Characterisation of chemical and electrochemical synthesis of polyaniline

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CHAPTER TWO: Experimental methods

Abstract

In this chapter the details of the various experimental and data analysis procedures used in this thesis are outlined. The equipment and experimental settings are supplied below.
This chapter has been partially published as:
To T., Kilmartin P. (2013) Electrochemically synthesised polyaniline on marine grade aluminium, Sixth International Conference on Advanced Materials and Nanotechnology, Auckland, New Zealand, in press.

Materials

Aniline, obtained from Sigma-Aldrich, was distilled under reduced pressure before use and stored in a refrigerator. Aminobenzoic acid (ABA), sulfuric acid (H2SO4), ammonium persulfate (APS), potassium iodide (KIO3) and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich, while solid oxalic acid was provided by Ajax Finechem and used as supplied. The solutions for the electrochemical polymerisation of PANI were kept at 0.1 M of monomer in 0.5 M oxalic acid (C2H2O4.2H2O), HCl and H2SO4 supporting electrolytes. All solutions were prepared using pure water and used as supplied.
Three types of aluminium alloy were used as working electrodes of PANI desposition, namely 1100 aluminium alloy (high purity Aluminium), 4043 alloy (high Si, ~5% Si content) and 5083 alloy (marine grade aluminium, with a thickness of 3 mm, from Austral Bronze Crane Copper Limited). The composition of the alloys is believed to have an affect on the rate and performance of electropolymerisation, therefore three different aluminium alloys were selected for testing. These three alloys were selected due to their high content level of Si (4043), Mg (5083) and aluminium (1100).
A new piece of aluminium was used for each experiment, and was pretreated prior to use, see Figure 1.3. The pretreated alloys were masked with insulating tape to get an effective working area of 1 cm2 before being used for electropolymerisation of PANI. A Pt wire was used as the counter electrode along with an Ag/AgCl reference electrode (0.207 V versus SHE), from BASi. To prepare electropolymerised samples for Raman spectroscopy a 1 cm2 Pt sheet was used as the working electrode. To clean each working Pt electrode between experiments, the electrode was cycled to 2 V in H2SO4 solutions until the breakdown of the polymer film was observed. The electrode was further cleaned by cycling between -0.1 to 1.3 V in 0.5 M H2SO4 until a typical CV of Pt in H2SO4 was obtained [175]. All potentials reported here are relative to the Ag/AgCl electrode scale.

Chemical synthesis

For aniline polymerisation, aniline (50 mmol) was dissolved in 300 mL acidic aqueous solution in beaker A. Beaker B contained 3.5 mmol (APS) in 200 mL acidic aqueous solution. The solution from beaker B was slowly added dropwise into beaker A with vigorous stirring. After adding, the mixture was stirred overnight for complete polymerisation.
Polymers were then filtered and washed with deionised (DI) water, followed by acetone to obtain green powder samples which were then dedoped with diluted ammonia solution. PANI was obtained by chemical oxidation of aniline in different acids, oxalic acid and H2SO4.
Different dopant concentrations (0.5 M and 1.5 M) were used, along with two different oxidants (APS or KIO3).
The chemical synthesis of poly-ABA was carried out in the manner reported earlier by Rivas et al [63]. The synthesis of poly-ABA (poly-2-ABA and poly-3-ABA) in two solutions was performed. Solution A contained 6 g (21.9 mmol) of monomer (ABA) dissolved in an acidic medium (HCl or H2SO4), for example 190 mL of 1 M HCl, pH = 0.8 (a clear solution).
Solution B contained 9.9 g (21.9 mmol) of APS dissolved in 28.8 mL of 1.5 M HCl. Each solution was heated to 50°C to dissolve the reactants prior to the reaction. The reaction was carried out in a 500 mL round-bottomed flask at 50°C. The solution of APS (solution B) was added dropwise with stirring into solution A. After 8 hours, the solution was filtered and washed with hot HCl followed by warm water. The filtered product was then dried in a vacuum oven for 2 days at 60°C [45]. The same chemical procedures were followed for 1:1 copolymers of 21.9 mmol of monomer dissolved in 190 mL of acidic solution.

Electrochemical instrumentation and cells

The electrochemical cell used for electropolymerisation consisted of a glass container and Teflon cap, with inlet sites to insert electrodes, and nitrogen supply for deaerating the solution, see Figure 2.1. The solutions used for the electrochemical polymerisation were 0.1 M of aniline, 0.1 M ABA or 0.05 M aniline + 0.05 M aniline monomer with the total monomer concentration kept at 0.1 M in 0.5 M oxalic acid (C2H2O4.2H2O), 0.5 M/ 1.0 M/1.5 M H2SO4 or 1 M HCl supporting electrolyte. A Pt wire was used as the counter electrode, along with an Ag/AgCl reference electrode in 3 M NaCl from BASi. Five types of working electrodes were used, including a Pt electrode coated with a Teflon holder from BASi for the electropolymerisation of PANI. A glassy carbon working electrode also from BASi was used for characterisation of chemically synthesised polymers. Moreover, three
grades of aluminium alloy were used as working electrodes for PANI growth: 1100 aluminium alloy (high purity Aluminium), 4043 alloy (high Si, ~5% Si content) and 5083 alloy (marine grade aluminium) as presented in Chapter Six. To prepare electropolymerised samples for SSNMR and Raman a 1 cm2 Pt sheet was used as the working electrode. Three instruments were used in this thesis, a CH Instruments electrochemical workstation (model 650C, CH Instruments, USA), VersaSTAT and EDAQ potentiostat with E-corder 410, with most of the electrochemically synthesised PANI carried out on the EDAQ potentiostat.

Electrochemical synthesis

A three electrode cell setup, see Figure 2.1, was used for the voltammetric preparation of PANI films. The solutions for the electrochemical polymerisation were 0.1 M of aniline monomer in 0.5 M oxalic acid (C2H2O4.2H2O) or H2SO4 supporting electrolyte. All electropolymerisation solutions were deaerated with oxygen-free nitrogen for at least 15 min prior to use.
The polymers were formed using cycling from -0.1 to 0.9 – 1.1 V at a scan rate of 50 mV s-1 and a film was formed on the surface of the electrode during polymerisation. The film was washed with electrolyte used in the polymerisation without monomer present for characterisation.
Poly-ABA polymer and its copolymers with aniline were all grown on a BASi Pt working electrode with a working area of 1 cm2 in H2SO4 and HCl electrolyte solution. All electrochemically synthesised polymers mentioned in this chapter were prepared using an EDAQ potentiostat with E-corder 410.

Electrochemical synthesis of a homopolymer of poly-ABA was prepared using two different methods:

1. Polymers were prepared directly on a Pt electrode in either 1 M HCl or 0.5 M H2SO4.
All poly-ABA electrochemically synthesised polymers were polymerised in the potential range between -0.1 to 1.2 V for the first two cycles, except for poly-2-ABA doped with 1 M HCl where the upper potential limited was lowered (-0.1 to 1.1 V) followed by another 2 cycles between -0.1 to 1 V, see Chapter Four for more details.
To build thicker films, the polymers were cycled for 100 more cycles between -0.1 to 1 V at a scan rate of 50 mV s-1.
2. Poly-ABA was grown on a PANI surface, which had been prepared already under the conditions mentioned above, on a Pt electrode in 0.5 M H2SO4 electrolyte, see Figure 2.2.
Electrochemical polymerisation was carried out by cycling the potential in 0.5 M H2SO4 containing 0.05 M of ABA and 0.05 M of aniline. The solutions used for the copolymerisation experiments had a total concentration of 0.1 M, i.e. 9:1 (ABA: aniline), and were prepared at a scan rate of 50 mV s-1 for 100 cycles -0.1 to 1.0 V.
The coated electrodes were transferred to a solution of pure electrolyte for electrochemical characterisation. Cyclic voltammetry was undertaken using the same potentiostat; three cycles were necessary to establish steady-state conditions.
Prior to electrosynthesis the aluminium was cleaned as mentioned in Chapter One. Electrochemically synthesised PANI on marine grade aluminium was studied using different methods. The formation of a PANI coating on a marine aluminium electrode was undertaken using both one-step and two-step processes at a fixed potential of 2 V (Ag/AgCl). Step 1 was an anodisation step with the formation of an oxide layer, Al2O3, by electrolysis carried out for an hour in the absence of the aniline monomer, which allowed the oxidation of metal to occur first (Figure 2.3). This was followed by an electropolymerisation of aniline for 3 hours (step 2). The experimental conditions are shown below in Figure 2.3.
A one-step process, involving only step 2 from Figure 2.3, was also applied. In this case both oxidation reactions will occur simultaneously, the oxidation of the metal with the formation of aluminium oxide and aniline monomer oxidation with the formation of PANI.
Electropolymerisation of aniline on marine grade aluminium was carried out by cycling from between -0.4 to -1 V for the lower potential, and an upper potential (Eλa) that varied from 1.5
to 2.4 V. The films were grown at a scan rate of 50 mV s-1 cycled for 170 cycles using a VersaSTAT 3 potentiostat.
The formation of PANI using potentiostatic polarisation of 2 V was undertaken in a one-step process with an Ag/AgCl reference and Pt counter electrode from an 0.5 M oxalic acid solution containing 0.1 M aniline monomer.

CHAPTER ONE: Introduction
1.1. Abstract
1.2. General background
1.3. Current antifouling coatings system
1.4. Conducting polymers
1.5. Aluminium substrates
1.6. Aims and objectives of this work
CHAPTER TWO: Experimental methods
2.1. Abstract
2.2. Materials
2.3. Chemical synthesis
2.4. Electrochemical instrumentation and cells
2.5. Electrochemical synthesis
2.6. Characterisation Methods
2.7. Performance properties
CHAPTER THREE: Characterisation of chemical and electrochemical synthesis of polyaniline
3.1. Abstract
3.2. Results and Discussions
3.3. Conclusions
CHAPTER FOUR: Characterisation of polyaminobenzoic acid and aniline-aminobenzoic acid copolymer
4.1. Abstract
4.2. Results and Discussions
4.3. Conclusions
CHAPTER FIVE: Structural studies of polyaniline and aniline-aminobenzoic acid copolymers
5.1. Abstract
5.2. Results and Discussions
5.3. Conclusions
CHAPTER SIX: Electrodeposition of polyaniline on aluminium
6.1. Abstract
6.2. Results and Discussions
6.3. Conclusions
CHAPTER SEVEN: Fouling resistance studies
7.1. Abstract
7.2. Experimental Procedure
7.3. Results and Discussions
7.4. Conclusions
CHAPTER EIGHT: Conclusions
8.1. General Conclusions
8.2. Future work
APPENDIX
REFERENCES
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