Initial states in the electropolymerization of aniline and p-aminodiphenylamine as studied by in situ FT-IR and UV-Vis spectroelectrochemistry

SYflTH|TII£ I I|TRILS ELSEVIER

Synthetic Metals 93 (1998) 17-25

Initial states in the electropolymerization of aniline and p-aminodiphenylamine as studied by in situ FT-IR and UV-Vis spectroelectrochemistry Angela Zimmermann a, Ulrich Ktinzelmann b,1, Lothar Dunsch b,, lnstitut fUr Physikalische Chemie und Elektrochemie der TU Dresden, Mommsenstrafle 13, D-01069 Dresden, Germany h IFW Dresden, lnstitutfUr Festkrrperforschung, Abt. Elektrochemie und Leitf~ihige Polymere, Helmholtzstrafle 20, D-01069 Dresden, Germany Received 26 June 1997; accepted 9 November 1997

Abstract

The study on the very first stages of electrochemical polymerization of aniline and p-aminodiphenylamine (p-ADPA) on electrodes by in situ FT-IR/ATR and UV-Vis transmission techniques is presented. The in situ FT-IR/ATR measurements indicate the highest polymer growth rate during the reversed cathodic potential scan in potentiodynamic electropolymerization. In this scan direction the p-ADPA radical is formed in a less anodic potential region by symproportionation of the soluble oxidized dimer N-phenyl-quinonediimine with p-ADPA formed by re-reduction. The resulting radical formed by symproportionation causes the polymer growth. The radical cations form the tetrameric Willstatter blue and red imine by dimerization and further oxidation. Therefore, the main step in polymer deposition is the radical reaction of p-ADPA. By further reactions of the first oligomers, insoluble higher oligomers are formed and deposited on the electrode. The preferred occurrence of these reactions is the reason for the more intense polymer growth by cycling of the potential in comparison to potentiostatic methods. Comparative studies of the p-ADPA polymerization were done under the same conditions. The comparison of FT-IR vibration modes of PANI and polymerized p-ADPA shows significant structural differences of both polymers. By FI'-IR and UV-Vis spectroscopy it is shown that the main structure of polymerized p-ADPA is the aniline tetramer Willstatter blue/red imine. © 1998 Elsevier Science S.A. Keywords: Electropolymerization; Spectroelectrochemistry; p-Aminodiphenylamine; Polyaniline

1. Introduction

Polyaniline (PANI) as an intrinsically conductive polymer shows a marked redox behaviour. Its electrical conductivity can be varied by a redox process [ 1 ]. Aniline as the starting material for PANI formation can be electrochemically polymerized on metal electrodes in aqueous media like HC104/ NaC104, HzSO4, HC1 [2,3] or in organic solutions like LiC104/CH3CN, NH4F/HF [4,5] by potentiodynamic or potentiostatic methods. The application of a short potentiodynamic pre-pulse at a positive potential higher than the aniline oxidation causes a strong acceleration of the initial polymerization [ 6]. To form polymer layers the potential of the electrode is scanned usually between 0 and 1 V (SHE) with scan rates between 10 * Corresponding author. t Present address: SENTRONIC GmbH, Gostritzer StralSe 61-63, D-01217 Dresden, Germany. 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved P l l S 0 3 7 9 - 6 7 7 9 ( 9 7 ) 0411 6-7

and 100 mV / s. This potentiodynamic aniline polymerization yields films with regular morphology and strong adherence [6,7] which are not formed by the potentiostatic method. During electrochemical oxidation of PANI structural changes occur in the polymer chain. While the fully reduced state of the polymer consists of benzoid units, quinoid units are formed with increasing potential. They play an important role, as the positive charges are delocalized over the polymer backbone and the conductivity increases. In the full oxidized state benzoid and quinoid structures alter in the polymer chain [8]. The nitrogen bonds change from N - C single bonds to N = C double bonds. By spectroscopic methods (IR, U V Vis) different structures can be observed [9]. First investigations concerning the mechanism of electrochemical aniline polymerization are presented by Mohilner et al. [ 10]. They proposed that polymer growth proceeds via formation of dimers, tetramers and octamers. Dunsch [ 11 ] and Stilwell and Park [ 12] showed that the polymerization

18

A. Zimmermann et al. / Synthetic Metals 93 (1998) 17-25

is accompanied by an ortho addition of aniline to oxidized structures beside the dimerization of the dimers and oligomers. At potentials more positive than those of the aniline oxidation a sufficient amount of nuclei for initiation of the polymerization is formed at the electrode. During the initial step the formation of the head-to-tail dimer p-aminodiphenylamine (p-ADPA) and tail-to-tail-dimer benzidine occurs depending on the polymerization conditions. The formation of p-ADPA predominates because of the higher rate of this dimerization reaction [ 13 ]. The dimeric radical cations form tetrameric Willst~tter blue/red imine by further dimerization and oxidation. Therefore, the main step in polymer deposition is the radical reaction ofp-ADPA [ 14]. This is suggested to be the reason for a better polymer growth caused by an anodic pre-pulse [ 15 ]. The nucleophilic addition of aniline on oxidized oligomers takes place at lower potentials. Orata and Buttry [ 16] and Inzelt [ 17] investigated the polymer deposition on the electrode during potentiodynamic polymerization. They combined cyclovoltammetry and the quartz microbalance technique to detect a significant amount of polymer deposited after the potential reversal in the cathodic scan of the potentiodynamic sweep. This was interpreted by a better solubility of oxidized oligomers (above 0.75 V (SCE)) so that the cyclic method captures more of the oxidized precursors of PANI before they can diffuse into solution [ 16]. Heinze et al. [ 18] assumed that the polymer formation leads to soluble oligomers which were deposited immediately at the electrode during discharge. The aim of this work is to clarify the very first states of PANI formation with in situ FT-IR/ATR and UV-Vis transmission techniques. These measurements should give evidence of intermediates and products during aniline oxidation to explain the polymer deposition during potentiodynamic electropolymerization. Comparative measurements with the dimer of aniline, p-ADPA, give further information on structure properties during polymer growth.

detector (600-7000 cm -~) and OPUS TRS time-resolved spectrometry software (Bruker). ATR spectroelectrochemical measurements were carried out in a cell constructed in our group and shown in Fig. 1 ~vhich was already successfully applied in spectroelectrochemical studies [ 19]. To obtain a reasonable conductivity of the working electrode and simultaneous IR detectability of the polymers we prepared the ZnSe-ATR crystal as follows. After polishing and rinsing the crystal with ethanol in an ultrasonic bath to remove polishing particles a grid structure at the crystal was prepared using a mask with gutters 50 I~m wide and gaps between them 100 I~m wide. The crystal was placed in an evaporating apparatus and glimmered at 3 X 10 ~ mbar and 200 mA discharge current. Gold was deposited without any adhesive layer. The evaporation of gold was done from a tungsten shuttle at 2-3 X 10 -5 mbar rest pressure with an evaporation rate of about 1 nm/s. The resulting gold grid structures had a thickness of 180 to 200 rim. The cell body is sealed to the ZnSe working electrode and to the Pt sheet counter electrode by rubber O-rings (Carl Freudenberg, Weinheim, Germany). Two metal foils served as electrical contacts. The cell has O-ring sealed connections to thin Teflon tubes to enable a rapid change of the electrolyte under inert conditions. A saturated calomel electrode (SCE) was situated outside the cell and connected with a Luggin capillary positioned close to the working electrode and filled with the same electrolyte as the cell. The dependence of the penetration depth of the evanescent wave from the wavelength is taken into account by the Bruker OPUS software using the formula [ AB ] x X~ 1000 = [ ATR] (X is the wavenumber). Fast UV-Vis measurements were carried out in transmission mode and performed with an Instaspec II diode array spectrometer (LOT, Darmstadt, Germany) with optical wave referenceelectrode

PTFEelectrolyteinput

2. Experimental O-ring

2.1. Electrochemical equipment For the electrochemical experiments in the three-electrode system a PG 285 potentiostat (Heka, Germany) was used which was either coupled via an IEC interface with the computer using the ASYST based software or connected by an ITC- 16 A D / D A plug-in board (Instrutech Corp., Greatpeck, NY, USA) with an Apple Macintosh system using Potpulse software (Heka, Germany).

2.2. Spectroscopic measurements The in situ FT-IR measurements were carried out with an IFS 66v spectrometer (Bruker, Karlsruhe, Germany) equiped with a fast liquid N2 cooled MCT/B semiconductor

ATR

~

I

p

~

crystal

cellbody PIcounterelectrode

? Fig. 1. Explosion scheme of the spectroelectrochemical FT-IR/ATR cell.

A. Zimmermannet al. / Synthetic Metals 93 (1998) 17-25

guides. The UV-Vis spectroelectrochemical cell was a quartz cuvette containing a Pt grid as working electrode (250 I~m mesh wide), a Pt wire served as counter electrode and a chloridized Ag wire as a quasi reference electrode (Ag/ AgCI). An additional quartz window inside the cuvette reduces the electrolyte volume (thickness of the slit roughly 2 mm). 2.3. Mass spectrometric measurements

The product was dissolved in acetone. The measurement was carried out with an FD MAT 711 A mass spectrometer (Finnigan). The input system was FD and the ion source temperature 32 °C. 2.4. Chemicals

Aniline was used for electropolymerization as aniline sulfate (C6HsNH2)2'H2504 (Aldrich, 98%) due to its higher stability against air oxidation, p-ADPA (Fluka p.a.) was freshly recrystallized three times from ligroin before use. Sulfuric acid (Laborchemie Apolda, p.a. 96%) was diluted with distilled water to 0.5 M solution. For in situ spectroscopic investigations a solution of 0.05 M aniline/0.5 M H2SO 4 was used. Due to the higher reactivity ofp-ADPA the electrolyte solution containing the dimer was diluted to 5 × 10 -3 M p-ADPA/0.5 M H2SO 4.

3. Results and discussion 3.1. Influence of pre-pulse

The study of the influence of high anodic potentials on the formation of initial products was carried out by in situ UVVis measurements at a Pt grid working electrode to obtain information on soluble species near the electrode surface. When aniline is oxidized at high anodic potentials during a pre-cycle with high scan rate (experimental conditions: 0.05 M aniline/0.5 M H2504, potential range - 3 0 0 to 900 mV (Ag/AgC1), scan rate 200 mV/s) a new UV-Vis absorption band at 425 nm is formed in the oxidized state of the system corresponding to benzidine quinoid structure (see also [ 20] ) (Fig. 2(a), (b)). This absorption cannot originate from the polaron state of PANI, because the corresponding broad and intense absorption at 750-800 nm disappears. It is known from the literature that the formation of the tail-to-tail dimer benzidine is supported by high anodic potentials of aniline oxidation. Yang and Bard [ 13] claimed that benzidine like p-ADPA is also an initial dimerization product for further polymer growth. It was observed recently that indamine radicals are formed during the electrochemical aniline oxidation in DMSO by the reaction of benzidine in its quinoid structure with aniline as detected by ESR spectroscopy [21]. The results give evidence that the formation of initial benzidine structures in the solution is enhanced at high positive

19

potentials. The oxidized benzidine also acts as nuclei for further aniline polymerization. So the application of high positive pre-pulses or pre-cycles is a way to increase the polymer growth rate by increasing the number of polymerization nuclei at the electrode surface. The high concentration of benzidine and of its oxidized form during the initiation cycle is clearly observed in the UV-Vis spectra. The simultaneous formation of the pendant dimer p-ADPA is perceptible only by a broad absorption shoulder at 300 and 500 nm. During further polymerization cycles the formation of p-ADPA and its tetrameric polymerization product becomes more dominant. The absorption peak of benzidine vanishes rapidly. It is clear that the early formed benzidine is incorporated into the polymer matrix by further reaction of benzidine quinoid with aniline to form trimeric structures [ 13,14]. Further studies are necessary to prove the completeness and irreversibility of incorporation, because the existence of this cancerogeneous substance is not acceptable for technical application of the polymer. 3.2. Polymer growth and mechanism of PANl formation 3.2.1. In situ FT-IR/ATR measurements

The study of the initial states of polymer growth was done by in situ FT-IR/ATR measurements during the first cycles of PANI formation on a gold grid-covered ZnSe reflection element (experimental conditions: electrolyte 0.05 M aniline, 0.5 M H2504, potential range 0-1 V (SHE), scan rate 10 mV/s). The penetration depth of the ATR wave is too small to determine soluble intermediates in low concentrations. Thus, only structures deposited at the electrode are detectable which enables us to determine the potential dependence of product deposition. Moreover, the detected products are slightly soluble and were deposited at the electrode, discussed as follows. The most important observation is the more intensive increase of IR intensity during reduction of the polymer film in the cathodic sweep (Fig. 3(a) first cycle, Fig. 3(b) ). The strong increase of the IR intensity occurs in the potential range close to 600 mV (SHE). There the main part of the absorption increase is caused by polymer deposition during the reversed scan. Fig. 3(b) shows the absolute IR intensity of different IR bands versus electrode potential. By increasing the polymer conductivity the IR intensity increases. It decreases again in the insulating states of PANI ('intensity waves'). The IR absorption of the polymer film is influenced by the following factors: 1. growth of the polymer layer at the electrode: the thicker the polymer film the more intensive is the IR absorption; 2. oxidation state of pol3,mer film: PANI in the middle oxidized conducting state absorbs more IR radiation due to the influence of the electronic absorption (free carrier absorption about 4000 c m - J [8,22] ). IR absorption bands of the polymer at the electrode surface were at first observed at about 800 mV (SHE) during the

20

A. Zimmermann et al. / Synthetic Metals 93 (1998) 17-25

o f the initial

Cyclovoltammogram cycle 2.5.

E e-

1.5.

0.5 ~ 0, -0.5 -301

I -100

100

300

500

700

g00

Potential I mV (AglAgCl)

o

Potential dependence

/n

of

absorbance at different wavelengths

- o - 2 6 0 nm ~

425 nm

0.06

-300 0.05 c

(al

mV

e

)

=

ouu 850

0.04

0.03 0.02 -300 -I00

100

300

500

700

g00

Potential I mV (AglAgCl) 0.2 o.~8

1 (b)

0.16 0.14

'~ 0.12

8

in situ

~o.1 V o

/

O.O8

/

'~ 0.06 0.04

/

\

0.02 0 200

. . . . /~k

ex situ ._.,- .--'~+J

x. t

i

300

400

I

I

i

i

500

600

700

800

"l[ gO0

W a v e l e n g t h I nm

Fig. 2. (a) UV-Vis spectrocyclovoltammogram of the initial cycle of aniline oxidation (electrolyte 0.05 M/0.5 M H2504, potential range - 3 0 0 to 900 mV (Ag/AgC1), scan rate 200 mV/s), with cyclovoltammogram of the initial cycle and potential dependence of absorbance. (b) Comparison of the in situ UVVis spectra of the aniline oxidation at 600 mV (backscan, (a)) with the quinoid benzidine structure, formed by electrochemical oxidation of benzidine in l M HCI.

cathodic sweep. At the end of the first cycle the vibration bands of the polymer are clearly developed. A band shift due to the formation of extended chain structures was not observed. No agreement between the vibration bands of polymer structures with those of aniline oligomers like dimers or tetramers were found. At the beginning of polymerization the obtained vibration bands are caused by PANI structures (Table 1). Therefore, the first polymerization intermediates like dimers and oligomers should be dissolved in the electrolyte and are not observable with this FT-IR/ATR technique.

The results are explained as follows. During the first step of aniline polymerization the dimerization product N-phenylquinonediimine (the fully oxidized form of p-ADPA) is formed by the radical reaction of the anodically formed anilinium cation radical with aniline and further oxidation [ 11 ]. N-Phenyl-quinonediimine and p-ADPA are highly soluble and therefore not detectable with the surface specific ATR method. During reduction of that intermediate about 600 mV (SHE) N-phenyl-quinon'ediimine is partially reduced by a two-electron transfer mechanism to p-ADPA. The oxidized

A. Zimmerrnann et al. / Synthetic Metals 93 (1998) 17-25

21

0.20- EImV

~

cathodic ~t~

200 C

015-

~ r.

~

0.10-

[g 0.05-

o

21111anodic

(a)

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(b)

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/

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number

o

o

o

d

: ,

,

¢D

o

to

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Fig. 3. (a) In situ FT-IR/ATR spectra of the first cycle of the electrochemical aniline oxidation at a gold grid/ZnSe reflection element (electrolyte 0.05 M aniline/0.5 M H2504, after a pre-cycle 0-1.2 V (SHE), 1st cycle 0-1 V (SHE), potential range - 2 0 0 to 800 mV (SCE), scan rate 10 mV/s; reference spectrum: uncovered grid). (b) In situ FT-IR spectroscopy during the potentiodynamic PANI formation in 1-3 cycles: ATR intensity of specific vibration modes vs. potential: O, 1485; ,1260; r-l, 1579; A, 1156 cm t.

and reduced form of the dimer symproportionize to form the radical cation [ 14]. In aqueous sulfuric acid the radicals form the reduced tetramer structures (see Scheme 1) which are less soluble. The octamers become totally insoluble and are deposited at the working electrode to show PANI-like IR spectra. To obtain more information on the mechanism of PANI formation we compared the polymerization of aniline with that of the head-to-tail dimer p-ADPA by the FT-IR/ATR technique (experimental conditions: 5 × 10 -3 M p-ADPA/ 0.5 M HeSO4, potential range 0-1.2 V (SHE), 10 , V / s ) . The comparison of the vibration modes of PANI and polymerized p-ADPA shows significant differences (Table 1 ). The band positions and intensities of polymerized p-ADPA are quite different from those of PANI structures. For example, the benzoid ring vibration, the C - N stretch vibration and

the C-H out-of-plane vibration are blue shifted up to 10 c m in comparison to PANI. It is important to note that the free carrier absorption of polymerized p-ADPA is blue shifted about 1000 cm ~. These observations point to products with shorter chain length. Therefore, the polymerization of p-ADPA in aqueous sulfuric acid leads preferably to oligo, e r i c structures. In the reduced state the vibration modes of polymerized p-ADPA agree very well with literature data of the aniline tetramer Willstatter blue imine [ 23 ]. Thus, it is concluded that the final product of p-ADPA polymerization is the tetramer structure. Due to the high concentration the product is deposited at the ATR crystal working electrode as an adhesive amorphous film. Thus, the conclusion must be drawn that the electrochemically formed PANI is not a linear chain of p-ADPA sequences but a more complicated two-dimensional structure

22

A. Zimmermann et al. /Synthetic Metals 93 (1998) 17-25

Table 1 FT-IR vibration modes of PANI and polymerized p-ADPA measured with the ATR technique in comparison to literature data Vibration mode

Measured data

Free carrier absorption - N - H stretch - C = N stretch Quinoid ring Benzoid ring - C = N stretch - C - H in plane - C - H out of plane Sulfate

Literature data

PANI

Polymerized p-ADPA

PANI [ 8 ]

7000, 4000 3300-3200 1650 1610 1480 1600 1500 1310 1250 1150 860, 800 1050

> 7000, 5400 3000-2860

7000, 4000

Tetramer WBI [23]

3400

1600 1500

1578 1502 1591 1502 1319 1250 1148 843

1510 1355, 1311 1260 1155 830, 810

1600 1500 1510 1380, 1310 1260 1160 830

H H

H"

H -N

. "-(=3=-N-Z=L-N

+

~'~/~

H -

~

\H

H

N

H"

~ H

\H

H ~-¢

H

H

2° 2.

-

H

~

N

xx~--# H x'M-g I~ x%--g H %--¢ h

H

H H

WBI Scheme 1.

where ortho-substitution products also play an important role [ 11,24]. 3.2.2. In situ UV-Vis measurements with Pt grid electrode

The structure of soluble species in aniline oxidation was studied by the UV-Vis transmission technique at the Pt grid electrode. Polymer species deposited at the mesh of the Pt electrode are not detectable as tested by an appropriate experiment with a polymer-covered Pt grid electrode. Beside the aniline polymerization (experimental conditions: 0.05 M aniline/0.5 M H2SO4, potential range - 2 0 0 to 900 mV (Ag/ AgC1), scan rate 10 mV/s) analogous measurements were done with p-ADPA (conditions: 5 X 10 - 3 M p-ADPA/0.5 M H 2 5 0 4 , potential range 0-600 mV, 10 mV/s) to compare the detected UV-Vis absorptions. During the polymerization of p-ADPA soluble products are detectable around the Pt meshes (Fig. 4 (a)). The absorption band ofp-ADPA at 290 nm (see also [23] ) decreases during the cycling under consumption of the educt. Starting

at 300 mV (Ag/AgC1) a very intense band at 750 nm is formed which reaches its maximum at 250 mV (Ag/AgC1) during the cathodic sweep. This band is an interband transition of diimine structures caused by the oxidized states of the tetrameric polymerization product. It decreases and shifts to higher wavelengths during further reduction. Additionally, a band at 390 nm of the reduced tetramer is formed. We found a good agreement with the chemically formed tetrameric Willst~itters blue imine (Fig. 4(b) ) (see also [ 11,23] ). This observation coincides with the symproportionation mechanism of p-ADPA with N-phenyl-quinonediimine to radical cations and the following reaction to tetramer structures [ 14]. During the oxidation ofp-ADPA in aqueous acid solution the symproportionation and radical reaction to the tetramer are preferred and higher oligomers are not found. The UV-Vis spectra of the soluble intermediates of aniline polymerization are nearly identical to those of p-ADPA polymerization (Fig. 5). During a polymerization cycle of aniline an absorption band at 290 nm is formed and vanishes

23

A. Zimmermann et al. / Synthetic Metals 93 (1998) 17-25

Cyclovoltammogram 300 • 250 • 200 • 150, 100 50. 0

0 -50 •

-100

I 100

I 200

Potential

I 300 I mV

I 400

e-, <

09

I 600

(Ag/AgCI)

Potential dependence of absorbance at different 1 wavelengths

g

I 500

".-o.--290nrn I i-a-390 nm b-~- 75o nm

>

0.8 0.7 0,6

°00/-

0.5

./ (a

~tential

ouu 850

.

0.4 0,3 i

I mV

0.2

(AglAgCI)

0.1 i

0]

t 100

I 200

Potential

I 300 / mV

I 400

t 500

I 600

(AglAgCI)

2,5

(b) /

\

/ c

= 1.5-

/

~ ' ~ " % . k _

"" \

<

./

250

/ /

ex situ 'f

~so

~"

\

in s i t u

\

0.5

0

\

I

450

I

' "~ '

~ 650 Wavelength I nm

;

' '1 '

rso

85o

Fig. 4. (a) UV-Vis spectrocyclovoltammogramof the anodic p-ADPA oxidation at a Pt grid (electrolyte 1.6 mM p-ADPA/0.5 M H 2 S O 4 , potential range 0-600 mV (Ag/AgC1), scan rate 10 mV/s), with cyclovoltarnmogramand potential dependence of absorbance. (b) Comparison of the in situ spectra of the anodic p-ADPA oxidation at 450 mV (backscan, (a)) with chemically formed tetramer Willst~itterblue imine in 0.5 M HzSO4.

with further increase of potential which is identified as pA D P A absorption. It is formed during the early states of aniline oxidation and disappears at higher potentials to form oligomer products. With decreasing p - A D P A concentration other band structures are visible. A band shift to 350 nm with a shoulder at 320 nm and the most intense band at 730 nm during further potential increase points to the oxidation of the tetramer to the quinoid Willstatter red imine [23,24]. The band at 390 nm is associated with the formation of reduced

tetramer structures. The polaron states of PANI are not detectable with this technique. With the transmission technique used only soluble intermediates were detected and the absorption must originate from the tetramer structures. Petr and Dunsch [ 14] discussed the formation of the radical cation p - A D P A °+ by symproportionizing the reduced p - A D P A with its oxidized form N-phenyl-quinonediimine and evaluating the equilibrium constant of this step. This reaction is the basis for further oligomer formation. The tad-

A. Zimmermann et al. / Synthetic Metals 93 (1998) 17-25

24

Cyclovoltammogram

250 ~

2°°t

1 / ./..,...," ...,e¢"

15o- 1l°°t

0.140.125-

-200

0.110.095 ¢-

~

200

400

0,08-

0.065<

0

600

800

Potential / mV (AoIAgCl)

0.050.035-

Potential dependence o f

--e--260 n m I - - e - 3 9 0 nm

a b s o r b a n c e at different 0.12 .wavelengths

~

750 nm

0,020.005-

170 -200

0.1

-0.012C

0

~ 0.08 0,06

. . . . . . . . al I mV

(AglAgCl)

<

0.04 0,02 0 -200

0

200

400

600

800

Potential I mV (Ag/AgCI) Fig. 5. UV-Vis spectrocyclovoltammogram of the aniline oxidation at a Pt grid (electrolyte 0.05 M aniline/0.5 M H2804, potential range - 300 to 900 mV (Ag/AgCI), scan rate 10 m V / s ) , with cyclovoltammogram and potential dependence of absorbance.

ical cations react with each other to the tetrameric form, what is proved by UV-Vis spectroscopy (Scheme 1). Aniline monomer units are able to attack these terameric polymer nuclei [ 11,13 ]. It should be noted that changes in the UV-Vis spectra are observed during the potential cycles in aniline polymerization (5-10 min break up to the next cycle) in the presence of monomer. This phenomenon is attributed to a further reaction, diffusion or deposition of polymerization products. The spectra of p-ADPA polymer did not change between the cycles. Therefore, the tetrameric structure is the final product ofp-ADPA polymerization. This assumption is confirmed by mass spectrometric measurements. The main peak of the spectra is found at M = 365, the mass of Willst~itters blue imine.

4. Conclusions

The very first structure in an anodic pre-pulse of the aniline oxidation at the electrode is the benzidine quinoid structure forming the nucleus for aniline oligomer deposition. The lower molecular weight oligomers are soluble and are deposited after further dimerization. The formation of higher oligomers takes place in the electrolyte solution. By in situ FT-IR/ATR measurements it is shown that electrochemical

polymer deposition occurs after the reversal of the anodic potential scan. In this region the diimine structures symproportionize with amine structures to radical cations which in turn may dimerize and polymerize forming longer oligomer and polymer chains. This is the reason for more intensive film formation in potentiodynamic methods. UV-Vis measurements supported the result that cyclic aniline polymerization forms at first p-ADPA/N-phenyl-quinonediimine. These dimers lead to the formation of the tetramers, Willst~itter blue/red imine. This is evidence that the polymer formation mechanism is more complex than simple addition of dimers. A further step of the polymerization is the formation of soluble oligomers which form longer oligomer chains. They are deposited on the electrode as poorly soluble products during the discharge. The structure of the electrochemical polymerization product of aniline is often assumed to be a long para-substituted polymer chain. The polymerization of the dimer p-ADPA should result in similar polymers. The comparison of FT-IR spectra of polymerization products of aniline and p-ADPA shows some differences. Therefore, electrochemically formed PANI is not only a chain with well-defined head-totail sequences but ortho-substituted and linked structures must be taken into account additionally. The FT-IR and U V Vis spectra of polymerized p-ADPA are similar to the aniline tetramer Willst~itter blue imine.

A. Zimmermann et al. / Synthetic Metals 93 (1998) 17-25

Acknowledgements T h e financial s u p p o r t o f the S~ichsisches S t a a t s m i n i s t e r i u m fur W i s s e n s c h a f t u n d K u n s t a n d the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t is g r a t e f u l l y a c k n o w l e d g e d . W e t h a n k D r K.M. M a n g o l d for c o o p e r a t i n g in the in situ U V - V i s m e a s u r e m e n t s , D r R. T h i e l s c h for p r e p a r i n g the g o l d c o a t i n g o f the A T R crystals a n d Drs A. P e t r a n d A. N e u d e c k for helpful d i s c u s s i o n s .

References [ 1] A.F. Diaz and J. Bargon, in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986, pp. 81116. [2] W.-S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 2385. [3] H. Nengebauer, A. Neckel, N.S. Sariciftci and H. Kuzmany, Synth. Met., 27-29 (1989) E185. [4] M.C. Miras, C. Barbero, R. K6tz and O. Haas, J. Electrochem. Soc., 138 (1991) 335. [5] E.M. Genibs and M. Lapkowski, J. Electroanal. Chem., 220 (1987) 67. [6] V. Tsakova, A. Milchev and J.W. Schultze. J. Electroanal. Chem., 346 (1993) 85.

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[7] C.Q. Cui, L.H. Ong, T.C. Tan and J.Y. Lee, J. Electroanal. Chem., 346 (1993) 477. [8] N.S. Sariciftci, H. Kuzmany, H. Neugebauer and A. Neckel, J. Chem. Phys., 92 (1990) 4530. [9] A. Neckel, Microchim. Acta, III (1987) 263. [ 10] D.M. Mohilner, R.N. Adams and W.J. Argersinger, Jr., J. Am. Chem. Soc., 84 (1962) 3618. [ 11 ] L. Dunsch, J. Electroanal. Chem. Interfacial Electrochem., 61 (1975) 61. [ 12] D.E. Stilwell and S.-M. Park, J. Electrochem. Soc., 135 (1988) 2254. [ 13] H. Yang and A.J. Bard, J. Electroanal. Chem., 339 (1992) 423. [ 14] A. Petr and L. Dunsch, J. Phys. Chem., 100 (1996) 4867. [ 15] R. Kostic, D. Rakovic, J.E. Davidova and L.A. Gribov, Phys. Rev. B, 45 (1992) 728. [ 16] D. Orata and D.A. Buttry, J. Am. Chem. Soc., 109 (1987) 3574. [ 17] G. lnzelt, personal communication. [ 181 J. Heinze, R. Bilger, C. Kvarnstroem and T. Kunz, in L, Dunsch, B. Klatt und W. Plieth (eds.), Elektrochemie und Werkstoffe, GDCh Monographie, Bd. 2, Frankfurt/M, 1995, p. 37l. [ 19] J. Sturm, U. Kunzelmann, S. Bischoff and L. Dunsch, Bruker Rep. No. 142, 1996, p. 8 [20] Y.-B. Shim, M.-S. Won and S.-M. Park, J. Electrochem. Soc., 137 (1990) 539. [21 ] A. Petr and L. Dunsch, J. Electroanal. Chem., 419 (1996) 55. [22] I. Harada, Y. Furukawa and F. Ueda, Synth. Met., 27-29 (1989) E303. [23] Y. Cao, S. Li, Z. Xue and D. Guo, Synth. Met., 16 (1986) 305. [24] L. Wang, X. Jing and F. Wang, Synth. Met., 41-43 ( 1991 ) 685.