Synthesis and properties of a mechanically strong poly(bithiophene) composite polymer containing a polyelectrolyte dopant

Synthetic Metals 110 Ž2000. 123–132 www.elsevier.comrlocatersynmet

Synthesis and properties of a mechanically strong poly žbithiophene / composite polymer containing a polyelectrolyte dopant J. Ding, W.E. Price, S.F. Ralph, G.G. Wallace

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Intelligent Polymer Research Institute, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia Received 19 October 1999; accepted 19 October 1999

Abstract Electrochemical polymerisation of 2,2X-bithiophene ŽBT. in the presence of a sulfated polyŽb-hydroxyether. ŽS-PHE. yielded thin films that displayed excellent mechanical properties. Tensile strength measurements gave breaking strains ranging from 98 to 128 MPa for free standing polyŽbithiophene.rsulfated polyŽb-hydroxyether. ŽPBTrS-PHE. films, while scanning electron microscopy ŽSEM. and atomic force microscopy ŽAFM. studies on this material revealed a uniform surface morphology, with an average distance between peaks and valleys on the surface of 1 mm. Four point probe measurements performed on free standing PBTrS-PHE indicated that they were semiconducting, with conductivities ranging from 0.2 to 0.8 S cmy1. In propylene carbonate solution PBTrS-PHE was electroactive, with overoxidation commencing at approximately q1.35 V. Although electroinactive in aqueous solutions, PBTrS-PHE-coated electrodes were able to be used to examine the aqueous electrochemistry of electroactive solutes, such as cupric ion and ferrocyanide. All results indicate that PBTrS-PHE composite films and films are interesting candidates for a number of applications. q 2000 Elsevier Science S.A. All rights reserved. Keywords: PolyŽbithiophene.; Sulfated polyŽb-hydroxyether.; Polypyrrole; Electroactivity; Tensile strength

1. Introduction Thiophene, substituted thiophenes and oligomeric thiophenes can be electrochemically or chemically oxidised to give conducting polymers with a range of useful properties that make these materials interesting candidates for a variety of potential applications w1,2x. However, developments in these areas have been limited by the generally poor mechanical properties and lack of solubility and electroactivity in aqueous solutions displayed by these materials. Incorporation of polyelectrolytes into conducting polymers has attracted increasing interest due to the favourable mechanical properties they often confer on these materials w3–6x, as well as other interesting features such as high water content and open porous structure w7x. Furthermore, when the conducting polymer is reduced, charge compensation is achieved predominantly by the incorporation of cations from the solvent owing to the bulk and lack of mobility of polyelectrolyte dopants w8,9x. By appropriate choice of polyelectrolyte dopant, it has also ) Corresponding author. Tel.: q61-242-213-319; fax: q61-242-213314; e-mail: gordon – [email protected]

been possible to produce conducting polymers that contain electroactive functional groups w10x, which are capable of complexing metal ions w11,12x, or are biocompatible w7x. The synthesis of a series of S-PHEs, and their subsequent use as dopants for both polypyrrole ŽPPy. and polyŽ3,4-ethylenedioxythiophene. ŽPEDT., has been reported w3,4,6,12–14x. PPyrS-PHE composites were shown to display excellent mechanical properties, as well as the ability to be moulded at elevated temperatures w3,4x. Similar favourable mechanical properties were displayed by PEDTrS-PHE composites w6,14x. In addition, the latter materials were found to be electrochromic and highly conducting, with conductivities in the range 150–180 S cmy1 . The formation of new polythiophenes has recently attracted a great deal of attention w2,6,14–24x. This is due partly to the fact that polythiophenes are generally chemically and electrochemically stable in air and moisture, irrespective of their level of doping, and undergo oxidation at potentials above q1.0 V w1x. In addition, the chemistry of thiophene itself lends itself more readily than other heterocycles to derivatisation, allowing a greater range of polymers with different properties to be produced w1,2,14–

0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 2 7 7 - 5

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16,18–24x. Most attention has focused on polymerisation of simple derivatives of thiophene such as 3-methylthiophene Ž3-MeT., 3-bromothiophene and 3,4-dibromothiophene. However, more complex starting materials, such as 3,6-dioxaheptylthiophene, have been used to prepare novel polymers that display electroactivity in aqueous solution w23,24x. Furthermore, a number of thiophene, bithiophene and trithiophene monomers with attached crown ether ligands have been polymerised to give materials that can be used for the electrochemical detection of cations w15x. We present here a report on the synthesis and characterisation of a PBTrS-PHE composite material. Both 3-MeT and 2,2X-bithiophene were initially selected for study, however, electropolymerisation using the former monomer did not readily give useful thin films and films under a variety of conditions. An advantage of using 2,2X-bithiophene as the monomer was that the lower potential required for its oxidation enabled polymerisation to be performed under milder conditions, reducing the probability of polymer overoxidation. The results of mechanical strength and surface morphological investigations on films composed of PBTrS-PHE are presented together with data obtained on PPyrS-PHE films. Also included is an investigation into the electrochemistry of aqueous solutions containing either Cu2q or FeŽCN. 64y performed using electrodes coated with PBTrS-PHE films.

2. Experimental 2.1. Reagents 2,2X-Bithiophene, 3-MeT and propylene carbonate were obtained from Aldrich, while tetrabutylammonium perchlorate ŽTBAP. and pyrrole were obtained from Fluka. All other inorganic reagents were of AR grade and obtained from Ajax Chemicals. Pyrrole was distilled before use while all other reagents were used as received. Deionised water Ž18 M V cm. was used throughout for preparing aqueous solutions. The structure of the S-PHE used, which was supplied by Dr. Wolfgang Wernet, is illustrated in Fig. 1. Published procedures describing the synthesis of this polyelectrolyte with different ratios of sulfate groups and free hydroxyl groups w MR s nrŽ n q m., where n s number of sulfate groups and m s number of free hydroxyl groupsx have

been reported elsewhere w25x. S-PHEs with MR values of 0.125, 0.20, 0.33 and 0.50 were used for preparing composite polymers with PPy, while for PBTrS-PHE composites, the polyelectrolyte used throughout had MR s 0.33. 2.2. Preparation of polymers Electropolymerisations were performed using an EG and G Princeton Applied Research Model 363 PotentiostatrGalvanostat together with a conventional one compartment electrochemical cell and three electrode system. All solutions were thoroughly deoxygenated with nitrogen prior to use, and experimental data was collected using a MacLab ArD and DrA data collection system ŽAD instruments.. Galvanostatic, potentiodynamic and, in some cases, potentiostatic methods were used for preparation of thin polymer films. Thin films composed of PPyrS-PHE were grown onto either platinum or glassy carbon electrodes from aqueous solutions containing 0.2 M pyrrole, 2% S-PHE and 1% H 2 O. A AgrAgClrNaCl Žaq. Ž3 M. reference electrode and a platinum mesh auxiliary electrode were also present in the electrochemical cell. Polymerisation was performed both potentiodynamically Žcyclic voltammetry; y0.2 to q1.0 V, 50 mV sy1 . and galvanostatically Žcurrent density s 2 mA cmy2 , 3 min polymerisation time.. Films composed of PPyrS-PHE were grown onto a polished stainless steel electrode from solutions with the same composition as that described above. A AgrAgClrNaCl Žaq. Ž3 M. reference electrode was again used, however, a reticulated vitreous carbon ŽRVC. auxiliary electrode was used instead of platinum mesh. Polymer growth was achieved by applying a constant current density of 2 mA cmy2 to the solution for 10 min. After polymerisation, films were washed thoroughly with distilled water, peeled off the electrode and allowed to dry. Electropolymerisation of 3-MeT was attempted using propylene carbonate solutions containing 0.5 M 3-MeT and 2% S-PHE Ž MR s 0.33.. Both glassy carbon and platinum electrodes were used as the working electrode in conjunction with a AgrAgClrNaCl Žaq. Ž3 M. reference electrode and platinum mesh auxiliary electrode. Polymerisation was commenced using both potentiodynamic Žcyclic voltammetry; 0 to q1.8 V or 0 to q1.5 V, 100 mV sy1 . and galvanostatic methods Žcurrent densitys 1 mA cmy2 , 3 min polymerisation time..

Fig. 1. Structure of the S-PHE used in this work.

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0.2 M 2,2X-bithiophene and 2% S-PHE Ž MR s 0.33. in propylene carbonate was electropolymerised to form thin films using potentiodynamic, potentiostatic and galvanostatic methods. A platinum working electrode was used throughout in conjunction with a AgrAgClrTBAP in CH 3 CN Ž3 M. reference electrode and platinum mesh auxiliary electrode. During galvanostatic growth, a constant current density of 1 mA cmy2 was applied, while polymer deposition under potentiostatic conditions was optimally achieved by applying a constant potential of q1.5 V to the system. Potentiodynamic growth of polymer was performed using cyclic voltammetry Ž0 to q1.5 V, 100 mV sy1 .. Microanalyses, conductivity and tensile strength measurements, atomic force microscopy ŽAFM. and scanning electron microscopy ŽSEM. studies were performed on free standing PBTrS-PHE films grown on a stainless steel electrode Ž18 cm2 area. using propylene carbonate solutions containing 0.2 M 2,2X-bithiophene and 2% S-PHE. A RVC auxiliary electrode and a AgrAgClrTBAP in CH 3 CN Ž3 M. reference electrode were also present. Growth of films was achieved under galvanostatic conditions by applying a constant current density of either 0.5 or 1 mA cmy2 for 5 min or longer. Examination of the chronopotentiogram obtained while polymerising for 10 min using a constant current density of 1 mA cmy2 revealed that the rest potential was q2.2 V. After growth, films were rinsed with propylene carbonate, then carefully peeled from the stainless steel electrode surface. They were then washed again with propylene carbonate before being washed with acetone and allowed to dry in a fume cupboard for 2 h. Absorption spectra were obtained using thin polymer films grown under identical conditions to those just described, except Indium–Tin-Oxide ŽITO.coated glass slides Ž10 V . were used as the working electrode and polymerisation was performed for 50 s. 2.3. Techniques and instrumentation Elemental analyses on PBTrS-PHE prepared galvanostatically by applying a constant current density of 1 mA cmy2 for 10 min were performed by the Microanalysis Unit at The Australian National University. AFM and SEM micrographs of PBTrS-PHE and PPyrS-PHE films were obtained using a Digital Multitude Nanoscope 3 and Leica Cambridge 440 Scanning Electron Microscope, respectively. Conductivity and thickness measurements were performed on films using the four-point probe method and digital vernier calipers, respectively. UV–VIS absorption spectra of PBTrS-PHE films grown onto ITO-coated glass slides were obtained using a SHIMADZU Model UV-1601 spectrophotometer. Tensile strength measurements of free standing PBTrS-PHE and PPyrS-PHE films were carried out using an Instron-4304. Cyclic voltammetry studies were conducted in a three electrode electrochemical cell using a platinum working

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electrode onto which the PBTrS-PHE composite had been deposited galvanostatically Ž1 mA cmy2 for 3 min.. A AgrAgClrTBAP in CH 3 CN Ž3 M. reference electrode and platinum mesh auxiliary electrode were used for studies conducted in propylene carbonate solution, with the former replaced by a AgrAgClrNaCl Žaq. Ž3 M. reference electrode for aqueous solution studies. Mechanical properties of PBTrS-PHE composites film were characterised by means of a stress–strain test. The samples used for the test were 3.5 cm long and 1 cm wide. Tensile strength results were obtained using an INSTRON4302 with a computer interface for date logging. For all of the trials, a 10 or 100-N-load cell was used.

3. Results and discussion 3.1. Polymer growth Takeshita et al. w12x and Chauvet et al. w13x have previously reported extensively on the electropolymerisation and characterisation of PPyrS-PHE composites grown using propylene carbonate as a solvent. We have also been able to grow thin films composed of PPyrS-PHE using solutions with a similar composition and galvanostatic, potentiostatic and potentiodynamic methods of electropolymerisation. Films composed of PPyrS-PHE were prepared as part of this work to allow a comparison with the morphological and mechanical properties of PBTrS-PHE films that are reported here for the first time. Electropolymerisation of propylene carbonate solutions containing 3-MeT and S-PHE under galvanostatic conditions Žcurrent densitys 1 or 2 mA cmy2 . failed to give a uniform film on either a glassy carbon or platinum electrode surface. Similar problems were noted when polymerisation was performed under potentiodynamic conditions Žeither 0 to q1.8 V or 0 to q1.3 V, 100 mV sy1 .. In contrast, growth of thin films composed of PBTrP-SHE on platinum electrodes was achieved using potentiodynamic, potentiostatic and galvanostatic techniques ŽFig. 2.. When potentiodynamic methods were employed, cyclic voltammetry showed an increase in current at positive potentials as the number of scans was increased, indicating that deposition of the polymer occurred ŽFig. 2a.. From the first scan, the minimum potential required to initiate monomer oxidation was determined to be q1.0 V. This value is similar to that used to initiate polymer growth from solutions containing 2,2X-bithiophene and small anions. For example, examination of the first cycle obtained during potentiodynamic polymer growth from an acetonitrile solution containing 2,2X-bithiophene and lithium perchlorate indicated that a potential of q1.0 V was also required to initiate polymerisation. Previous work on polythiophene films had shown that optimal conductivity and uniformity were obtained by growing the polymer under potentiostatical conditions at

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X Fig. 2. Ža. Cyclic voltammogram of a propylene carbonate solution containing 0.2 M 2,2 -bithiophene and 2% S-PHE at a platinum electrode. Scan y1 Ž . rate s 100 mV s . b Chronoamperogram for PBTrS-PHE grown potentiostatically at a platinum electrode using constant potential of q1.5 V and a X propylene carbonate solution containing 0.2 M 2,2 -bithiophene and 2% S-PHE. Žc. Chronopoteniogram for PBTrS-PHE grown galvanostatically at a X platinum electrode using a current density of 1 mA cmy2 and a propylene carbonate solution containing 0.2 M 2,2 -bithiophene and 2% S-PHE.

potentials 0.5 V more positive than that required to oxidise the monomer w26–32x. Similar results were also reported when a significantly higher potential than that required to initiate polymerisation was briefly applied w30,31x. The increased conductivity observed as a result of following either of these procedures has been ascribed to the simultaneous formation of a large number of nucleation sites for polymer growth w2x. Potentiostatic growth of PBTrS-PHE films was performed by applying a constant potential of between q1.1 and q1.5 V. When a constant potential of

q1.1 V was used, the resulting chronoamperogram showed that only a small, constant current was passed between the electrodes. In subsequent experiments, the magnitude of the current being passed increased as the potential applied was raised. In addition, as the current passed in any of these subsequent experiments was increased in magnitude, the longer polymerisation was allowed to continue. Fig. 2b illustrates the chronoamperogram obtained while polymerising using a constant potential of q1.5 V, which is 0.5 V more positive than that shown by cyclic voltamme-

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try to be necessary to initiate polymerisation. Inspection of the chronoamperogram reveals a steady increase in current with time that is consistent with polymer growth and deposition. The chronopotentiogram obtained during polymer growth under galvanostatic conditions is illustrated in Fig. 2c. Initiation of polymer growth required a potential of q1.6 V, substantially higher than that indicated by cyclic voltammetry to be necessary for polymer formation. Subsequently, the potential decreased only slightly as polymer deposition occurred. After 2 min, a steady state potential of q1.5 V was attained, identical to that shown to be optimal for polymer growth under potentiostatic conditions. 3.2. Chemical and physical characterisation of composites In most cases, electropolymerisation of 2,2X-bithiophene or terthiophene in the presence of small anions, such as perchlorate or hexafluorophosphate, leads to a powder rather than a compact film w2x. Powders and brittle films are also obtained when thiophene or 3-MeT are polymerised in the presence of small anions in acetonitrile w33–35x, whereas reactions performed in benzonitrile, nitrobenzene or propylene carbonate give films with good mechanical and electrical properties w26–29,36–41x. When propylene carbonate solutions containing 2,2X-bithiophene and S-PHE were polymerised under galvanostatic conditions using a stainless steel electrode, stand-alone films with exceptional strength were obtained. Previous studies by other workers had shown that electropolymerisation of either pyrrole or 3,4-ethylenedioxythiophene in the presence of S-PHEs yielded composite materials with favourable mechanical properties w3,4x. Tensile strength measurements performed on PBTrS-PHE films prepared galvanostatically gave breaking strains ranging from 98 to 128 MPa ŽTable 1.. These may be compared with values ranging between 17 and 80 MPa for PPy doped with small, sulfonated aromatic anions w42x. The tensile strength of the PPyrS-PHE grown galvanostatically was found to increase slightly as the MR value of the polyelectrolyte dopant increased. Composites prepared using the S-PHE Table 1 Effect of growth time and current density on conductivity and tensile strength of PBTrS-PHE films prepared galvanostatically from a propylene carbonate solution containing 0.2 M PBT and 2% S-PHE Current density used ŽmA cmy2 .

Growth time Žmin.

Film thickness Žmm.

Conductivity ŽS cmy1 .

Tensile strength ŽMPa.

1 1 1 1 1 0.5

5 10 40 60 90 10

3 6 33 68 123 4

0.3 0.8 0.4 0.2 0.5 0.3

128 122 118 109 98 111

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with an MR value of 0.125 gave a breaking strain of 25 MPa, while for PPyrS-PHE prepared under identical conditions using the S-PHE with MR s 0.33 this value had increased to 34 MPa. When the composite was prepared using the S-PHE with MR s 0.50, tensile strength increased only slightly further Ž36 MPa.. The above results indicate that the PBTrS-PHE composite has very useful mechanical properties. Other workers have also sought to improve the strength of polybithiophene films and films through formation of composite materials with other polymers including polyvinyl alcohol, polyvinyl chloride and polycarbonate w43x. Examination of Table 1 indicates that the conductivities of galvanostatically prepared PBTrS-PHE films ranged from 0.2 to 0.8 S cmy1 . These values are typical for polybithiophenes, which generally show conductivities of at most a few S cmy1 , several orders of magnitude lower than what is usually found for polythiophenes and polyŽalkyl.thiophenes w2x. However, it is also noteworthy that the conductivities obtained for PBTrS-PHE are considerably lower than those reported for PEDTrS-PHE composites Ž150–180 S cmy1 . w6x. Table 1 also illustrates that the film thickness per amount of charge passed increases with longer polymerisation time. This is probably due to the fact that the density of the material deposited decreases with increased polymerisation time. Despite this compact, coherent, mechanically stable films were obtained under all conditions investigated. The number of monomer units present per dopant in a conducting polymer is typically 3:1 or 4:1 for polymers derived from heterocyclic monomers and small anionic dopants w42x. However, in some instances, doping levels as high as 9:1 have been reported for polymers doped with bulky anions w44x, while PPyrpolyvinylphosphate ŽPVP. composite films exhibited a ratio of 10:1 w45x. Examination of microanalytical results obtained for PBTrP-SHE gave a value of 4:1 for the ratio of carbon to sulfur atoms present. From this ratio, it can be calculated that there were approximately two bithiophenes present per dopant unit in the composite material. This may be compared with a value of 3:1 found for PPyrS-PHE and PEDTrS-PHE composites w3,4,6x. SEM micrographs of the solution side of PBTrS-PHE and PPyrS-PHE films grown using the S-PHE with MR s 0.33 are illustrated in Fig. 3. Inspection of the micrograph of the PBTrS-PHE film reveals it to have a uniform surface morphology characterised by regions of deposited polymer generally - 3 mm in diameter. This contrasts significantly with the SEM image of the PPyrS-PHE film, which shows that polymer coverage of the surface is not as extensive. Instead, the SEM of the latter deposit indicates that the polymer surface consists of a smaller number of regions deposited polymer ranging in size from just under 1 mm in diameter to over 3 mm. A number of darker areas with little or no polymer deposited are also apparent in the micrograph of PPyrS-PHE, possibly indicating the pres-

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Fig. 3. Scanning electron micrographs of the solution sides of: Ža. a PBTrS-PHE film galvanostatically deposited Ž1 mA cmy2 for 10 min. onto stainless X steel using a propylene carbonate solution containing 0.2 M 2,2 -bithiophene and 2% S-PHE; and Žb. a PPyrS-PHE film prepared galvanostatically Ž2 mA cmy2 for 10 min. onto a stainless steel electrode from a propylene carbonate solution containing 0.2 M pyrrole, 2% S-PHE and 1% H 2 O.

ence of pores on the polymer surface. The micrograph of the PBTrS-PHE film did not provide any evidence for similar structures, while the micrographs of the electrode sides of both films revealed smooth surfaces that contrasted considerably to those of the solution sides. The surfaces of the solution sides of PBTrS-PHE and PPYrS-PHE films grown using the S-PHE with MR s 0.33 were also characterised by AFM ŽFig. 4.. Both films appeared to have very uniform polymer coverages, with the surface of the PBTrS-PHE film apparently only slightly

lumpier. A section of the PBTrS-PHE film measuring 900 mm2 was shown by AFM to have a true surface area of 1191 mm2 , a difference Žsurface roughness. of 32%. This may be compared with a value of 1% calculated from AFM data for the PPyrS-PHE film. The average distance between the peaks and valleys of nodes present on the surface of the PBTrS-PHE film was approximately 1 mm, significantly greater than approximately 0.1 mm, which was determined for the PPyrS-PHE film. Overall, these results indicate that the solution sides of PBTrS-PHE

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Fig. 4. Atomic force micrographs of the solution sides of: Ža. a PBTrS-PHE composite film galvanostatically deposited Ž1 mA cmy2 for 10 min. onto X stainless steel from a solution containing 0.2 M 2,2 -bithiophene and 2% S-PHE; and Žb. a PPyrS-PHE film prepared galvanostatically Ž2 mA cmy2 for 10 min. onto a stainless steel electrode from a propylene carbonate solution containing 0.2 M pyrrole, 2% S-PHE and 1% H 2 O.

films are rougher than the corresponding surfaces of PPyrS-PHE films.

Fig. 5 illustrates the absorption spectra of a PBTrS-PHE thin film deposited onto ITO glass immediately after

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Fig. 6. Cyclic voltammogram ŽCV. of a PBTrS-PHE-coated platinum electrode obtained after immersion in a propylene carbonate solution containing 0.5 M LiClO4 , scan rates100 mV sy1 . PBTrS-PHE was prepared galvanostatically Ž1 mA cmy2 for 3 min. using a solution X containing 0.2 M 2,2 -bithiophene and 2% S-PHE.

observations are consistent with overoxidation of the polymer and a consequent reduction in its conductivity. 3.3. Electrochemical characterisation Fig. 5. UV–VIS spectra of PBTrS-PHE composite galvanostatically deposited Ž1 mA cmy2 for 50 s. onto ITO-coated glass from a solution X containing 0.2 M 2,2 bithiophene and 2% S-PHE: ŽA. polymer after Ž . preparation; B–D an applied potential for 60 s in propylene carbonate solution containing TBAP. ŽB. 0 V, ŽC. q1.3 V, ŽD. q1.5 V.

preparation, after it was subsequently reduced, and after applying increasingly positive potentials to reoxidise the film. The spectrum of the polymer in its initial, oxidised state contained two peaks at 480 and 713 nm. Subsequent reduction of the polymer in a propylene carbonate solution containing TBAP by applying a potential of 0 V for 60 s resulted in the complete disappearance of the peak at 713 nm, and the growth of a band centred at 486 nm. Although this band is in a similar position to that of a band in the spectrum of the initial compound, its significantly greater intensity suggests that it clearly is attributable to reduction of the polymer. Coinciding with the appearance of this new absorption band was a change in colour of the polymer film from green to red. Similar electrochromic properties were noted previously for PEDTrS-PHE composites w6x. Subsequent application of a ramp of increasingly positive potentials for 60-s intervals to the reduced film resulted in a decrease in intensity of the band at 486 nm, and the reappearance of a band characteristic of the oxidised polymer at 756 nm, some 43 nm to higher wavelengths compared to the initial spectrum. The spectrum of the film after a potential of q1.2 V had been applied most closely resembled that of the original oxidised film. Application of even more positive potentials resulted in a shift in the position of the lower energy band to shorter wavelengths and the disappearance of the second absorption band. Both

Cyclic voltammetry was used to examine the electroactivity of PBTrS-PHE, which had been galvanostatically deposited onto a platinum electrode. When an aqueous solution containing 0.1 M NaNO 3 was used as the supporting electrolyte no electroactivity was detected. This is in keeping with what has been found for other polythiophenes, and attributed to the generally hydrophobic character of these polymers w2x. However, the cyclic voltammogram of a propylene carbonate solution containing 0.5 M LiClO4 ŽFig. 6. clearly illustrates that PBTrS-PHE was

Fig. 7. Cyclic voltammograms Ž100 mV sy1 . of PBTrS-PHE-coated platinum electrodes obtained after immersion in aqueous solutions containing: Ža. 0.1 M K 4 wFeŽCN. 6 x; Žb. 0.1 M CuŽNO 3 . 2 .

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Acknowledgements Partial financial support of this project by Rio Tinto is gratefully acknowledged. GGW and WEP acknowledge the continued support of the Australian Research Council. We are grateful to Wolfgang Wernet ŽCiba Geigy. for the generous supply of the polyelectrolytes used in this work.

Fig. 8. Cyclic voltammogram Ž100 mV sy1 . of bare platinum electrode in 0.1 M K 4 wFeŽCN. 6 x aqueous solution.

electroactive in this solvent. The principal features of the cyclic voltammogram are a reduction wave at q0.85 V and associated oxidation wave at q1.00 V. Also present were two smaller features at q0.60 and q0.45 V attributable to additional reduction and oxidation processes, respectively. By extending the potential range examined to q2.0 V and recording the cyclic voltammogram for several cycles, overoxidation of the polymer was estimated to commence at approximately q1.35 V. Despite the lack of electroactivity in aqueous solution exhibited by PBTrS-PHE, the polymer was sufficiently conducting to allow the cyclic voltammograms of electroactive species to be obtained. Fig. 7 illustrates the cyclic voltammograms obtained after PBTrS-PHE-coated platinum electrodes were immersed in aqueous solutions containing either Cu2q or FeŽCN. 64y. Each cyclic voltammogram clearly contains features attributable to oxidation and reduction of the metal ion. In the case of the solution containing Cu2q Žaq. reduction to elemental copper occurred at a potential of y0.25 V, with subsequent reoxidation to form Cu2q Žaq. taking place at q0.30 V. Peaks due to oxidation and reduction of the ferrocyanide ion were present in the cyclic voltammogram at q0.60 and q0.10 V, respectively. Clearly, this redox couple is not as reversible as when examined using a bare metal electrode ŽFig. 8., possibly owing to the lower conductivity of the polymer layer. Despite this, it is clear that the lack of electroactivity in aqueous solution exhibited by PBTrS-PHE does not preclude the use of electrodes coated with this material for electrochemical detection of electroactive species.

4. Conclusion The polyŽbithiophene.rsulfated polyŽb-hydroxyether. ŽPBTrS-PHE. films described in this report offer several important properties. These include mechanical strength, a high overoxidation potential and a significant level of electrical conductivity. Although the PBTrS-PHE is electroinactive in aqueous solution, other electrochemical probes do display oxidationrreduction responses.

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