In-Situ Cyclic Voltammetry and Cyclic Resistometry Analyses of conducting Electroactive polymer membranes

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1 Iteratioal Joural of ChemTech Research CODEN( USA): IJCRGG ISSN : Vol.1, No.4, pp , Oct-Dec 2009 I-Situ Cyclic Voltammetry ad Cyclic Resistometry Aalyses of coductig Electroactive polymer membraes Reza Asari Uiversity of Guila, Chemistry Departmet, Rasht, Ira, POB: Corres. rasari@guila.ac.ir ABSTRACT: Orgaic coductig polymers are usually used as electrode deposited film, powder or free stadig films. The electrochemical properties (e.g. electroactivity) of coductig polymers are usually studied as very thi deposited films (thickess<1micro). I this research, a special cell desig was employed for simultaeous characterizatio of electroactivity ad resistometry of the coductig electroactive polymers prepared as free stadig film or membrae. PPy membraes doped with some bezee sulfoates were sythesized electrochemically, the the electrical coductivity ad electroactivity of the films was measured after sythesized. Key words: Polypyrrole, electropolymerisatio, electroactivity, cyclic resistometry, cyclic voltammetry. INTRODUCTION Polypyrrole coductig polymers are easily sythesized usig a broad rage of electrolytes (as dopat) from aqueous ad o-aqueous solvets electrochemically [1, 2]. Amog the coductig polymers kow to date, oes based upo PPy have attracted special iterest because of their high coductivity ( Scm -1 ), their ease ad high flexibility i preparatio, high evirometal ad thermal stabilities ad good mechaical properties. The electrical coductivity of PPy is the product of two importat factors, the umber of carriers (e - or holes) ad charge carrier (polaro ad by polaros) mobility. The positive charges created o the polymer backboe (commoly termed polaros) are the charge carriers for the electrical coductio. Higher mobilities will occur with more crystallie, better orieted, defect free materials. The bipolaro model has bee show to provide a coheret ad uified picture of the properties of doped coductig polymers ad the possibility of a small bad gap has bee poited out. I order to prepare PPy, the moomer is first dissolved i a salt solutio (where the electrolyte is highly dissociated ad which are slightly acidic) with low ucleophilicity ad solutio resistace. Whe the solvet is ucleophile, a mild acidic coditio is preferred for electropolymerisatio. Oxidatio of pyrrole yields catioic charged polymer products with icorporated aios. Durig polymerizatio of pyrrole, electroeutrality of the polymer matrix is maitaied by icorporatio of aios (called dopat) from the reactio solutio, correspodig to oe aio for every 3-4 moomer uits, makig up 30-40% of the fial weight of the polymer depedig o the type (size, charge) of the dopat aio. The overall electropolymerisatio of polypyrrole ca simply be show as i Figure 1: N H Oxidize _ A N H + A _ Fig. 1 Overall polymerizatio reactio of polypyrrole coductig polymers

2 Reza Asari /It.J. ChemTech Res.2009,1(4) 1399 Where A - is the couterio icorporated ito the polymer durig polymerizatio i order to achieve electroeutrality [3]. A couterio (i this case a aio, typically termed a dopat aio) stabilises the charge o the polymer but is ot very mobile withi the dry material. Thus these polymers are truly electroic ad ot ioic coductors. Electropolymerisatio of PPy ca be carried out usig differet techiques icludig potetiostatic, galvaostatic ad potetiodyamic (e.g. pulsed potetial). More reproducibility i sythesis is ormally obtaied usig galvaostatic method. Fial properties (e.g. coductivity, mechaical) of the resulted polymer are depedet o various parameters such as preparatio or sythesis coditios (e.g. techique of sythesis, curret desity or potetial of electrodepositio, temperature, solvet, thickess of the film, ature of dopat, ad electrodes). Eve a mior chage i sythesis coditios may result i a great chage i the fial properties of the polymer product. For example we have frequetly observed that eve the cell desig ad geometry or cofiguratio ad the sequece of depositio all ca affect the quality of the product. Therefore, it is ot easy to optimize all of these parameters. For example it was frequetly observed that durig preparatio of polymer membrae from a sigle cell, the first grow polymer films or membrae showed higher coductivity tha the others. The cell solutio is colored after a few depositios, which is mostly due to the soluble oligomers produced durig polymerisatio. Trapig of these uwated byproducts ito the polymer matrix is believed to decrease the coductivity ad mechaical properties of the prepared polymer. Multicharge aios are ot usually suitable or preferred dopats because of disturbace of charge balace i polymer ad also ther higher ucleophilicity of the aios with icreasig the charge. As a result poor quality of the polymer with regard to electrical coductivity ad electroactivity, the two basic properties of these materials are obtaied which is ot desirable. PPy coductig polymer ad ca also udergo good reversibility betwee its coductig (oxidized form) ad isulatig state (reduced or eutral form).electroactive behaviour of the PPy is uique because the redox of polymer is accompaied by both electro ad io exchage properties as well as a chage i the electrical properties of the polymer from isulatig (reduced) to a electrical coductor (oxidized)forms as show i Figure 2. Polypyrrole coductig electroactive polymers udergo io exchage durig oxidatio/reductio process for maitaiig charge balace i the polymer. The io exchage properties of these polymers durig redox reactio proposed may applicatios such as drug release, sesors, rechargeable batteries ad cotrolled potetial separatio [4-8]. Cyclic voltammetry (CV) techique has bee widely used for measurig the electroactivity of coductig polymers [9]. The ature of io exchage is also depedet o the size of the dopat [9-11]. Fig. 2 Neutral ad oxidized redox states of PPy. EXPERIMENTALS Pyrrole (obtaied from Sigma) was used after distillatio. Electropolymerizatio was carried out i aqueous solutio. Electropolymerizatio was carried out galvaostatically usig a curret desity of 2 ma/cm 2, from a solutio cotaiig 0.2M pyrrole as moomer ad 0.05M of sodium salts of p-toluee sulfoate (PTS), p- hydroxybezee sulfoate (HBS), ad p- dodecylbezee sulfoate (DBS) as dopat electrolytes i distilled water. The time of polymerisatio was 12 mi. A sigle compartmet electrochemical cell employig a three electrode system was used for preparatio of ppy membraes [10]. Reticulated vitreous carbo (RVC), a carboeous, chemically iert coductig material with a large surface area was used as the auxiliary electrode. Mirror fiished stailess steel plate was used as the workig electrode, ad a silver/silver chloride referece electrode was used as referece electrode for moitorig potetial of workig electrode. The electrolyte ad moomer solutios were prepared freshly ad the cell solutio was deoxygeated usig high purity itroge gas before electropolymerizatio. The electrode coated by polymer was peeled from the electrode after soakig i distilled water for 15 miutes usig a sharp blade. The membraes washed thoroughly with distilled water i order to remove ay remaiig moomer, electrolyte, ad ay impurities such as oligomers ad fially dried at room temperature. The thickess of the polymer was 6 micros. Istrumetatio A Pricto Applied Research (PAR) potetiostat/ Galvaostat Model 363 was employed for electropolymerizatio. Electrical stimuli were applied to the polymer via rig shaped metal cotacts (stailess steel). A Resistometer was used for the i-situ measuremet of the resistace chages of polymers durig oxidatio/reductio. A solutio of 0.2 M KCl was used as supportig electrolyte for CV ad CR aalyses. Cyclic voltammetry experimets (CV) were carried out usig a Bio-Aalytical Systems (BAS) 100A Electrochemical Aalyser, a Electrolab, or a BAS CV-27 with the MacLab ad a recordig system (Macitosh computer). The electrochemical cell used for characterizatio of the polymer membrae is show i Figure 3. Electrical stimuli were applied to the polymer membrae via metal cotacts (rig shaped stailess steel).

3 Reza Asari /It.J. ChemTech Res.2009,1(4) 1400 Fig. 3 Electrochemical cell set up used for i-situ cyclic voltammetry ad resistometric characterizatios of coductig polymer membraes. RESULTS AND DISCUSSIONS The cell desig showed i Figure 3 permits the polymer to be characterized for its electrochemical properties as free stadig films. Electroactvity, oe of the most importat properties of coductig polymers is usually determied as a thi film coated o the surface of electrodes such as glassy carbo or platium. However; i some applicatios of these polymers such as membrae separatio, the polymer is used as free stadig films, so it is eeded to be characterized as it is applied. Simultaeous i-situ CV ad CR recorded for PPy/PTS membrae is show i Figure 4. Resistometry was used to measure the resistace chages of the polymer as it is switched betwee the oxidised ad reduced states (Fig. 4). Usig these techiques the suitable switchig potetials ca also be ascertaied. Fig. 4 Simultaeous CV ad CR of PPy/PTS membrae. Sca rate = 5mV/sec. The film thickess was 6micros. Resistace (ohm) was measured usig a resistometer.

4 Reza Asari /It.J. ChemTech Res.2009,1(4) 1401 The redox peaks labelled A/A 1 ad B/B 1 is attributed to aio ad catio movemet respectively. The cyclic voltammograms (CV) of electropolymerised polypyrrole revealed the ormal oxidatio/reductio processes accordig to: N H + A - +e -e N H Fig. 5:Redox behaviour of PPy coductig electroactive polymers o + Ā The auxiliary electrode reactios, durig the redox reactios of polymer at the workig electrode, will be the reductio of water or oxidatio of water. The electrochemical reactio of PPy ivolves the trasport of ios i ad out of the polymer i order to compesate the catioic ature of oxidised polymer [9]. Therefore, the redox reactio of PPy ivolves both electro-trasfer reactio ad mass trasport [11]. As our CV aalysis shows, the peak separatio ad broadeig icreases with icreasig the thickess. Sice the redox reactio of the film becomes more difficult, as the thickess icreases. Well defied redox peaks are usually observed whe a thi electro deposited film is aalyzed. I this work it was also foud that i spite of the great differeces i the surface morphology of the growth (porous structure) ad plate side (very smooth) i PPy films, the CV data obtaied from both sides of the polymer did ot show ay sigificat differeces. I resistometry, the resistace chages of polymers may be measured as the potetial of the electrode is varied [12]. This ew techique allows resistace chages durig dopig/udopig to be measured i-situ. Pulse resistometry aalysis of PPy/PTS membrae is show i Figure 6. As our aalysis shows, the resistace of polymer icreases whe the polymer is switched to egative potetials (reductio) ad back to its iitial value upo subsequet oxidatio. The icrease i resistace is attributed to the chagig of PPy from oxidised (PPy + ) form to reduced (PPy o ) which is accompaied by aio expulsio ad/or catio icorporatio. The large icrease i resistace of the polymer occurs at a potetial where the catio isertio peak appears ad the resistace decreases as the catio is expelled. The resistace chages with potetial is reversible if reasoable aodic potetials is applied (E < 1.0V). At aodic potetials more positive tha 1.0V will result to a irreversible coductivity or electroactivity decay [9, 12]. CONCLUSIONS Electrical coductivity of PPy films is strogly affected by the ature of its electrolyte dopat solutio used for preparatio. The cell desig itroduced i this paper make it possible to i-situ aalyses of cyclic voltammetry ad resistometry (pulsed or cyclic) of coductig electroactive polymers prepared ad used as free stadig films or membraes. Therefore, the proper potetials eeded to be applied o the polymer sample used as stad aloe membrae i a specific applicatio ca be ascertaied usig the itroduced cell desig. Fig. 6: The resistace chages (Ohm) recorded for PPy/PTS membraes durig applicatio of pulse potetials (pulse width=25sec). Electrochemical cell used was as show i Figure 3. Pulsed potetial applied were +0.6 V ad -1.0 V. 0.2 M KCl was used as supportig electrolyte.

5 Reza Asari /It.J. ChemTech Res.2009,1(4) 1402 REFERENCES 1. Diaz A.F. ad Lacroix J.C., Sythesis of electroactive/coductive polymer films: Electro-oxidatio of heteroaromatic compouds, New J. Chem., 1988, 12, Skotheim T., Hadbook of Coductig Polymers, 2th Editio, New York, Mirmohsei A., Price W.E., Wallace G.G., ad Zhao H., Adaptive membrae systems based o coductive electroactive polymers, It. Mater. Sys. Stru., 1993, 4, Weidlich C., Magold K.M., ad Jutter K., Coductig polymers as io-exchager for water purificatio, Electrochem. Acta, 2001, 47, Io R.M., Scarlat F., ad Niculescu N.I.R., Methylee blue modified polypyrrole film electrode for optoelectroic applicatios, Optoelec. Adv. Mater, 2003, 5, Rowley N.M. ad Mortimer R.J., New electrochromic materials, Sci. Prog., 2002, 85, Hosseii S.H. ad Etezami A., Polypyrrole based toxic gas sesors by mass ad coductivity measuremets, Ira. Polym. J., 1999, 8, Asari R, Alikhai, Amir H., Applicatio of polyailie/nylo composites coatig for corrosio protectio of steel, J. Coat. Techol. Res. 2009, 6 (2), Asari R., Electrochemical sythesis ad characterizatio of electroactive coductiig polypyrrole polymers, Russia Joural of Electrochemistry, 2005, 41(9) Asari R, effect of thermal treatmet o electrical coductivity of polypyrrole coductig polymers, Iraia polymer Joural, 2004, 13 (1) Asari R, Usig quartz crystal microbalace techique for ivestigatio of the effect of solutio temperature o io exchage properties of polypyrrole coductig polymers, Joural of Reactive & Fuctioal Polymers, 2003, 56, Asari R, PhD Thesis, Uiversity of Wollogog, N.S.W., Australia, *****