Electrochemical and spectroscopic dynamics of ...

Materials Science Forum Vol. 657 (2010) pp 231-248 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.657.231

Electrochemical and spectroscopic dynamics of nanostructured polynuclear sulphonic acid-doped poly(2, 5-dimethoxyaniline) Michael Klink, Richard Akinyeye, Vernon Somerset, Mantoa Sekota, Priscilla Baker and Emmanuel Iwuoha* SensorLab, Department of Chemistry, University of the Western Cape, Bellville, Cape Town, South Africa. Email: [email protected]

Keywords: Anthracene sulphonic acid, phenanthrene sulphonic acid, polyaniline nanotubes, poly (2, 5-dimethoxyaniline), spectroelectrochemistry.

Abstract. Conducting and electroactive nanostructured poly(2, 5-dimethoxyaniline), PDMA, doped with anthracene sulphonic acid, ASA, and phenanthrene sulphonic acid, PSA, respectively, were prepared by oxidative polymerisation of 2, 5-dimethoxyaniline, DMA, with ammonium persulphate as oxidant. Scanning electron microscope, SEM, images of the polymers showed well defined nanotubes and fibrils with diameters of between 50 to 100 nm and 200 to 300 nm for PDMA-ASA and PDMA-PSA, respectively. Evidence of the incorporation of ASA and PSA into the PDMA backbone was provided by UV-Vis and FTIR analyses. Electrochemical interrogation of the sulphonic acid-doped polymers by cyclic voltammetry showed that both PDMA-ASA and PDMAPSA exhibit quazi-reversible electrochemistry. The standard rate constant, ko, for the charge transfer reactions of PDMA-ASA and PDMA-PSA were 3.81 x 10-4 cm s-1 and 3.27 x 10-5 cm s-1, respectively. There was a relationship between the ko value and the formal potential, E0ʹ, of the polymeric nanomaterial. PDMA-ASA that had larger ko value gave an E0ʹ value of 134 mV which was lower than that of PDMA-PSA by 19 mV, indicating that PDMA-ASA has lower activation energy than PDMA-PSA for the electron transfer process Electrochemical impedance spectroscopy over a range of potentials showed that the polymeric nanotubues exhibited high conductivities, though the ASA-doped polymer was more conducting.

Introduction Tremendous advances have been made during the last two decades in our understanding of the chemistry, structure, electrochemistry, electrical and optical properties, processing routes and applications of conducting polymers of which polyanilines is the most widely exploited [1–16]. Traditionally over the years, polymers have been used as inactive materials in semi-conducting devices due to their chemical inertness, electrical insulation and ease of processing. However, since the discovery of conducting polymers, it has generated much more interest in applied science and technology for their unique electrical and physical properties, chemical stability and low cost for various applications [1−2]. Conducting polymers are characterised by a conjugated structure of alternating single and double bonds. This feature shared by all of them originates from the common nature of their -electron system which creates chromophoric character in the visible and near infrared region, an enhanced conductivity in oxidized or in reduced state and reversible redox activation in a suitable electrochemical environment [3]. Among the conducting polymers, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 196.11.235.119-07/06/10,15:47:17)

232

Current Application of Polymers and Nano Materials

polyaniline, PANi, and its substituted polyanilines (e.g. poly-(2, 5-dimethoxyaniline), PDMA, are being studied extensively due to the good environmental stability, ease of synthesis and affordability. Conducting polymers synthesised in the form of nanostructures (nanotubes, nanocomposites, etc.) are of particular interest since their properties significantly differ from the properties of corresponding macroscopic materials. The great potentialities of polyanilines are however that of its infusibility and insolubility in common organic solvents which hinders its wide scope of applications. This problem originates from great aromaticity of PANi coupled with existence of inter chain hydrogen bonding and effective charge delocalisation in its structure [11,15]. There is therefore the need to change the surface properties to overcome some of these problems. The change in surface properties is commonly achieved by surrounding a conductive polymer by another material, usually a bulky dopant acting as a polymeric host stabiliser. For example various authors have employed this route to synthesise water soluble PANi composites/blends [7-12]. Anionic/cationic surfactants have been preferred as a better route to conducting polymer modification as they combine various roles such as doping of the polymer, solubilising the synthesised material and helping to nano-structurise the copolymer produced [7– 15]. Morphological examination have shown that this doping with anionic surfactants is accompanied with fusing the cationic organic polymer backbone to the inside while the anionic surfactant dopants are net-worked around it which consequently create enhanced morphology in the resultant secondary material [4]. In conducting polymers, the incident infrared (IR) radiation interacts not only with the vibrational excitations of the material but also with free carriers and their electronic structures. These interactions create phenomena such as free-carrier absorptions and excitation across the energy gap [5]. The fundamental process of doping is a charge-transfer reaction between an organic polymer and a dopant. When charges are removed from (or added to) a polymer upon chemical doping, geometric parameters, such as bond lengths and angles are changed. The charge is localized over the region of several repeated units. Since the localized charges can move along the polymer chain, they are regarded as charge carriers in the polymer chain. These quasi-particles are classified into polarons and bipolarons according to their charge [6]. Evidence of existence of these particles is usually provided by UV-Vis measurements and FTIR or SNIFTIR spectroscopic investigation. The new materials could be further interrogated by voltammetric and impedimetric measurement which will provide information about the polymer’s electroactivity and conductivities at different potentials and conditions. In this study, we report the chemical synthesis and characterization of nanostructured, conducting poly (2, 5-dimethoxyaniline) doped with either anthracene sulphonic acid, PDMA-ASA, or with phenanthrene sulphonic acid, PDMA-PSA, (Fig. 1). The structure could be simplified by considering the poly (2, 5-dimethoxyaniline) dimers as hosting the bulky dopant [10,14]. ASA and PSA were chemically prepared by the sulphonation of anthracene and phenanthrene, respectively.

Materials Science Forum Vol. 657

233

Fig. 1, Schematic representation of (a) Poly (2, 5-methoxyaniline) doped with anthracene sulphonic acid (PDMA-ASA) and (b) Phenanthrene sulphonic acid (PDMA-PSA).

ASA and PSA are coal tar hydrocarbons which are isomeric and have some similarities but differ in there physical properties and chemical reactivities. For example while both are solids at room temperature, the melting point for anthracene is 216 °C whereas that for phenanthrene is just 101°C. [17]. They do readily undergo oxidation or reduction than naphthalene. Both are oxidised to the 9, 10-quinones and reduced to the 9, 10-dihydro compounds. Electrophilic substitution takes place at the 9-position and thus ensures that two benzene rings are kept intact and still conjugated which helps in stabilising the product (18, 19] The scheme (Fig. 2) shows the synthesis and polymerisation stages involved in the preparation of the PDMA nanostructures from the monomer and respective dopants. SEM, UV-Vis and FTIR spectroscopy were used to study the morphology, doping process and structural transitions of the different polymeric nanostructures. Electrochemical characterisation of the nanostructured polymer pastes was performed in acidic medium of 1M HCl where polyanilines are known to exhibit maximum conductivity [8,15,16]. Cyclic, Oysteryoung square wave and differential pulse voltammetric techniques and impedance spectroscopy were employed for the polymers electrochemical interrogation.

234


Experimental Materials. Anthracene sulphonic acid (ASA) and phenanthrene sulphonic acid (PSA) were chemically synthesized in the laboratory. The reagents 2, 5-dimethoxyaniline, phenanthrene (98%), anthracene (98%) and potassium bromide were obtained from Sigma-Aldrich (Germany). Fluka (Germany) supplied the dimethyl sulfoxide (DMSO), dimethyl ether, methanol and ammonium persulphate. The electrolyte used in the polymerization and all electrochemical characterization experiments was prepared from hydrochloric acid (32%) (Fluka) and distilled water (specific resistance 18 MΩ, Milli-Q, Millipore). All reagents were of analytical grade and were purchased in Cape Town (South Africa), and were used as obtained without further purification.

Synthesis of Anthracene sulphonic acid (ASA) and Phenanthrene sulphonic acid (PSA). Typical sulphonated anthracene and phenanthrene were prepared as follows. A 10 ml portion of fuming H2SO4 was diluted with 10 ml 6 M H2SO4 and the mixture was diluted to 100 ml in a volumetric flask. 50 ml of the above solution was added to a round bottom flask that contained 2 g of anthracene. The contents were heated to boiling in an oil bath (temperature between 100 – 140 o C) fitted with a condenser and thermometer. The mixture was refluxed for 3 hrs with constant stirring to immerse reactants into solution. The mixture was then poured into crushed ice for 20 minutes and the un-reacted anthracene was filtered off. 10 ml of a 50% NaOH was added to the mixture and put in a refrigerator to crystallize, to form a white anthracene sulphonic salt. The salt was then hydrolysed to form the anthracene sulphonic acid. The same procedure and quantities was used for the synthesis of phenanthrene sulphonic acid [20].

Synthesis of nanostructured poly (2, 5-dimethoxyaniline)/Anthracene sulphonic acid and poly (2, 5-dimethoxyaniline)/Phenanthrene sulphonic acid: A typical polymerization procedure for PDMA-PSA nanostructures is as follows: Using a 100 ml beaker, a 0.3340 g (2 mmol) of 2, 5dimethoxyaniline (monomer) was dissolved in 20 ml of de-ionized water alongside 0.259 g (1 mmol) of anthracene sulphonic acid or phenanthrene sulphonic acid (dopant). The mixture was heated for 30 min at 50 oC while stirring vigorously. 10 ml of an aqueous solution of ammonium persulphate (APS) (0.1 M) was added drop-wisely to the hot solution. Then water was added to make up the total volume of the liquor to 50 ml. The mixture was cooled down to room temperature while continuously been stirred for 15 h. The product was filtered and washed with deionised water, methanol and dimethyl ether three times consecutively, to remove impurities such as APS, free PSA and un-reacted 2, 5-dimethoxyaniline. The resultant polymer powder was vacuum dried for 24 h prior to characterisation. A scheme for the synthesis is shown in Fig. 2. The same procedure was used for the synthesis of PDMA-ASA nanostructures [21,22].


235

phenanthrene 120 - 140 deg C OH O

S

1. Fuming H2SO4 / SO3 2. NaOH / H2O

OO

O

S

O

H+/OH-

phenanthrene-9-sulphonate

phenanthrene-9-sulphonic acid (dopant)

+ NH2

NH3+

H+/OHH3CO

OCH3

H3CO

2, 5-dimethoxy aniline (monomer)

OCH3

2, 5-dimethoxy anilinium ion

H+/-H2O (dimerisation and polymerisation)

OCH3

OCH3

O O

S

O

-

+ H N

N

H3CO

*

H3CO

n

x

Poly (2, 5-dimethoxyaniline-phenanthrene 9-sulphonic acid)

Fig. 2, Schematic representation of the synthesis and incorporation of phenanthrene sulphonic acid (PSA) in the polymerisation of 2, 5- dimethoxyaniline (DMA) to form PDMAPSA.

236


Instrumentation. Scanning electron microscopy was performed with a Hitachi X650 scanning electron microscope with an operating voltage of 10-30 kV window. In the SEM experiment, about 0.01 g of the polymer sample was spiked onto a carbon coated sample holder and charged with gold to improve surface electrical conductivity. This was subsequently placed in the microanalyzer where it was scanned by electron beams at selected voltage(s) for optimum resolution. The signal generated was then amplified for image display at different magnifications. UV/Vis absorbance measurements were recorded at room temperature on a UV/Vis 920 spectrometer (GBC Scientific Instruments, Australia) with a 1 cm quartz cuvette and using 99.6% dimethyl sulphoxide (DMSO) as reference solvent. UV-Vis measurements were made with a 0.002 g fine powder of the polymer sample dissolved in 5 ml DMSO. The FTIR measurements in the range 400-4000cm-1 were recorded as a mixture of KBr (99%) and sample (1%) pellet using a Perkins Elmer, Paragon 1000 PC, FTIR spectrometer. In each case, 0.001 g of polymer was ground with 0.1 g of potassium bromide and placed in the pallet maker to obtain a fairly transparent pellet used in the FTIR experiment. All electrochemical experiments were performed with and recorded on a BAS 50W integrated automated electrochemical workstation (Bioanalytical Systems, Lafayate, IN, USA). Alumina micropolish and polishing pads (Buehler, IL, USA) were used for electrode polishing.

Electrochemical measurements. A conventional one compartment and three electrode configuration cell was employed for the voltammetric measurements. The working electrode (WE) was a platinum disc electrode with a surface area of 0.0201 cm2, the reference electrode was a silver/silver chloride (Ag/AgCl) and a platinum gauss was used as the auxiliary electrode. A paste of the different polymers (0.4 g) in 2 ml HCl (1 M) was first prepared and degassed with argon for 20 minutes to exclude any oxygen from the paste. Polymer film was electrochemically adsorbed on the working electrode by performing a 20-cycle cyclic voltammetry (CV) at a scan rate of 50 mV/s from an initial, Ei, to a switch, Ef, potentials of -200 and 700 mV, respectively, The PDMA-ASA (or PDMA-PSA)-modified Pt working electrode was characterized by multi scan rate CV, square wave voltammetry (SWV), differential pulse voltammetry (DPV) and EIS in de-aerated 1 M HCl. Square wave voltammetry was performed at 50 mV square wave amplitude and 2 mV potential step from an initial, Ei, to a final, Ef, potentials of -200 mV and 700 mV, respectively. For convention, a negative oxidation current was used for the display of all voltammograms. Electrochemical impedance spectroscopy measurements were performed at potentials of -300 to 500 mV (using 100 mV intervals), perturbation voltage amplitude of 10 mV and temperature of 25 °C for frequencies 105 to 10-1 Hz. Results and Discussion Scanning Electron Microscopy (SEM). Fig. 3, shows the SEM micrographs of PDMA doped with ASA and PSA. The effect of the different dopants on the monomers prepared under the same conditions was clearly observed in the SEM images. Fairly uniform nanotubes or nanofibres to microtubes were observed when PDMA was doped with PSA (Fig. 3a) with diameters between 50 nm to 300 nm. In micrograph (b) for PDMA doped with ASA, bigger lumps of tubes were seen with diameter between 200 – 300 nm [23]. Offset each micrograph is presented the cross sectional magnification of the observed fibrous lumps of the polymers for better viewing. The nanostructured morphology observed for both polymers is an attestation that their surface is suited for electron shuttling. Similar morphology including thin sheets, fibers, micro rods/tubules, nano-micelles and nano-rods/tubules have been reported for polyanilines and polypyrrole doped with naphthalene sulphonic acid prepared under different conditions [13-15, 22,23].


237

100nm

(a) PDMAPSA

200nm

(b) PDMAASA Fig.3, SEM images of PDMA-PSA (a) and PDMA-ASA (b) at different magnifications.

Spectroscopic Characterisation UV-Vis spectroscopy. Fig. 4, shows the UV-visible spectra of PDMA-ASA (a) and PDMAPSA in DMSO to that of PDMA doped with HCl in DMSO. The UV-vis absorption spectra of PDMA doped with ASA showed 3 clear absorption bands at 3 selected wavelengths, 350, 600 and 800 nm respectively. The same 3 absorption bands were seen for PDMA doped with PSA, but in this case the band at around 800 nm was broadened. The band at around 800 nm was not seen for

238


PDMA doped with HCl. The band at 350 nm corresponds to the reduced state (leucoemeraldine) of PDMA (π-π* transition). The band at 600 nm corresponds to partial oxidation of PDMA and can be assigned to represent the intermediate state between leucoemeraldine form containing benzenoid rings and emeraldine form containing conjugated quinoid rings in the backbone of the PDMA (polaron). The emeraldine form transforms into fully oxidized pernigraniline form and characterized by a broadened band at around 800 nm (bipolaron). Shoulder bands observed around 200 to 230 nm for PDMA-HCl, PDMA-ASA and PDMA-PSA can be assigned to the π-conjugated dienes of the PDMA, PSA and ASA species. The red shift in this wavelength for PDMA-ASA and PDMA-PSA from that of the dopants could be attributed to stronger interaction (small bond lengths) between composite PDMA polymer with the ASA and PSA units respectively. The band at around 800 nm (bipolaron) was not seen for PDMA doped with HCl (Fig 4a), which suggested that the use of sulphonated polycyclic aromatic hydrocarbon enhances better doping of polyanilines [14, 24, 25]. This is an attestation to the fact that better doping occurred in the PDMAPSA than PDMA-HCl leading to the formation of the prominence of the charged excitons observed at < 800 nm has been shown to give proportionate relationship to the charge transportation ability for different polypyrroles [14, 16 and 26]. Thus PDMA-ASA having the most prominent peak at < 800 nm should give proportionate charge transportation ability.

1.4 1.2 1.0

Absorbance

0.8 0.6 a 0.4 b 0.2 c

0.0 -0.2 -0.4 -0.6 0

200

400

600

800

1000

Wavelength (nm)

Fig.4, UV-visible absorption spectra of the PDMAASA (a), PDMAPSA (b) and PDMA-HCl (c).

polymers

dissolved

in

DMSO:

FTIR spectroscopy. The structural characteristics of the polymers were investigated by FTIR spectroscopy from 4000 to 400 cm-1. The major differences in the spectra were in the fingerprint region (2000 to 400 cm-1). The result for the finger print region for PDMA doped with HCl (1), PDMA doped with ASA (2) and PDMA doped with PSA (3) are presented in Fig. 5 and


239

compared with those for the dopants ASA (4) and PSA (5) respectively. PDMA-HCl has the major bands at 1594, 1435, 1277, 1115 and 617 cm-1, which are attributed to the stretching vibrations of quinonic-type rings, benzenic-type rings and the characteristic of the p-substituted chains in PANIHCl polymers, except for a few shifts in the wavenumbers for PDMA-HCl. For PDMA-ASA, the peaks of the quinoid units’ shift from 1594 and 1152 to 1598 and 1159 cm-1 respectively and the stretching vibrations of benzoid ring to1465 and 1285 cm-1. These shifts suggested changes of environment at the molecular level [27]. The bands at 1115, 1026 and 812 cm-1 are related to the 14 substitution on the benzene ring in PDMA. FTIR spectrums of PDMA-PSA and PDMA-ASA have absorptions between 1600 and 1450 cm-1, which are related to the stretching of C-N bonds of benzenic and quinonic rings in the polymer. Absorptions between 1285 and 1154 cm-1 related to the asymmetric and symmetric stretching of =C-O-C- bonds. The absorption band around 1080 cm-1 attributes to the S=O group in the sulphonated PDMA, which is not seen for PDMA-HCl, also suggesting the incorporation of PSA and ASA respectively into the PDMA polymer backbone [4,28]. All these results confirm the existence of modified polyanilines with characteristic features.

Fig.5, FTIR spectra of the polymers: PDMA-HCl (a), PDMA-PSA (b), PDMA-ASA (c), ASA (d) and PSA (e).

Electrochemistry Voltammetric characterization of the polymers on platinum electrode. A comparison was made between the voltammetric behaviour of PDMA modified with ASA with that from PSA. Typical multi-scan rate voltammograms of a paste made of PDMA-ASA in HCl (1 M) on a Pt electrode with scan rates of 10, 20, 30, 40 and 50 mV s-1 are shown in Fig. 6 while that for PDMAPSA system is shown in Fig. 7.. The peak potentials and corresponding currents in the CV’s vary as the scan rates value varies indicating that the polymeric nanostructures are electroactive and that

240


charge transportation was taking place along the polymer chain [16,26]. A careful inspection of the CV’s show that there are 4 anodic and 4 cathodic peaks in the voltammograms of PDMA-ASA (Fig. 6) and three anodic and 3 cathodic peaks for the PDMA-PSA polymer (Fig. 7). These peaks were confirmed by both SWV and DPV voltammetric modes. Emphasis was placed on the first redox couple (most electroactive and conductive) observed during oxidation from -200 mV to the switching potential of 700 mV and the subsequent reverse scan. The first redox couple (a/a), with oxidation peak (peak a) at +163.3 mV relates to the transition from the fully reduced polyleucoemeraldine state to the half oxidised poly-emeraldine salt which is the most conductive redox transition in polyanilines. This transition reveals a CV’s formal potential, E0ʹ, which is the average of the cathodic and anodic peak potentials, of 134 mV for PDMA-ASA and 153 mV for PDMAPSA. This was corroborated by DPV’s measurement that gave E0ʹ value of 136 mV and 159 mV for the first redox couple of PDMA-ASA and PDMA-PSA respectively. Higher E0ʹ values had been reported for Iwuoha research group for ASA modified PANI (190 mV) and POMA (210 mV) [15]. The order of relative energies (mV) required to oxidise the polymers is hereby proposed as follows: PDMA-ASA < PDMA-PSA < PANi-ASA < POMA-ASA. This result is in line with our prediction from UV-Vis spectroscopic investigation.

Fig. 6, Multi-scan cyclic voltammograms for Pt/PDMA-ASA in 1 M HCl at 25 oC. The CV’s scan rates outwards are 5, 10, 20, 30, 40 and 50 mV s-1. The arrows indicate increase in scan rate.


241

Fig.7, Multi-scan cyclic voltammograms for Pt/PDMA-PSA in 1 M HCl at 25 oC. The CV’s scan rates outwards are 5, 10, 15, 30, 40 and 50 mV s-1. The arrows indicate increase in scan rate.

There are two middle redox couples with E0ʹ values of about 247 mV (b/b) and 347 mV (c/c) for PDMA-ASA while PDMA-PSA had only one redox couple with E0ʹ values of about 364 mV. This ill-formed middle redox couple(s) has been assigned differently by various authors as originating from either side chain reactions, redox processes from degradation products of pbenzoquinone or to defects in the linear structure of the polymer [15, 26]. The fact that we have two middle redox couples in PDMA-ASA is an indication that more of these combinations are involved in it. The last redox couple (d/d) in the CV for PDMA-ASA and that of c/c in the CV for PDMA-PSA represents the emeradine salt/pernigraniline transition with E0ʹ value of 488 mV for PDMA-ASA and 513 mV for PDMA-PSA. These values are lower compared to the reported values of 700 mV and 600 mV for anthracene sulphonic acid modified PANi and POMA respectively [15]. The compliment of structural symmetry in the combination of PDMA and ASA moieties to form PDMA-ASA must have contributed to a lower steric strain in the polymer leading to lower formal potential in the redox couples.

Sequential measurement of the cathodic peak potentials for the redox couples a/a and b/b in PDMA-ASA at the different scan rates gave potential values which do not significantly change. The independence of cathodic peak potentials on changes in scan rate is an indication that a surface

242


bound thin film of an adsorbed electroactive species (stationary paste) undergoing fast electron transfer at the working electrode. No charge transfer accompanied the reduction process, only absorption. The oxidation peaks c and d however show progressive increase in peak potential with scan rate. This is an indication of charge transportation along the polymer chain and further confirms that the polymer is conducting in its oxidised state [16,26]. Fig. 7, illustrates the multi-scan voltammograms of a thin polymer film generated from a paste of PDMA-PSA in HCl (1 M) on a Pt electrode at scan rates 5, 10, 15, 30, 40 and 50mV s -1. Analysis of the voltammograms shows that the peak potentials and corresponding currents vary progressively as the scan rates value varies. This indicates that the nanostructured polymer is electroactive and that the electron transfer processes are coupled to a diffusion process namely, charge transportation along the polymeric nanotube. Analysis of the cyclic voltammograms (Fig. 7) established 2 anodic and 3 cathodic peaks. The first oxidation peak at +172.1 mV is the emeraldine (peak a), which is further oxidized at higher potential +577.0 mV (peak c) to the pernigraniline state. Upon reduction, the pernigraniline radical cation is formed at +354.0 mV (peak c) which is subsequently reduced to the fully reduced leucoemeraldine state at +119.3 mV (peak a) [16, 23, 26]. Kinetic studies of the different polymers on Pt electrode in 1M HCl. The kinetic evaluation of voltammetric data of the cyclic voltammograms at different scan rates for PDMAASA and PDMA-PSA is presented in Figs. 6 and 7 respectively. The PDMA-ASA gave more interesting electrochemistry than that from PDMA-PSA. From the CV’s for PDMA-ASA shown in Fig. 6, the anodic and cathodic currents were virtually equal with ratio (Ipa/Ipc) of 1.1 ± 0.1 and with peak separation values (ΔEp) between the anodic peak potential (Epa) and the cathodic peak potential (Epc) of 61 ± 3 mV and a formal potential of 134 ± 1 mV for all the scan rates investigated. This is typical of quasi-reversible systems. The trend differs from the PDMA-PSA polymer (Fig. 7) with an Ipa/Ipc ratio of 0.6 ± 0.1, a formal potential of 153 ± 5 mV and peak separation in the range of 55 to 163 mV. This trend suggests that the cationic species of the PDMA-PSA polymer carries more current than the anionic species indicative of more cationic adsorption of the polymer. The deviation from unity is also indicative of the contribution of kinetic or other complications to the electrode process. Also there is significant drifting in peak potential at the different scan rates investigated. This feature greatly undermines the polymers electrochemistry. The number of electrons transferred for each of the polymers PDMAASA and PDMAPSA was estimated from the peak (a) of the CV’s using the equation: { ǀEp – Ep1/2ǀ = 2.20 R T / n F = 56.5 /n } where Ep is the maximum peak potential, Ep1/2 is half the maximum peak potential, R is the gas constant (8.314 J. mol. K-1), T is the absolute temperature (298 K) of the system, F is the Faraday constant (96,584 C mol-1) and n represents the number of electrons transferred. It was found to be one electron transfer system for both polymers [16, 26 and 29]. The linear dependence of peak currents on the scan rate for the various polymers showed that we have a stationary paste of conducting electroactive polymers on the electrode, which undergo rapid charge transfer reactions. This is typical of a quasi- reversible reaction of surface confined species. The surface molar concentration (Ґ*) of the adsorbed electroactive species was estimated from a plot of Ipc versus υ in accordance with the Brown Anson model [29, 30]. The Brown Anson equation, {Ip = n2 F2 * A  / 4 R T}, was therefore used to estimate the surface molar concentration (*) of the polymers PDMA-PSA and PDMA-PSA using a linear plot of the cathodic peak currents (Ipc) obtained at different scan rates () between 5 mVs-1 and 50 mVs-1 for each polymer. The notations F, A, R and T are constants for the Faraday’s constant (96584 C mol1 ), working electrode area, molar gas constant and room temperature of 298 K respectively. The surface concentration of PDMA-ASA was calculated to be 4.01 x 10-9 mol cm-2 which was about 10 times higher than that obtained for PDMA-PSA (0.36 x 10-9 mol cm-2).


243

The Randles-Sevcik equation of analysis of voltammetric data {Ip / 1/2 = 2.686 x 105 n3/2 A * De1/2} was used to determine the rate of charge transport coefficient (De) along the different polymer chain [29]. The De was evaluated from the slope of the straight line graph obtained from the plot of Ipc versus ν1/2. While the De value of 1.15 x 10-6 cm2 s-1 was estimated for PDMA-ASA, a value of 1.70 x 10-8 cm2 s-1 was obtained for PDMA-PSA. Since De depends on the character (electro-activity, homogeneity, etc.) of the bound species, there is therefore a higher conductivity in the PDMAASA polymeric nanostructure that is having a higher value of De compared to the PDMAPSA. Thus, the charge transfer along the PDMA-ASA polymeric nanotube chain is about sixty eight times faster than that in the PDMA-PSA polymeric nanotubes. Our group and others have reported similar De values for different chemically synthesised polyanilines; PANI (8.68 x 10-9 cm2 s-1) and PANI-PVS (6.46 x 10-8 cm2 s-1) in 1 M HCl [26,29,31]. It is obvious from these results that the PDMA-ASA polymer is an improved polyaniline over its contemporaries. This trend of charge transportation is hereby proposed for the polymers, thus: PDMA-ASA (133) > PANi-PVS (7) > PDMA-PSA (2) > PANI (1). These findings were further confirmed by calculation of the standard rate constant (kº) using the Nicholson’s treatment for quasi-reversible system [16,29,32–34]. The standard rate constant for PDMA-ASA was estimated as 3.81 x 10-4 cm s-1 which was 1 order of magnitude higher than that for PDMA-PSA, being 3.27 x 10-5 cm s-1. Thus, the rate of electron transfer along the polymeric nanotube chain of PDMAASA is about ten times faster than that in PDMA-PSA. It could be seen from these results that synergy between the monomer units with that of the dopant is very vital for good charge and electron transportation in conducting polymers. Electrochemical Impedance Spectroscopy of PDMA-ASA and PDMA-PSA. Electrochemical impedance spectroscopic investigation of the modified polyanilines offers further information about the anodic behaviours in the electrolyte medium of 1M HCl. When they were perturbed at different potentials covered in the CV studies by small AC voltages of low amplitude at selected frequency ranges, the data generated at the low frequency limit provided information about the relative conductivities and capacity of the double layer at the polymer-electrolyte interface. Within the frequency range of -300 to ca 500 mV, the impedance responses for the polymers did not give the usual semicircular arc(s) at the low frequency domain. Rather, straight lines were provided by the Nyquist plots at the low frequencies and there were no semicircle at both high and low frequencies. This usually indicates diffusion of protons (ions) through the polymer layers which is often indicative of infinite arrays of RC elements made up of Resistor and Capacitors [14,36] Consequently, the magnitudes of these elements (resistors and capacitors) could be used to interpret the electrochemical processes taking place at the interface when perturbed at different potentials [14]. Fig. 8, shows the Nyquist plots for PDMA-ASA at different stepping potentials. The closing of the impedance arc in the low frequency region at potentials between 0 to 300 mV and the comparative lower real impedance (Zr) values quantitatively indicates an oxidative electron transfer process. Similar trend to Fig. 8 was observed for PDMA-PSA. An estimate for the real impedance and capacitance in the system at the different stepping potentials was provided with Zr and capacitance (C) values read from the Voltalab Impedimetric Analyser at 0.1 Hz. The more resistive system is indicated by a higher magnitude of Zr which also indicates that the system is less conductive and vice versa. From the Nyquist plot (Fig. 8) for data recorded at potentials of -300 to 500 mV in steps of 100 mV, least resistances were recorded at the region corresponding to the transition from the leucoemeraldine to the emeraldine state of polyanilines. For PDMA-ASA, this was at between 100 to 300 mV with real impedance values of less than 10 k.Ohms.cm2, while that for PDMA-PSA was between 0 to 200 mV with Zr values of less than 13 k.Ohms.cm2.

244


Fig. 8, Nyquist plot for PDMA-ASA taken at different stepping potentials


245

Fig. 9, Real Impedance (Zr) and Capacitance (a) plots for (a) PDMA-ASA and (b) PDMA-PSA at different potentials (E) These regions represents potentials where the polymers were most conductive and coincidentally also more capacitive than at other potentials with capacities of about 0.75 F for PDMA-ASA and 1.0 to 3.8 F for PDMA-SA. Fig. 9 gives the 2-dimensional plots of Capacitance and Real Impedance versus Potential for PDMA-ASA (a) and PDMA-PSA (b) showing the observed trends. These results give clue to proper choice of potentials where electron shuttling would be maxima (i.e. at the most conductive states of the polymer) for potentiometric applications of these polymers in biosensors or chemical sensors.

246


Conclusions The different derivatised polyaniline composites of Anthracene sulphonic acid and phenanthrene sulphonic acid modified poly (2, 5-dimethoxyaniline), PDMA-ASA and PDMA-PSA have been chemically synthesized in aqueous acid medium. We have shown that the dopants, anthracene sulphonic acid (ASA) and phenanthrene sulphonic acid (PSA) could be incorporated into the poly (2, 5-dimethoxyaniline), PDMA polymer backbone. Polymeric tubular morphology for PDMAASA and PDMA-PSA were ascertained by SEM graphs with diameters of 50 – 100 nm for the PDMAP-SA and bigger diameters of 200 to 300 nm for the PDMA-ASA which offers good platform for fast electron transfer at both surfaces. FTIR spectrophotometric analysis shows existence of characteristic features such as the quinoid and benzoid bands typically of polyaniline as well as the sulphonated polycyclic aromatic hydrocarbon (dopant) species. Furthermore UV-Vis bands and shifts showed that ASA and PSA were incorporated into the PDMA backbone respectively with indication of more charge carriers in the PDMA-ASA than in the PDMA-PSA. Cyclic voltammetric characterisation of the polymer pastes showed four distinctive redox states exists for the PDMA-ASA polymer and three for the PDMA-PSA polymer. The CV’s reveals a formal potential, E0ʹ, of 134 mV for PDMA-ASA and 153 mV for PDMA-PSA for the transition from the fully reduced poly-leucoemeraldine state to the half oxidised poly-emeraldine salt (first redox couple). The experimental values obtained for the charge transportation coefficient and the electron transfer coefficient for each polymer composite are quite facile and comparable with those reported for other conducting polymers. The PDMA-ASA polymer showed significant faster charge transportation and electron transfer ability than that for PDMAPSA and those hitherto observed for other polyanilines in line with the findings from UV-Vis analysis. Electrochemical impedance spectroscopic interrogation showed that the polymer is most conductive at potentials between 100 and 300 mV for PDMAA-SA and 0 to 200 mV range for PDMA-PSA. Thus by applying appropriate potential, the polymeric nanostructures can be stabilized at required oxidation states and used as may be required. These results are confirmation that PDMA-ASA and PDMA-PSA could prove promising for developing novel electrocatalysts for use in biosensor and chemical sensor devices. These results encourage us to continue in the development of electronic devices based on this material. The measurement of the sensor responses and transduction characteristics of the polymer composites is in progress.

Acknowledgements This work was financially supported by the National Research Foundation (South Africa) and Dr. Miranda Wallace and Dr. Gerald Malgas of the Microscopic Units at the University of Cape Town and the University of the Western Cape respectively are also gratefully acknowledged for the reported SEM analysis. References [1] C. Johans, J. Clohessy, S. Fantini, K. Kontturi and V.J. Cunnane: Electrochem. Commun. Vol. 4 (2002), p. 227 [2] A.R. Hopkins, R.A. Lipeles and W.H. Kao: Thin Solid Films, Vol. 474 (2004), p. 447 [3] P. Bernier, S. Lefrant and G. Bidan: Advances in Synthetic Metals – Twenty years of Progress in Science and Technology (Elsevier 1999). [4] M.G. Han, S.K. Cho, S.G. Oh and S.S. Im: Synth. Met. Vol. 126 (2002), p. 53


247

[5] B. Schrader: Infrared and Raman Spectroscopy – Methods and Applications (VCH 1995). [6] G. Zerby: Modern Polymer Spectroscopy (Wiley-VCH 1999). [7] Z. Wei, Z. Zhang and M. Wan: Langmuir Vol. 18 (2000), p. 917 [8] S.J. Su and N. Kuramoto: Synth. Met. Vol. 108(2000), p. 121 [9] I. D. Norris, L.A.P. Kane-Maguire and G.G. Wallace: Macromolecules Vol. 33 (2000), p. 3237 [10] G. Appela, R. Mikaloa, K. Henkela, A. Opreaa, A. Yfantisa, I. Paloumpaa and D. Schmeiûera: Solid-State Electronics Viol. 44 (2000), p. 855 [11] A. Pud, N. Ogurtsov, A. Korzhenko and G. Shapoval: Prog. Polym. Sci. Vol. 28 (2003), p. 1701 [12] L. Zhang and M. Wan: Thin Solid Films Vol. 477 (2005), p. 24 [13] R.O. Akinyeye, M. Sekota, P. Baker and E. Iwuoha: Fullerenes, Nanotubes and Carbon Nanostructures Vol. 14 (2006), p. 49 [14] R.O. Akinyeye, I. Michira, M. Sekota, A. Al-Ahmed P. Baker and E. Iwuoha: Electroanalysis Vol. 18(24) (2006), p. 2441 [15] I.N. Michira, M. Klink, R.O. Akinyeye, V. Somerset, M. Sekota, A. Al-Ahmed, P.G.L. Baker and E.I. Iwuoha, in: Recent Advances in Analytical Electrochemistry edited by K. Ozoemena, Transworld Research Network, India (2007). [16] E.I. Iwuoha D.S. de Villaverde, N.P. Garcia, M.R. Smyth and J.M. Pingarron: Biosens. and Bioelectr. Vol. 12 (1997), p. 749 [17] J. McMurry: Organic Chemistry, 2nd Edition (Brooks/Cole publishing company, California, USA 1988). [18] R.T. Morrison and R. N. Boyd: Organic Chemistry, 5th Edition (Allyn and Bacon Inc., USA 1987). [19] T.N. Sorrell: Organic Chemistry (University Science Books, California, USA 1999). [20] A.I. Vogel: Vogel’s Text Book of Practical Organic Chemistry, 5th Edition (Wiley, New York 1989). [21] Z. Zhang and M. Wan: Synth. Met. Vol. 128 (2002), p. 83 [22] Z. Zhang and M. Wan: Synth. Met. Vol. 132 (2003), p. 205 [23] K.R. Prasad and N. Munichandraiah: Synth. Met. Vol. 123 (2001), p. 459 [24] L. Huang, T. Wen and A. Gopalan: Synth. Met.Vol. 130 (2002), p. 155 [25] L. Huang, T. Wen and A. Gopalan: Mat. Chem. & Phys. Vol. 77 (2002), p. 726

248


[26] N.G.R. Mathebe, A. Morrin and E. I. Iwuoha: Talanta Vol. 64 (2004), p. 115 [27] F. Cataldo and P. Maltese: Euro. Polym. J. Vol. 28 (2002), p. 1791 [28] W.A. Gazotti and M. dePaoli: Synth. Met. Vol. 80 (1996), p. 263 [29] A.J. Bard, L.R. Faulkner: Electrochemical methods - Fundamentals and Applications, 2nd edition (John Wiley & Sons, Inc. USA 2001). [30] A.P. Brown, F.C. Anson: Anal. Chem. Vol. 49 (1977), P. 1589 [31] S. Brahim, A.N Wilson, D. Nariesingh and E. Iwuoha, A. Guiseppi-Elie : Michrochim. Acta Vol. 143 (2003), p. 127 [32] P. Zanello, Inorganic Electrochemistry (Theory, Practice and Applications), Royal Society of Chemistry, Cambridge, UK, 2003, 49. [33] R.S. Nicholson: Anal. Chem. Vol. 37 (1965), p. 1351 [34] R.S. Nicholson and I. Shain: Anal. Chem. Vol. 36 (1964), p. 706 [35] P.M.S. Monk: Fundamentals of Electroanalytical Chemistry (John Willy and Sons Ltd, England 2005).