigned to the chloride species denoted as Cl*, which has been widely observed in doped polyaniline with
122 Chapter 5 chloride counterions and is attributed to the charge-transfer interactions between the halogen and the conducting state of the polymer chain (Mirrezaei et al., 1988; Tan et al., 1991; Kang et al., 1991). In the present case, this species is likely to result from the charge-transfer interactions between the chlorine and the gold ions and is an intermediate state, unlike covalent chlorine (-Cl) which gives a component at 200.2 eV in the Cl 2p spectrum (Moulder et al., 1992; Kang et al., 1992).
The photo-induced reduction of gold ions on the surface of the VBV-LDPE film is likely to involve the photo-reduction of the viologen as the first step. The photo- reduction process involving the transfer of one electron from the counterion (Cl- in this case) to the viologen is known to be rapid (Kamogawa et al., 1984). This results in the conversion of the viologen dications to radical cations. The radical cation may further react with the gold ion absorbed on the VBV-LDPE film resulting in the reduction of the gold ion, and concomitantly, the radical cation itself undergoes oxidation back to the dication state. Hence, no significant changes in the N 1s core-level spectra of the
VBV-LDPE films are observed after the treatment with the gold salt solution. It would be desirable if the photoinduced redox reactions of the VBV can be sustained to result in continual gold reduction. However, as the coverage of the elemental gold on the
VBV increases, the UV irradiation on the VBV-LDPE film may be reduced and the accessibility of the gold ions to the active sites will be increasingly more difficult.
Moreover, the redox reactions may be limited by the availability of the counterions to transfer electrons to the viologen and also by the dimerization of the radical cation
(Monk, 1998). The presence of Cl* and –Cl species indicates that interactions between gold and the Cl- counterion have also occurred. The proportion of Cl* appears to be well correlated with the proportion of Au+ on the VBV-LDPE film surface. As can be
123 Chapter 5 seen from Figure 5.2 and Figure 5.3, at a gold salt solution concentration of 1000ppm, if the UV irradiation time is increased, the proportion of the Au+ decreases (with a corresponding increase in Au0), the proportion of Cl* also decreases. Other experiments using different concentrations of gold solution also indicate that with increasing solution concentration, the proportions of both Au+ and Cl* on the film surface increase. The data on the distribution of Au species is given in Figure 5.2e to
5.2g and 5.2c, and the corresponding data on Cl is shown in Figure 5.4. In a case where minimal Au+ is present, for example, for 100ppm gold salt solution under UV irradiation for 10min, there is no significant Cl* present. The presence of –Cl species is likely to be due to the formation of the gold complexes on the surface of the VBV-
LDPE film.
A similar series of experiments was carried out to investigate the reaction of the VBV-
LDPE film with platinum salt solution. Figure 5.5 shows the XPS Pt 4f core-level spectra of the VBV-LDPE film after reaction with platinum chloride standard solutions
(1000ppm) for different periods of time (Figure 5.5a-5.5d) and of different concentrations under 15 min of UV irradiation (Figure 5.5e-5.5g). Figure 5.5a shows the Pt 4f core-level spectrum of the VBV-LDPE film after immersion in 1000ppm platinum standard solution for 15 min without UV irradiation. The spectrum can be curve-fitted with two spin-orbit-split doublets, having BEs at about 73.6 eV and 76.9
2+ eV assigned to Pt species and at about 71.2 eV (Pt 4f7/2) and 74.5 eV (Pt 4f5/2) attributable to Pt0 species (Moulder et al., 1992). It can be seen that the peaks of Pt2+
4+ species are predominant and no peaks for Pt species (with BEs at 75.5 eV (Pt 4f7/2) and 78.8 eV (Pt 4f5/2)) (Moulder et al., 1992) exist. Thus, platinum metal ions were
124 Chapter 5
(a) Cl 2p Cl- -Cl
(b)
(c) Cl* Intensity (arb. units)
(d)
194 196 198 200 202 204 Binding Energy (eV)
Figure 5.4 XPS Cl 2p core-level spectra of the VBV-LDPE film after reaction with gold chloride solutions of concentrations of (a) 100 ppm, (b) 200 ppm, (c) 500 ppm, and (d) 1000 ppm under UV irradiation for 10 min.
125 Chapter 5
(a) 2+ Pt 4f (e) Pt 4f Pt Pt0
0 Pt2+
Pt
(b) (f)
Intensity (arb. units) (c) (g)
(d) 70 74 78 82
70 74 78 82 Binding Energy (eV)
Figure 5.5 XPS Pt 4f core-level spectra of the VBV-LDPE film after reaction with a 1000 ppm platinum chloride solution for (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, and with platinum chloride solutions of concentration of (e) 100 ppm, (f) 200 ppm, (g) 500 ppm under UV irradiation for 15 min.
126 Chapter 5 also readily adsorbed onto the VBV-LDPE film surface. Similar to the reduction of gold complexes on the surface of the VBV-LDPE film as proposed in the earlier section, the reduction of the Pt4+ ions adsorbed from the salt solution (Cotton and
Wilkinson, 1980) by viologen also occurs. A comparison of Figure 5.5a to Figure 5.2a shows the presence of elemental Pt, while elemental Au was not observed in the absence of UV irradiation. This implies that platinum ions are easier to be reduced than gold ions by the VBV-LDPE film. This can be ascertained further by the spectra of the
VBV-LDPE film after reaction with platinum salt solution under UV irradiation. After
UV irradiation of the VBV-LDPE film in the 1000ppm platinum salt solution for 5 min, the Pt/N ratio on the film surface is 0.14, which is higher than the corresponding Au/N ratio (0.08) of the VBV-LDPE film after reaction with 1000ppm gold salt solution under UV irradiation for 5 min. In addition, the increase in the proportion of the Pt0 species (Figure 5.5b) is substantially higher than the proportion of the Au0 species in
Figure 5.2b. These results are reasonable since the standard reduction potentials of
-2 - - - - - PtCl6 + 4e Æ Pt + 6Cl and AuCl4 + 3e Æ Au + 4Cl are 1.47V and 0.994V, respectively (Weast and Astle, 1982). Chen et al. (2002) had reported that in mixed
- -2 HAuCl4 and H2PtCl6 ethanol-water system AuCl4 ions was reduced earlier than PtCl6 .
However, the reduction mechanism is different from the present one since ethanol
-2 participated in the reduction of the PtCl6 anions. The detection of the N 1s signal in the XPS analysis of the VBV-LDPE film after reaction with platinum salt solution suggests that the platinum coating is also less than 7.5 nm, the probing depth of the
XPS technique (Clark and Dilks, 1979; Tan et al., 1993; Briggs, 1998).
127 Chapter 5
From Figure 5.5b-5.5d it can be seen that increasing the UV irradiation time from 5 min to 15 min results in an increase in the proportion of Pt0 and a corresponding decrease in the proportion of the Pt2+ species. This is similar to the trend observed in the reduction of gold salt solutions except that a further increase in platinum reduction is achieved when the UV irradiation time is extended beyond 10 min. Thus, 15 min of
UV irradiation was used in the study of the effect of platinum salt solution concentration on the formation of a platinum coating on the VBV-LDPE film (Figure
5.5e-5.5g). It is obvious that at concentrations of 100ppm (Figure 5.5e) and 200ppm
(Figure 5.5f) there is no significant amount of Pt2+ species and almost all of the platinum ions absorbed on the surface of the film were reduced to the elemental state.
With the increase in the platinum salt solution concentration to 500 ppm and higher, the proportion of Pt2+ species becomes substantial (Figure 5.5g and Figure 5.5d). This inability to achieve complete conversion of the Pt ions to the elemental state is similar to that observed in the gold reduction experiments, and is again attributed to the presence of the Pt deposits on the VBV and the limitation of the VBV redox reactions.
Figure 5.6 shows the XPS Pd 3d core-level spectra of the VBV-LDPE film after reaction with palladium chloride solutions of 100ppm (Figure 5.6a) and 1000ppm
(Figure 5.6b) under UV irradiation for 15 min. Due to the low intensity of the signal and the high noise, the peak fitting in Figure 5.6a is only an indicative fitting with some uncertainty. The Pd 3d core-level spectra of the VBV-LDPE films treated with
PdCl2 solutions can be curve-fitted with three spin-orbit-split doublets, having BEs at
0 about 335 eV (3d5/2) and 340 eV (3d3/2) for the Pd species, at about 337 eV (3d5/2) and
342 eV (3d3/2) for the intermediate species (represented as Pd*), and the most prominent doublet having BEs at about 338 eV (3d5/2) and 343 eV (3d3/2) is assigned to
128 Chapter 5
(a) Pd 3d Pd2+
Pd0 Pd*
(b) Intensity (arb. units)
332 336 340 344 348
Binding Energy (eV)
Figure 5.6 XPS Pd 3d core-level spectrum of the VBV-LDPE film after reaction with palladium chloride solutions of concentration of (a) 100 ppm and (b) 1000 ppm under UV irradiation for 15 min.
129 Chapter 5 the Pd2+ species (Zhang et al., 2001; Dressick et al., 1994; Charbonnier et al.,
1996;Wang et al., 2000). There is still some uncertainty in the assignment of the Pd* and Pd2+ components. Although the second and third doublets have been assigned to palladium in the Pd-Cl and Pd-N environment respectively in an earlier work (Dressick et al., 1994), some other investigators have attributed the Pd* species to Pd atoms surrounded by both metal and non-metal atoms (Charbonnier et al., 1996). It is clear that Pd2+ is the main state of the palladium deposited on the surface of the VBV-LDPE films regardless of the concentration of the PdCl2 solution. The reduction of the
-2 palladium ions from PdCl2 aqueous solutions (PdCl4 exists as the dominant species in the acidic solution when Cl/Pd > 4) (Drelinkiewicz et al., 1998; Droll et al., 1957) is thus more difficult than the equivalent process observed with gold and platinum ions
-2 - - and this is consistent with the standard reduction potential of PdCl4 + 2e Æ Pd + 4Cl being 0.623V (Weast and Astle, 1982), which is lower than those for Pt and Au.
The scanning electron microscopy image of the VBV-LDPE film after reaction with a
500 ppm gold salt solution for 10 min under UV irradiation is shown in Figure 5.7a.
From this figure it can be concluded that the gold coating on the surface of VBV-
LDPE film is very homogeneous. A similar conclusion can be drawn regarding the
VBV-LDPE film after reaction with 500ppm platinum salt solution for 15 min (Figure
5.7c). From AFM analysis, the average surface roughness of the gold coated and platinum coated VBV-LDPE films are 6.5nm and 3.6nm, respectively, compared to
1.9nm for the pristine VBV-LDPE film. This confirms the smooth surface morphology of the metal coatings on the VBV-LDPE films. EDX analysis of the VBV-LDPE film surface was carried out to obtain information on the surface composition, and Figure
5.7b and 5.7d show the EDX spectrum of a point on the surface of the gold coated and
130 Chapter 5
(a) (c)
Spectrum 1 Spectrum 2
(b) (d)
Figure 5.7 (a) Scanning electron microscopy image and (b) EDX spectrum of the VBV-LDPE film after reaction with 500 ppm gold chloride solution under UV irradiation for 10 min; (c) scanning electron microscopy image and (d) EDX spectrum of the VBV-LDPE film after reaction with 500 ppm platinum chloride solution under UV irradiation for 15 min.
131 Chapter 5 platinum coated VBV-LDPE films. Similar spectra were achieved with several points on the surface of the films. The signal at 2.2 KeV in Figure 5.7b and the signal at 1.5
KeV in Figure 5.7d confirm that gold and platinum, respectively, are deposited on the
VBV-LDPE film (Weast and Astle, 1982).
5.3.2 Colloidal/nanosized metal particles in BV-PVA matrix
Figure 5.8 shows the UV-visible absorption spectra of the BV-PVA film before and after UV irradiation for 10 min. Before irradiation, the film is transparent and there is no intense absorption band in the wavelength range between 400 nm and 800 nm.
After 10 min of UV irradiation in air, the BV-PVA film turns intensely blue and three bands at 405 nm, 550 nm and 605 nm are observed in the absorption spectrum. These bands and the intense blue color are characteristics of the radical cation (in both the monomer and dimer forms) produced by the one electron reduction of the dication
(Monk, 1998). Among these bands, the one at 550 nm attributed to the dimer form is more intense than the band at 605nm. This is consistent with a previous report for benzyl viologen radical cations with the occurrence of dimerization (Bird and Kuhn,
1981). When the radical cation is exposed to air, it will be rapidly reoxidized to the dication state. However, in this case, the intense blue color due to the radical cations can persist for more than 5 days in the PVA matrix because of the negligible permeability of O2 in PVA (Ohtani et al., 1991).
When the PVA matrix containing BV+• radical cations is exposed to the gold salt solution, the solution results in the dissolution of the surface of the PVA matrix and the reaction between the gold ions and the exposed BV+• is expected to occur, as described
132 Chapter 5 in the earlier section. In order to observe the dispersions of the gold particles within the
PVA matrix, transmission electron microscopy was used. The high electron optical
2.0
Before UV Af ter UV irradiation 1.5 . units) b r a 1.0
ce ( an rb 0.5 Abso
0.0 400 600 800 1000
Wavelength (nm)
Figure 5.8 UV-visible absorption spectra of the BV-PVA film before and after UV irradiation for 10 min.
133 Chapter 5 density of the gold particles gives a high contrast when the particles are dispersed in the PVA films, and this renders them particularly amenable to TEM analysis. Figure
5.9a shows a transmission electron micrograph of the gold colloid particles formed in the BV-PVA film exposed to 1000ppm gold salt solution. From Figure 5.9a, the small colloidal gold particles can be clearly seen to be quite evenly distributed in the polymeric matrix. The average diameter of the colloidal gold particles is about 50-60 nm, as estimated from an image with higher magnification (Figure 5.9b). Some bigger particles can be observed in Figure 5.9a which may be the result of agglomeration of the small particles. To achieve smaller particles, a lower concentration of the gold salt solution was used. Figure 5.9c shows the particles obtained with a 100ppm gold salt solution. The particles are in the range of 10 to 20 nm. A lower concentration solution will also result in a lower number density of particles. A similar procedure can be used to obtain nanosized platinum particles within the BV-PVA matrix.
134 Chapter 5
(a)
(b)
(c)
Figure 5.9 Transmission electron micrographs of the gold particles formed in the BV-PVA matrix after reaction with gold chloride solutions of concentration of (a) and (b) 1000 ppm, (c) 100 ppm. The BV-PVA matrix was subjected to UV irradiation for 5 min prior to reaction with gold chloride solutions.
135 Chapter 5
5.4 Conclusion
Nanoscaled gold and platinum coatings on the surface of VBV grafted LDPE films have been successfully achieved via the photoinduced reduction of the corresponding metal salt solutions. The distribution of gold or platinum in the elemental and ionic state on the VBV-LDPE films is dependent on the UV irradiation time and the concentration of the metal salt solution used. The existence of these metals primarily in the elemental state on the VBV-LDPE film surface, as confirmed by XPS analysis, can be achieved with a lower concentration metal salt solution and longer irradiation time.
The results indicate that platinum ions are more readily reduced than gold ions by the
VBV-LDPE film. The reduction of palladium salt solution is much more difficult with the resultant coating comprising mainly Pd2+ ions rather than Pd metal. For gold and platinum solutions, a smooth and highly homogeneous coating can be achieved on the
VBV-LDPE film. Well-dispersed gold and platinum particles ranging from 10 nm, as determined from TEM analysis, can also be readily obtained via the reduction of the corresponding salt solution in a BV-PVA matrix.
136
CHAPTER 6
FORMATION OF CONDUCTING PATTERNS USING
PANI-VIOLOGEN COMPOSITE FILM AND
METALLIZED VIOLOGEN FILM
137 Chapter 6
6.1 Introduction
As mentioned in Section 2.2.4, the potential for using electroactive polymers as active materials in optoelectronics (Braun et al., 1994; Berggren et al., 1994), microelectronics (Dodabalapur et al., 1995), sensors (Lonergan et al., 1996), and related areas (Chen et al., 1993) has recently stimulated significant research interest.
The doped, conductive forms of conducting polymers are being evaluated as possible alternatives to metals as connecting wires and conductive channels. A number of techniques, such as photolithography (Venugopal et al., 1995; Schanze et al., 2000), e-beam writing (Cai et al., 1992), and laser writing (Abdou et al., 1992) and surface-templated deposition (Rozsnyai and Wrighton, 1994), have been successfully demonstrated for the formation of patterned microfeatures based on conductive polymers such as polyaniline, polypyrrole, and polythiophene. More recently, micropatterns of polyaniline were also realized by photopolymerization of aniline derivatives via photoinduced electron transfer between tris(2,2-bipyridyl ruthenium
2+ complex [Ru(bpy)3 ] and electron acceptors such as methyl viologen. (Sei et.al, 1999,
2001a, and 2001b).
In Chapter 3 of current work, we showed that the polyaniline layer coated on the viologen grafted LDPE film can undergo the photo-induced doping under UV irradiation. We also presented our initial results on the interaction of polyaniline in different oxidation states with viologen under UV irradiation. Herein, we expanded
138 Chapter 6 those studies to investigate the potential applications of such photo-induced reaction on the formation of conducting polymer patterns. Metal incorporation on the conducting polymer patterns can be realized further, which gives rise to potential applications in sensors, nanoelectronics, and catalysis.
Miscellaneous applications of viologens have also been extensively investigated as summarized in Section 2.1.4. Our work reported in Chapter 5 illustrates the success of gold and platinum coatings on the surface of VBV grafted LDPE films via the photoinduced reduction of the corresponding metal salt solutions. Hence, this novel technique of direct reduction of noble metal solutions using viologen grafted film can also be employed in the fabrication of conducting patterns.
In this chapter, we present two methods for fabricating conducting patterns on polymeric substrates. In the first method, the pattern formation was developed from photoinduced reaction between polyaniline (PANI) and viologen through a mask.
Conventionally, photolithographic patterning methods of conducting polymer films require the coating of a resist layer, exposure and development of the photoresist
(Makela et al., 1999). In the present work, through proper surface functionalization of the polymeric substrates, a photoresist was not required, which allows for simplicity and cost saving. UV irradiation of the PANI-viologen film through a mask resulted in the doping of the exposed parts and the corresponding solubility change in
N-methylpyrrolidinone (NMP). Selective areas of conductivity can be developed by
139 Chapter 6 dissolving away the more soluble and non-conducting part. Therefore, micropatterns using doped PANI-viologen film were successfully generated on LDPE and polytetrafluoroethylene (PTFE) substrates. Metal incorporation on the conducting polymer patterns can be realized further, which gives rise to potential applications in sensors, nanoelectronics, and catalysis. The second approach takes advantage of the redox property of viologens to achieve the conducting patterns through the reduction of noble metal solutions. Characterization of the conducting patterns was carried out using optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS).
140 Chapter 6
6.2 Experimental
Preparation of PANI-viologen film
PANI-viologen LDPE film was prepared according to the method described in Section
3.2. A similar surface modification procedure was carried out in preparing
PANI-viologen PTFE film except for the plasma pretreatment step. In the case of
PTFE film, the cleaned PTFE films were pretreated by argon plasma for 90s using an an Anatech SP100 plasma system. A longer plasma pre-treatment was used compared to that for LDPE film (60s) due to the inert nature and strong covalent bonds between
C and F atoms of the PTFE material.
Preparation of patterns on PANI-viologen film and pattern-grafted
VBV-LDPE film
Photopatterning on the PANI-viologen films on the LDPE substrates was carried out by exposing the films through a mask to UV irradiation between 24°C to 28°C for 1h, using a 1 kW Hg lamp in the Riko rotary reactor. The masks used were a commercial
photomask (PHOTRONICS Singapore Pte Ltd.) and a 0.2 µm Al2O3 disc (Whatman
International Ltd, Maidstone, England). After irradiation, the samples were rinsed with copious amounts of N-methylpyrrolidinone (NMP) followed by washing with doubly-distilled water and dried under reduced pressure. NMP was chosen as the solvent since it can readily dissolve the unexposed parts which remained in the EB state but not PANI in the doped salt form. Pattern-grafted VBV-LDPE film was
141 Chapter 6 prepared in a way similar to the techniques mentioned in Section 5.2. However, in this experiment, the pristine LDPE film was sandwiched between the photomask mentioned above and quartz plate during the surface photografting process under the
UV irradiation for 20 min. The pattern-grafted VBV-LDPE film was then washed thoroughly with doubly-distilled water and dried under reduced pressure.
Incorporation of metals/metal ions onto the PANI patterns
The PANI-viologen LDPE film was treated with hydrazine for 10 min to reduce the polyaniline followed by thorough washing with water. The reduced films were then immediately pumped dry and immersed in AuCl3 or Pd(NO3)2 acid solutions for 10 min followed by thorough washing with water. Standard AuCl3 and Pd(NO3)2 acid solutions (100 ppm of metal cations in 0.5 M HCl or HNO3, respectively) were obtained by diluting the concentrated metal acid solutions (1000 ppm of metal cations in 0.5 M HCl or HNO3 from Merck) with the respective acid.
Metallization of pattern-grafted VBV-LDPE film
The pattern-grafted VBV-LDPE film was cut into pieces of 0.5cm×4cm and put into a quartz cell. The gold salt solution (1000 ppm) was injected into the quartz cell to displace the air and the cell was then sealed with a stopper. This assembly was exposed to UV-irradiation in a Riko rotary photochemical reactor (RH400-10W) at
24~28°C for 15 min to facilitate the gold reduction. The pattern-grafted VBV-LDPE
142 Chapter 6 film was then extracted from the quartz cell and subjected to thorough washing with doubly-distilled water to remove any remaining metal ions.
Sample characterization
Microscopic images were captured by an optical microscope (Olympus BX 60). SEM images and energy dispersive X-ray (EDX) spectrum of the micropatterns on the films were obtained using a scanning electron microscope (Jeol JSM 5600LV). The surface morphology of the patterned films was analyzed using an atomic force microscope
(Nanoscope IIIa). The AFM image was obtained in the air using the tapping mode under a constant force at a scan rate of 1.0 Hz. XPS analysis of the films was carried out as described in Section 3.2.
143 Chapter 6
6.3 Results and Discussion
6.3.1 Photo-irradiated PANI-viologen system
The surface graft copolymerization of VBC and viologen on the LDPE substrate, and the photo-induced doping of PANI deposited on the LDPE substrate with surface graft copolymerized viologen moieties were described in detail in Chapter 3. In Section 3.3, it is shown that the sheet resistance of the EB-viologen film decreased sharply from >
1010 Ω/sq before UV-irradiation to 5×104 Ω/sq after 30 minutes of irradiation. The success of surface grafting of VBC and viologen on the PTFE films can be ascertained by XPS analysis in the same manner as described in Chapter 3 for the VBC and viologen grafted LDPE films.
After UV irradiation of the PANI-viologen film through a mask for 1h, the parts of the
PANI-viologen film which were exposed to UV irradiation changed from blue to green, indicating the conversion of the PANI coating in the EB state to the doped state, whereas the unexposed parts remained blue. After the film was immersed in NMP, the unexposed parts dissolved readily leaving only the irradiated region since NMP is a good solvent for EB (Angelopoulos et al., 1992) but not the doped salt. An image of a circuit pattern on the surface of the LDPE substrate obtained using the above described process is shown in Figure 6.1. However, after the NMP treatment, the doped PANI may undergo some loss of dopant anions. A simple post-treatment with protonic acid can restore the doping level and recover the conductivity of the PANI pattern. The
144 Chapter 6 developed circuit represents the parts exposed to the UV irradiation while the background is the viologen grafted LDPE film without the EB coating which had been dissolved away. The pattern matches that of the commercial mask used during the selective UV irradiation of the PANI-viologen film and exhibits well defined edges.
The diameter of the fine lines is about 50 µm.
1 mm
Figure 6.1 Image of developed circuit pattern via photo-irradiation of a PANI-viologen LDPE film through a commercial mask and using NMP to dissolve away the unexposed parts.
In the next part of the investigation, an Al2O3 porous mask with the pore size of 0.2
µm was employed to achieve finer resolution. The SEM images of the pristine Al2O3 mask and the PANI-viologen film after UV irradiation for 1 hr without the mask and without NMP post-treatment are shown in Figures 6.2a and 6.2b, respectively. It can
be seen that the pristine Al2O3 mask is alveolate with an average pore size of 0.2 µm
145 Chapter 6 a) b)
15kv x 15,000 1µm 17 16 SEI 15kv x 15,000 1µm 17 16 SEI c) d)
Figure 6.2 SEM images of a) pristine Al2O3 mask and b) PANI-viologen film after UV irradiation for 1 hr without mask; c) 3-D and d) 2-D AFM images of the developed PANI-viologen film after UV irradiation for 1 hr
through the Al2O3 mask and subsequent treatment with NMP.
146 Chapter 6 and the surface of the PANI-viologen film after UV irradiation for 1hr without the mask is relatively smooth. The surface morphology of the PANI-viologen film after
UV irradiation for 1hr through the Al2O3 mask can be delineated by AFM. Figures
6.2c and 6.2d show the 3-D and 2-D AFM images of the PANI-viologen film after UV irradiation through the Al2O3 mask and subsequent treatment with NMP. Relief images were formed on the surface of the PANI-viologen film which corresponded to the
pattern of the Al2O3 mask. However, the average size of the “dots” is about 0.5 µm, in contrast to the 0.2 µm pore of the mask. This difference is due to the diffraction of the
UV irradiation around the holes of the Al2O3 mask, which resulted in a region of doped
PANI which was larger than the pore size.
The techniques used in forming conducting patterns on the PANI-viologen LDPE film can be extended to the PTFE film well. UV irradiation of the PANI-viologen PTFE film through a photomask for 1 hr, followed by the development of the latent image with sequential immersion in NMP and deionized water also affords sharp and well-matched conducting patterns on the surface of PTFE film as shown in Figure 6.3.
By comparing the corresponding width of the same sections on mask and film, an average reproducibility error of – 6% is obtained.
After the formation of the patterns on the PANI-viologen film, metals can be selectively incorporated on the patterns. To facilitate the metal deposition, the
PANI-viologen film was first treated with hydrazine to reduce PANI to its fully
147 Chapter 6
(a) (d)
(b) (e)
(c) (f)
Figure 6.3 Microscopic images of (a) to (c) patterns of the commercial photomask, (d) to (f) developed PANI patterns on the surface of PTFE films.
148 Chapter 6 reduced form, leucoemeraldine (LM). The immersion of the PANI-viologen film (with the PANI in the LM state) into gold chloride solution resulted in the reduction of
0 AuCl3 to Au and the deposition of the elemental gold on the patterns. Figure 6.4 shows the SEM image and energy dispersive X-ray (EDX) spectrum of the microdots patterned PANI-viologen LDPE film after reduction with hydrazine for 10 min and reaction with a 100ppm AuCl3 solution for 10 min. From Figure 6.4a it can be seen that the distribution of those spots on the film is again consistent with the pristine
Al2O3 mask. However, a comparison of Figure 6.4a and Figure 6.2d shows that the average size of the spots has decreased to about 0.2 µm after treatment with hydrazine and gold chloride solution. As mentioned above, the diffraction of the UV irradiation around the edges of the holes resulted in the doping of a wider area than the actual hole.
However, it is possible that the reactions in the peripheral region did not proceed to the same extent as that in the center region. Hence, the interactions between PANI and viologen chains may be weaker in the former, resulting in the loss of the PANI during treatment with hydrazine and gold chloride solution. Figure 6.4b shows the EDX spectrum of a point on one of the microdots on the PANI-viologen film after treatment with gold chloride, and a strong gold signal at 2.2 KeV (Weast and Astle, 1982) was obtained. On the other hand, there is no discernible gold signal for the parts outside the microdots. These results confirm that the microdots are polyaniline incorporated with gold.
149 Chapter 6
Palladium was incorporated on the microdot pattern of the PANI-viologen film using a method similar to that for the incorporation of gold, as mentioned above. The XPS Au
4f and Pd 3d core-level spectra of the microdot patterned PANI-viologen film after
(a)
(b)
Figure 6.4 a) SEM image and b) EDX spectrum of the patterned PANI-viologen film after reduction with hydrazine for 10 min followed by reaction with a 100ppm
AuCl3 solution for 10 min.
treatment with hydrazine and AuCl3 and Pd(NO3)2 solutions are shown in Figure 6.5a and 6.5b, respectively. In Figure 6.5a, the spectrum can be resolved into 2 component peaks (Au 4f7/2 and Au 4f5/2) at 84 eV and 87.7 eV (Moulder, 1992) This confirms the presence of elemental gold. In Figure 6.5b the Pd 3d core-level spectrum can be
150 Chapter 6 curve-fitted with three spin-orbit-split doublets. The most prominent doublet having
0 BEs at about 335.0 eV (Pd 3d5/2) and 340.0 eV (Pd 3d3/2) is attributed to the Pd species. The doublets having BEs at about 338.0 eV (Pd 3d5/2) and 343.0 eV (Pd 3d3/2) is assigned to the Pd2+ species, whereas the intermediate doublet is assigned to the
Pd-N complex (represented as Pd*) (Zhang et al., 2001; Wang et al., 2000). The
(a) Au 4f Au0
80 84 88 92 (b) Pd 3d Pd0 Intensity (arb. units) Pd2+ Pd*
332 336 340 344 348
Binding Energy (eV)
Figure 6.5 XPS Au 4f and Pd 3d core-level spectra of the patterned PANI-viologen film after reduction with hydrazine for 10 min followed by reaction with a) 100 ppm
AuCl3 solution, and b) 100 ppm Pd(NO3)2 solution, for 10 min.
151 Chapter 6 dominant Pd0 peaks confirm that palladium ions in solution were reduced to the elemental state and deposited on the surface of microdot pattern on the PANI-viologen film.
6.3.2 Reaction of VBV-LDPE Patterned Films with Metal Solutions
Based on the studies in Section 5.3, gold coating was achieved on the surface of the
VBV-LDPE film after the film was immersed in gold salt solution and subjected to
UV-irradiation for over 10 min. The sheet resistance of the VBV-LDPE film with gold coating was about 1.8×106 Ω/sq, implying that the film became conductive due to the gold coating. Similarly, the VBV parts of pattern-grafted VBV-LDPE film could react with gold salt solution to form a gold coating on the surface of the patterns when the film was immersed in gold salt solution and subjected to UV-irradiation. Figure 6.6 shows the SEM image of the pattern-grafted VBV-LDPE film after gold deposition, and the corresponding gold, carbon and chlorine EDX mapping images, respectively.
The gold (Figure 6.6b) and chlorine (Figure 6.6d) images exactly match the patterns on the surface of the VBV-LDPE film. In contrast, in the carbon EDX mapping image
(Figure 6.6c) the parts which were initially grafted with VBV and then covered with a gold coating are black, implying the absence of the carbon element. The resolution of the carbon EDX mapping image is not as good since EDX is much more sensitive to heavy metal elements than nonmetal elements. The EDX results give further support for the successful formation of the gold coating on the VBV pattern since it is covered totally with gold and the carbon signal is not discernible.
152 Chapter 6
(a) (b)
(c) (d)
Figure 6.6 (a) SEM image of patterned VBV-LDPE film after gold deposition, and corresponding (b) gold, (c) carbon and (d) chlorine EDX mapping images of the patterned VBV-LDPE film after gold deposition.
153 Chapter 6
6.4 Conclusion
Two methods for fabricating conducting patterns on polymeric substrates were demonstrated. In the first method, through proper surface functionalization of the polymeric substrates, micropatterns with good resolution can be successfully generated on the surfaces of LDPE and PTFE substrates. UV irradiation of the PANI-viologen film through a mask resulted in the doping of the exposed PANI part and the subsequent solubility change in NMP solution. Selective areas of conductivity can be developed by dissolving away the more soluble and non-conducting part. The patterns fabricated from the PANI conducting polymer can be treated with metal salt solutions to incorporate of metal or metal ions. Another patterning approach takes advantage of the redox property of viologens. Through the UV-induced reduction of gold salt solution by the viologen, elemental gold can be successfully deposited on the pattern-grafted VBV-LDPE film to form the conducting patterns, as confirmed by
EDX analysis.
154
CHAPTER 7
CO-IMMOBILIZATION OF ENZYME AND ELECTRON MEDIATOR ON CONDUCTING POLYMER FILM FOR GLUCOSE SENSING
155 Chapter 7
7.1 Introduction
Research in the field of enzyme electrode is expanding rapidly. The development of an electrochemical glucose sensor with an immobilized enzyme on a chemically modified electrode surface has become an attractive area (Mell and Maloy, 1976; Umana and
Waller, 1986; Malitesta et al., 1990; Hale et al., 1991; Bartlett and Birkin, 1994; Yang et al., 2002). Traditionally, glucose sensors have relied on the immobilization of glucose oxidase (GOD) onto various metallic or carbon transducers and the monitoring of the current associated with the oxidation of the liberated hydrogen peroxide. The current flow at the electrode is directly proportional to the hydrogen peroxide concentration, and hence to the dextrose concentration. Various methods have been used for the immobilization of GOD enzyme in the fabrication of glucose sensors.
Most of the conventional procedures used for biomolecule immobilization are cross-linking (Kajiya et al., 1993; Qian et al., 2002), covalent bonding (Parente et al.,1992; Ying et al., 2002), and entrapment (Foulds and Lowe, 1986; Compagnone et al., 1995; Reddy and Vadgama, 2002) in gels, membranes or films. Recently，the immobilization of biomolecules in electropolymerized films by entrapment is gaining importance due to the increasing demand for miniaturized biosensors. However, this method suffers from the problem of low loading and low relative activity of the immobilized enzyme (Cosiner, 1999). An attractive alternative to enzyme entrapment involves the covalent binding of the biomolecule to conducting polymer films bearing appropriate functional groups. A range of functional groups can be used in the covalent
156 Chapter 7 immobilization of enzymes, including amino, hydroxyl, carboxyl and phenolic groups
(Sano et al., 1993).
In the present work, electroactive polypyrrole (PPY) film surface-modified via graft copolymerization with acrylic acid (AAc) was used. The AAc grafted PPY film was subsequently functionalized via simultaneous immobilization of GOD and viologen, whereby the viologen functions as an electron mediator for coupling the electrochemical and enzymatic reactions. A previous work in our group has shown that the presence of viologen in the close proximity with immobilized GOD enabled the
GOD-catalyzed oxidation of glucose to proceed under UV irradiation in the absence of
O2 (Cen et al., 2003). In this study, a simpler one step immobilization method was applied in the co-immobilization of the enzyme and electron mediator. This method also allowed different amounts of redox mediator and enzyme to be immobilized. A further detailed investigation about how viologen and GOD act in tandem for glucose oxidation was carried out.
157 Chapter 7
7.2 Experimental
Materials
4, 4'-bipyridine, methyl iodide, 3-bromopropyl-amine hydrobromide(98%) and pyrrole(99%) were obtained from Aldrich Chemical Co. The pyrrole was distilled before use. Toluene-4-sulfonic acid (TSA) was from Fluke Chemie GmbH.
Water-soluble-1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (WSC) was purchased from Dojindo Chemical Co. (Japan) and was used as received. Glucose oxidase (GOD, Type II, 15500 units g-1 from Aspergillus niger) was purchased from
Sigma Chemical Co.. Dulbecco's phosphate buffer solution (PBS) (containing 8000 mg of sodium chloride, 200 mg of anhydrous monopotassium phosphate per liter of water), used for the enzyme immobilization work, was freshly prepared. Bio-Rad dye reagent for protein assay (Catalog No. 500-0006) was obtained from BioRad (USA). The glucose solution prepared was left at room temperature for 24 h prior to use, to ensure the presence of the β-D(+)-glucose form. The solvents and other reagents were of analytical grade and were used without further purification.
The viologen, N-methyl-N’-(3-aminopropyl)-4,4’-bipyridinium (MAV), was synthesized in two steps according to the method reported in the literature (Katz et al.,
1993). First, monomethylviologen was prepared by reacting a 1:2 molar ratio of iodomethane and 4,4’-bipyridine in toluene at room temperature for 4 days. The precipitate was filtered and recrystallized. In the second step, a mixture of 1:2 molar ratio of monomethylviologen and 3-bromopropyl-amine hydrobromide in dried
158 Chapter 7 methanol was refluxed at 60°C for 48 h with stirring. The resulting precipitate was filtered and recrystallized from water by addition of ethanol. The elemental analysis of
MAV gave the following weight percentages: C, 31.78; H, 3.57; N, 8.37. These values are close to the theoretical values expected for C14H19N3BrI (C, 32.50; H, 3.68; N,
8.12).
Electrochemical synthesis of PPY film
Electrochemical polymerization of pyrrole was carried out in an Autolab-PGSTAT30
(Metrohm-Schmidt Ltd) at 0°C under an argon atmosphere in a conventional three-electrode cell. An electrolyte solution of 0.1M pyrrole and 0.1M TSA in acetonitrile containing 1 vol. % water was used. A highly polished stainless steel plate
(4cm×10cm) served as the working electrode (anode) while a platinum wire gauze served as the counter electrode (cathode), and an Ag/AgCl electrode was used as the reference electrode. PPY was electrodeposited on the stainless steel plate by applying a constant voltage of 8 V for 40 min to obtain doped PPY films of approximately 50 µm in thickness (as measured by a micrometer caliper), which is consistent with previous report (Choi and Tachikawa, 1990). After these films were removed from the stainless steel plate, they were washed thoroughly with acetonitrile and water and dried by pumping under reduced pressure.
Immobilization process
PPY film strips of about 2×2 cm2 were used in all experiments. In the case of graft copolymerization with AAc, each PPY film was immersed in an aqueous AAc solution
159 Chapter 7 of a predetermined concentration between 4 and 10 vol.% in a Pyrex glass tube.
Degassing of the solutions was achieved by bubbling nitrogen vigorously into the solution in the tubes for 30 min before stoppering and sealing with silicon rubber stoppers. The tubes of AAc solution containing PPY films were then exposed to near-UV irradiation in a Riko rotary photochemical reactor (RH400-10W) for 40 min at about 25°C. The graft-copolymerized films were finally subjected to washing with doubly-distilled water at 50°C for 50 h to remove the residual homopolymer.
The simultaneous covalent immobilization of GOD and MAV onto the AAc-graft copolymerized film was facilitated by the activation of the carboxylic groups grafted on the film surface. The COOH groups were preactivated for 1 h with WSC at 4°C in
0.1 M PBS, containing 5mg/ml of WSC. The films were then transferred to the 0.1 M
PBS(+) (pH 7.4, with 0.02 M CaCl2 added) containing GOD at a concentration of 4 mg/ml and MAV of different concentrations ranging from 3mM to 12mM. The immobilization was allowed to proceed at 4°C for 24 h. After that, the reversibly bound GOD was desorbed in copious amounts of PBS(+) for 1 h at 25°C (Kulik et al.,
1993). The process for the immobilization of GOD and MAV is summarized in Figure
7.1. The PPY films surface-modified with GOD and MAV will be denoted as
GOD-MAV-g-PPY in the following discussion. The GOD-MAV-g-PPY film was used as an enzyme electrode to test its response to glucose solutions. The enzyme electrode served as the anode in the electrochemical set-up mentioned earlier. The cyclic
160 Chapter 7 voltammetric (CV) experiments were carried out at room temperature under an argon atmosphere.
PPY C N R’ COOH UV ^^^^^^^ N R (WSC) + AAc 40 min, 25 °C ^^^^^^^ COOH
a) Graft copolymerization of b) Preactivation with WSC AAc
O O
C NH GOD ^^^^^^^ C O C N R’ ^^^^^^^ H2N GOD O O HN R C NH H2N MAV ^^^^^^^ C O C N R’ ^^^^^^^ MAV HN R
c) Co-immobilization of GOD and MAV on AAc grafted PPY film
Figure 7.1 Schematic presentation of the co-immobilization of GOD and MAV on AAc grafted PPY film.
Testing and characterization
Surface compositions were monitored with X-ray photoelectron spectroscopy (XPS) on an AXIS HSi spectrometer (Kratos Analytical Ltd.) as described in Section 3.2.
The amount of GOD immobilized on AAc-grafted PPY film was determined by the modified dye-interaction methods (Bonde et al., 1992; Kang et al., 1993), using the
BioRad protein dye reagent. For the preparation of the dye solution, the Bio-Rad stock
161 Chapter 7 dye solution was diluted five times with doubly-distilled water. GOD solution (100 µl) of known concentration was added to 5 ml of the dye solution. The GOD-dye solution was kept for 3 h and centrifuged at 5000 rev/min for 15 min. In the latter process, the
GOD-dye complexes were precipitated and the free dye remained in the upper layer.
The absorbance of the supernatant at 465 nm was used for the standard calibration. For the quantitative determination of immobilized GOD, the dye solution (5 ml) was added to a test tube and the GOD-functionalized PPY film (2×2 cm2) was immersed into the dye solution. After 3h of reaction, the film was removed and the absorbance of the dye solution was measured at 465 nm. The amount of GOD immobilized on the surface of
PPY film was calculated on the basis of the standard calibration.
Cyclic voltammetry experiments were performed using a computer-controlled electrochemical system Autolab-PGSTAT30 (Metrohm-Schmidt Ltd) at room temperature. A conventional three-electrode system was used throughout. All potentials were recorded and reported versus an Ag/AgCl/saturated KCl reference electrode. The working electrode was either a bare platinum or a GOD and/or MAV functionalized PPY film, and a platinum wire electrode served as the counter electrode.
All solutions were thoroughly deoxygenated by purging with argon gas, and during the electrochemical experiments an argon atmosphere was maintained above the solution.
162 Chapter 7
Assay of GOD activity
For the investigation of the activity of immobilized GOD, 5 ml β-D(+)-glucose solution (18 wt. %) was used as the assay mixture and the activity was calculated by measuring the difference in the concentrations of the β-D(+)-glucose solution before and after a GOD-MAV-PPY film was immersed for a predetermined time. The concentration of the β-D(+)-glucose solutions were measured by the YSI Model 2700
SELECT Biochemistry Analyzer (YSI Incorporated, USA). The activity of the immobilized GOD in Units is defined as the number of µmol of β-D(+)-glucose oxidized to D-gluconolactone per minute. The relative activity (RA) is defined as the ratio of the observed surface enzyme activity over the activity obtained from an equivalent amount of the free enzyme. For the investigation of the involvement of viologen in the GOD-catalyzed oxidation of glucose in the absence of O2 and under
UV irradiation, the assay mixture was first bubbled with argon for 30 min before immersing the GOD-MAV-PPY film and sealing the tube with a silicon rubber stopper.
After that, the assembly was exposed to near-UV irradiation at about 25°C for different periods of time. The characterization of the GOD activity in these experiments was also carried out using the YSI Biochemistry Analyzer.
163 Chapter 7
7.3 Results and Discussion
7.3.1 Co-immobilization of GOD and MAV
After the surface graft copolymerization of AAc on the PPY film, the carboxyl groups of the grafted AAc polymer were activated by the water-soluble carbodiimide. Since both MAV and GOD have amine groups, they can be co-immobilized on the AAc grafted PPY film in one step through the formation of the peptide linkage (CONH).
The success of the covalent immobilization of GOD and MAV onto the surface of the
AAc graft-copolymerized PPY film can be ascertained from the XPS N 1s and I 3d spectra of the film. The XPS N 1s and I 3d core-level spectra of the as-synthesized
MAV powder are shown in Figure 7.2(a) and 7.2(b). The N 1s core-level spectrum of
MAV show an individual peak at 401.7 eV attributed to the positively charged nitrogen species, indicating that the 4,4’-bipyridine has been totally diquaternized during the MAV synthesis process. The I 3d core-level spectra of MAV are resolved into two spin-orbit split doublets with BEs for the I 3d5/2 peaks at 618.5 eV and 620.6
- - eV (Figure 7.2(b)) attributable to ionic iodine species I3 and I5 , respectively
(Salaneck et al., 1980; Hsu et al., 1978). As shown in Figure 7.2(b), the ionic iodine
- - (I3 and I5 ) is the dominant feature which confirmed that the iodine species exist as the counterions of MAV after the viologen synthesis reaction. Figure 7.2(c) and Figure
7.2(d) shows the XPS N 1s and I 3d core-level spectra of the PPY film graft-modified with MAV and GOD (10 vol% AAc monomer concentration in the graft-copolymerization step, 4mg/ml GOD and 9mM MAV in the co-immobilization
164 Chapter 7 step). The N 1s core-level spectrum of the GOD-MAV-PPY film (Figure 7.2(c)) can be deconvoluted into two peaks with BEs at 399.5 eV and 401.7 eV. The peak at 399.5 eV is attributed to the NH species of the immobilized GOD and PPY substrates, and the peak at the highest BE is attributed to the positively charged nitrogen of both MAV grafted onto the surface of the PPY film and the doped PPY substrates. The N+ species
(a) N 1s (b) I 3d - I3 N+
- I5
(c) (d) -NH- Intensity (arb. units)
396 398 400 402 404 612 618 624 630 636
Binding Energy (eV)
Figure 7.2 XPS N 1s and I 3d core-level spectra of (a) and (b) as-synthesized MAV powder, (c) and (d) GOD-MAV-PPY film. Surface modification is carried out with 10 vol% AAc monomer concentration in graft-copolymerization step, and 4mg/ml GOD and 9mM MAV in co-immobilization step.
165 Chapter 7 from MAV is expected to be the dominant contributor since the N1s signal of PPY film after the AAc graft copolymerization is very weak due to the coverage of the AAc copolymer, and the N+/N ratio is < 0.14 due to some degree of undoping during the washing process. After MAV and GOD co-immobilization, the N 1s signal increases in intensity and the N+/N ratio is about 0.22. The observation of iodine signal on the surface of the GOD-MAV-PPY film confirms that MAV was successfully grafted onto the AAC grafted PPY film. The I 3d core-level spectrum of the GOD-MAV-PPY film can be fitted with two spin-orbit split doublets in a similar manner as that used in
Figure 7.2(b) although the intensity is much lower.
The surface concentration of immobilized GOD on the GOD-MAV-PPY film is expressed as the weight of immobilized GOD per surface area of the surface modified
PPY film, as determined using the protein-dye interaction method (Kang et al., 1993;
Bonde et al. 1992). Figure 7.3 shows the amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the AAc monomer concentration used in the graft copolymerization process (4mg/ml GOD and 9mM MAV in co-immobilization step). The increase in AAc monomer concentration results in increasing surface graft concentration of AAc on the PPY substrate, as shown by the previous work in our group (Cen et al., 2003)). It can be seen from Figure 7.3 that the
[I]/[N] ratio of the GOD-MAV-PPY films increases almost linearly with increasing
AAc monomer concentration. Therefore, the result indicates that the amount of MAV immobilized on the AAc grafted PPY films increases with the surface graft
166 Chapter 7 concentration of AAc on the PPY film. The covalent coupling of the GOD molecules to the AAc grafted PPY film through the WSC intermediate also results in an increase in the amount of immobilized GOD with the surface graft concentration of AAc, albeit in a non-linear manner.
Figure 7.3 Amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the AAc monomer concentration used in the graft copolymerization process (4mg/ml GOD and 9mM MAV used in the co-immobilization step).
167 Chapter 7
Figure 7.4 Amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the MAV monomer concentration used in the co-immobilization process (GOD concentration was 4mg/ml and the AAc grafted PPY films were prepared with 10 vol% AAc monomer concentration in the graft copolymerization step).
It can be expected that a competing reaction exists during the co-immobilization of
MAV and GOD when the AAc grafted PPY films were immersed in the PBS solution containing both MAV and GOD. Figure 7.4 shows the amount of immobilized GOD and [I]/[N] ratio of the GOD-MAV-PPY films as a function of the MAV monomer concentration used in the co-immobilization process (GOD concentration was 4mg/ml and the AAc grafted PPY films were prepared with 10 vol% AAc monomer
168 Chapter 7 concentration in the graft copolymerization step). It is obvious that the decrease in the amount of immobilized GOD on the GOD-MAV-PPY films is almost proportional to the increase in the MAV monomer concentration. As expected, the [I]/[N] ratio indicating the amount of MAV immobilized via the covalent coupling increases almost linearly with the increase in the MAV monomer concentration. Therefore, through the adjustment of the ratio of MAV monomer concentration to GOD concentration, the relative amounts of the immobilized enzyme and mediator can be manipulated.
7.3.2 Effect of viologen on enzyme activity
After the co-immobilization of MAV and GOD on the AAc grafted PPY film, the
GOD-MAV-PPY film can be used for the glucose sensing. Generally, amperometric glucose sensors based on glucose oxidase and non-physiological redox mediators use the following mechanism (Hale et al., 1991) to shuttle electrons between the reduced flavin adenine dinucleotide center of the enzyme (FADH2) and the electrode:
Glucose + GO(FAD) gluconolactone + GO(FADH2)
+ GO(FADH2) + 2 Mox GO(FAD) + 2 Mred + 2H
2 Mred 2 Mox + 2e- (at the electrode) where GO(FAD) and GO(FADH2) represent the oxidized and reduced forms of glucose oxidase, respectively. The mediator Mox/Mred is assumed to be a one-electron couple. Since the prosthetic group of enzymes is often deeply located within the protein shell, the mediator plays an important role in enhancing the efficiency of the electron transfer of the system. In our case, MAV was used as the mediator. The
169 Chapter 7 involvement of the viologen in the activity of the GOD was studied under several circumstances. Figure 7.5 shows the enzymatic activity of the GOD-MAV-PPY film
(4mg/ml GOD and 9mM MAV in co-immobilization step) grafted with different AAc concentrations after reaction with glucose solution (a) under UV irradiation for 30min in the absence of O2 (b) with O2 for 30 min and without UV irradiation (c) for 30 min without UV irradiation and O2. The activity of the immobilized GOD is defined as the number of µmols of β-D-glucose oxidized to D-gluconolactone per minute. From curve
(a) in Figure 7.5, it can be seen that the enzyme still retains a certain amount of activity under UV irradiation in the absence of oxygen. Hence, the MAV must be acting as the mediator in this process. The activity of the immobilized GOD increases quickly with the increase of the AAc graft concentrations in the graft copolymerization step, which is equivalent to the increase of the amount of GOD and MAV immobilized on the AAc grafted PPY film as shown from the data above. The effect of AAc grafting concentrations on the conventional GOD catalyzed glucose oxidation process using
O2/H2O2 couple as the mediator was shown as curve (b) in Figure 7.5. In this case,
MAV is unlikely to serve as the mediator, as shown below. There is a steady increase in the activity of the immobilized GOD with the increase of the AAc graft concentration. In comparison with curve (a) in Figure 7.5, the activity of the immobilized GOD using viologen as the mediator under UV irradiation is higher than that of the immobilized GOD with oxygen as the mediator. This is probably because the GOD and redox mediator (MAV) are immobilized in very close proximity during the co-immobilization process, which may facilitate the penetration of the MAV chains
170 Chapter 7
4.0
3.5
(a) ) 3.0 2
2.5
2.0
1.5 (b) 1.0
Activity (µmol/min.cm 0.5 (c)
0.0
4 5 6 7 8 9 10 AAC concentration (%)
Figure 7.5 Enzymatic activity of the GOD-MAV-PPY film (4mg/ml GOD and 9mM MAV in co-immobilization step) grafted with different AAc concentrations after reaction with glucose solution (a) under UV
irradiation for 30min in the absence of O2 (b) with O2 for 30 min and without UV irradiation (c) for 30 min without UV irradiation and O2.
into the active enzymatic site. Consequently, a better electrical communication with the enzyme could be achieved resulting in a higher enzyme activity. On the other hand, the reaction of enzyme with oxygen may be constrained by the oxygen diffusion resistance.
171 Chapter 7
A similar experiment was carried out without UV irradiation and in the absence of oxygen. As shown in Figure 7.5 (c), a very low enzyme activity was detected at all
AAc graft concentrations. This implies that MAV in the absence of UV irradiation could not serve as a mediator. In this case, without the O2/H2O2 couple or MAV as a mediator, the enzymatic reaction is impeded.
172 Chapter 7
7.3.3 Electrochemical characterization of the GOD-MAV-PPY film
The effect of varying glucose concentration was studied using the GOD-MAV-PPY film prepared with 10 vol% AAc monomer concentration in the graft copolymerization step, and 4mg/ml GOD and 9mM MAV solution in the co-immobilization step. Figure
7.6(a) shows the cyclic voltammograms of GOD-MAV-PPY film in PBS buffer
0.004 (a) ) 2
m (b) 0.003
0.002 nt (A/c
0.001
Curre Increasing 0 0 0.2 0.4 0.6 0.8 1 glucose Glucose Concentration (mM) concentration
2mA
Potential (V)
Figure 7.6 (a) Cyclic voltammograms of GOD-MAV-PPY film in glucose solution of 0, 0.2, 0.4, 0.6, 0.8, 1.0mM glucose. Scan rate =0.1V/s. PBS buffer solution with 0.1M NaCl was used as supporting electrolyte. (b) Peak currents at 0.435V as a function of glucose concentration.
173 Chapter 7 solutions containing 0, 0.2, 0.4, 0.6, 0.8, 1.0mM glucose and 0.1M NaCl as supporting electrolyte at a scan rate of 0.1V/s. The change in the shape of the CVs as the glucose concentration increases substantiates the fact that GOD is active on the electrode and the MAV is able to mediate the GOD-glucose reaction. Figure 7.6(b) illustrates the peak current response of the GOD-MAV-PPY film at 0.435V as a function of glucose concentration. A linear relationship is obtained at low glucose concentrations from 0.2 to 1.0 mM while the response levels off for glucose concentrations larger than 1.0 mM.
These results are also comparable with the work done by our other group member. Liu et al. (2003) reported a technique for the covalent binding of GOD onto an electrically conductive PPY surface via a viologen such that the viologen served not only as an anchor for the immobilization of GOD on the PPY surface, but also as an electron mediator between the modified electrode and the enzyme. In this method, a linear relationship of the amperometric response of the GOD and viologen modified PPY electrode as a function of glucose concentration was observed up to 20 mM glucose with current responses up to 5 mA/cm2. Both the linear range and current response are higher than the value obtained in the current co-immobilization method (linear response up to 1.0 mM glucose and current responses up to 4 mA/cm2). The advantage of Liu’s method is that viologen mediator is located between GOD and the conducting
PPY film. Thus, a better and direct electron transfer from the electrode to the GOD active site can be realized through the covalent bonds of viologen with PPY film and viologen with GOD. This direct link enhances the amperometric response as well as the stability of the electron mediators.
174 Chapter 7
Nevertheless, compared with other previous work on the glucose sensors, the present approach gives a much higher response signal. Hale et al. (1991) have reported that several water-soluble viologens can efficiently mediate electron transfer from reduced glucose oxidase to a conventional carbon paste electrode, where the current response was in the range of 20 to 80 µA/cm2 for glucose concentration of 10mM to 30mM.
Similarly, the current response of another glucose sensor made of an enzymatic clay-modified electrode and methyl viologen (Zen and Lo, 1996) was from 20 to 60
µA /cm2 for glucose concentration up to 6mM. These differences are possibly due to the different sensor fabrication methods. In the methods of Hale and Zen, both GOD and the mediators were embedded in the electrode matrix, which restrict the functionality of GOD and the electron transfer of the mediators. In contrast, in the present work, the GOD and viologen mediator were co-immobilized together on the surface of AAc graft copolymerized PPY film, which facilitate the contact between
GOD and glucose solution as well as the mediating effect of the viologens. Therefore, through co-immobilization of GOD and MAV on the PPY film, the surface modified conducting PPY film may have potential advantage for use as sensors for glucose sensing. However, since a competing reaction exists during the co-immobilization of
MAV and GOD, the amount of each component immobilized on the PPY film is affected by the other component.
175 Chapter 7
7.4 Conclusion
Through a one-step co-immobilization of GOD and MAV, the AAc graft copolymerized PPY film can serve as an electrode for glucose sensing. The amount of the immobilized GOD and MAV could be controlled by changing the AAc graft concentration on the PPY film and the ratio of GOD to MAV in the co-immobilization step. A high graft concentration of AAc will lead to more GOD and MAV immobilized on the PPY film. For a fixed AAc graft concentration, a higher MAV monomer concentration will result in a lower amount of immobilized GOD. Since the
GOD and MAV were co-immobilized simultaneously in very close proximity on the
PPY film, MAV can effectively serve as the mediator for electron transfer from the active sites of GOD to the surface of electrode in the absence of oxygen and under UV irradiation. The electrochemical response of the as-functionalized enzyme electrode changes linearly in the range of 0 to 1.0 mM of glucose in solution, showing good potential for use as glucose sensors.
176
CHAPTER 8
CONCLUSIONS AND RECOMMENDATIONS
177 Chapter 8
8.1 Conclusions
Through the study of the interactions of viologens with conducting polymers, metal solutions and glucose oxidase, various potential applications of viologens have been highlighted based on the interesting redox properties of viologens. First, polyaniline
(PANI) can be converted from the insulating state to the doped and conductive state through the photo-induced reaction of PANI with viologen in the solid state. The photo-sensitive films consisting of PANI coatings on viologen-grafted low density polyethylene (LDPE) substrates was fabricated using a 3-step process whereby vinylbenzyl chloride (VBC) was first graft copolymerized onto the LDPE substrate, followed by the linking of the viologen moieties to the VBC and finally the deposition of the PANI coating onto the viologen-grafted film. The density of the grafted VBC and viologen does not play an important role in the doping of PANI under UV irradiation since the reactions are confined mainly to the interfacial region between
PANI and VBC or viologen. But the photo-induced doping of PANI shows a strong dependence on the UV intensity. PANI in different intrinsic oxidation states from leucoemeraldine (LM) to nigraniline (NA) can be doped by this method as well. PANI shows marked improvement in the adhesion with viologen-grafted LDPE film as compared to the pristine LDPE. The photo-irradiated films show good electrical stability in air up to 75°C, but undope rapidly in water. The conductivity of the irradiated films decreases sharply after the films were immersed in water possibly due to the loss of the HCl dopant.
178 Chapter 8
Since both the doped PANI and the viologen in the radical cation state are unstable in the aqueous environmental or ambient environment, a radio frequency sputtering technique to deposit a layer of fluorinated ethylene propylene copolymer (FEP) coating of controllable thickness was developed to enhance the electrical stability of conducting polymers and prolong the photochromic effect of viologens. The electrical stability of the PANI in water is substantially enhanced via the sputtering of a layer of
FEP on the order of 10nm thickness on its surface. This technique can be applied to both conventional acid protonated PANI-LDPE film and photoinduced doped
PANI-viologen film. Both assemblies remain conductive even after 3h in water, although some changes in the surface morphology of the FEP coating are observed.
With a thicker FEP coating, the stability enhancement can also be achieved in basic solutions of pH up to 12. This technique of FEP sputtering can also be used to prolong the photochromic effect of viologen films by retarding the diffusion of O2 to the viologen radical cations formed under UV irradiation.
Viologens can also be employed to reduce noble metal solutions via a photo-induced reaction. Nanoscaled gold and platinum coatings on the surface of
1,1’-bis(4-vinylbenzyl)-4,4’-bipyridinium dichloride (VBV) grafted LDPE films have been successfully achieved. The distribution of gold or platinum in the elemental and ionic state on the VBV-LDPE films is dependent on the UV irradiation time and the concentration of the metal salt solution used. The existence of these metals primarily in the elemental state on the VBV-LDPE film surface can be achieved with a lower
179 Chapter 8 concentration metal salt solution and longer irradiation time. The results indicate that platinum ions are more readily reduced than gold ions by the VBV-LDPE film. The reduction of palladium salt solution is much more difficult with the resultant coating comprising mainly Pd2+ ions rather than Pd metal. For gold and platinum solutions, a smooth and highly homogeneous coating can be achieved on the VBV-LDPE film.
Well-dispersed gold and platinum particles ranging from 10 nm can also be readily obtained via the reduction of the corresponding salt solution in a poly(vinyl alcohol)
(PVA) matrix containing benzyl viologen (BV).
With the understanding of the interactions of viologens with polyaniline and metal solutions, two methods for fabricating conducting patterns on polymeric substrates were demonstrated. In the first method, patterns were developed from photo-induced reaction between PANI and viologen. UV irradiation of the PANI-viologen film through a mask resulted in the doping of the exposed PANI and the corresponding solubility change in N-methylpyrrolidinone (NMP). Selective areas can be developed by dissolving away the more soluble and non-conducting part. Therefore, micropatterns using doped PANI-viologen film were successfully generated on the
LDPE or polytetrafluoroethylene (PTFE) substrate. Conducting patterns also can be formed through the reduction of noble metal solutions using viologen grafted films. In this method, pattern-grafted VBV-LDPE film was prepared first, followed by reaction with gold salt solutions under UV irradiation. A layer of gold coating was deposited on the surface of the VBV patterns, imparting conductivity to the patterns.
180 Chapter 8
The good electrochemical reversibility between the first two redox states of viologens makes them suitable as electron mediators in enzyme electrodes for biomedical sensing and analysis. Our studies show that polypyrrole (PPY) film surface-modified through the co-immobilization of glucose oxidase (GOD) and
N-methyl-N’-(3-aminopropyl)-4,4’-bipyridinium (MAV) can work as an electrode for glucose sensing. MAV can serve as the mediator for effective electron transfer from the active sites of GOD to the surface of the PPY electrode in the absence of oxygen and under UV irradiation. The amount of the immobilized GOD and MAV could be controlled by changing the AAc graft concentration on the PPY film and the ratio of
GOD to MAV in the co-immobilization step. The electrochemical response of the as-functionalized enzyme electrode changes linearly in the range of 0 to 1.0 mM of glucose in solution.
181 Chapter 8
8.2 Recommendations
Based on the investigation we carried out for this research work, the attractive redox properties of viologens may be employed in a wider range of potential applications.
The following are recommendations for further investigations related to this topic.
(i) We have shown in Chapter 5 that under UV irradiation, metal ions can be reduced directly from the metal salt solutions to result in a thin metal coating on the surface of the viologen grafted polymeric substrates. The development of a technique or experimental condition to achieve good control of the size of the colloidal gold is a topic needed for further study. In addition, this technique of metal reduction via reaction with viologen may also be extended to the micellar system. If viologens can be successfully grafted onto polymeric micelles, these functionalized micelles may serve as the carrier for the recovery of precious metals from primary and secondary sources in a less energy intensive and more environmentally-friendly process.
(ii) Long chain viologens can undergo molecular conformational changes between the dication state and radical cation state. Okahata and Enna (1988) reported that viologens can be graft copolymerized onto the surface of nylon microcapsules to control the permeability of the salts inside the capsules. Investigations on the graft polymerization or other surface modification methods of long chain viologens onto different membranes can be carried out. Functionalized membranes can serve as
182 Chapter 8 redox-sensitive gate for permeability control to meet different application requirements. This may find important applications in designing and constructing sustained drug release devices and artificial cells. Viologens may also be copolymerized with the membrane materials to achieve a copolymer membrane with redox property, which could be used in selective microfiltration and ultrafiltration.
(iii) The studies in Chapter 7 show that viologen can be used in fabricating electrode for glucose sensing through a one-step co-immobilization of enzyme and viologen onto the surface of AAc graft copolymerized PPY film. However, the stability and capability of such functionalized electrode need to be improved further so that the electrode can be used for a wider range of glucose detections. The function of viologen as a mediator can be further extended to enzymes other than GOD, such as nitrate reductase. A better understanding and control of the electron transfer from the electrode to the enzymes attached on the surface of the electrode via different immobilization methods is also needed.
183 References
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204 Appendix
PATENT AND PUBLICATIONS
Zhao, L.P., K.G. Neoh and E.T. Kang. Method of Enhancing the Stability of an Electroactive Polymer and a Redox Active Material. US Patent Application. Filed on 13 Feb 2003. Application No., 10/367,180.
Zhao, L.P., K.G. Neoh and E.T. Kang. Photo-induced and Thermal-activated Doping of Polyaniline. Chem. Mater., 14, pp.1098-1106. 2002.
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Zhao, L.P., J. G. Wang, K.G. Neoh and E.T. Kang. “Electroactive Polymer Patterns with Metal Incorporation on a Polymeric Substrate”, Polymer Engineering and Science, 44, pp.2061-2069, 2004.
Liu, X., K.G. Neoh, L.P. Zhao and E.T. Kang. Surface Functionalization of Glass and Polymeric Substrates via Graft Copolymerization of Viologen in an Aqueous Medium. Langmuir, 18, pp.2914 -2921. 2002.
Wang, J.G., K.G. Neoh, L.P. Zhao and E.T. Kang. Plasma Polymerization of Polyaniline on Different Surface Functionalized Substrates. J. Colloid. Interf. Sci., 251, pp.214-224. 2002.
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