Electropolymerization of aniline modified by para-phenylenediamine

Electrochimica

Acra. Vol. 40. No. 7. pp. 849-857.1995 Copyright Q 1995 Elsevier Science Ltd. Printed in Great Britain. All rights remrvcd 0013-4686/95 S9.50 + 0.00

0013-4686(94)00370-x

ELECTROPOLYMERIZATION OF ANILINE PARA-PHENYLENEDIAMINE

MODIFIED

BY

HEQINGTANG,* AKIRA KITANI, SADATOSHIMAITANI, HIROSHIMUNEMURAand MASARUSHIOTANI Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Kagamiyma 1-4-1, Higashihiroshima 739, Japan (Received 27 July 1994; in revised form 17 October 1994)

Abstract-Electropolymerization of aniline in perchloric acid solutions was investigated with presence of pam-phenylenediamine (PPDA). Experimental results showed that the presence of PPDA promoted markedly the growth of the polymer. The catalytic effect of PPDA on the polymerization of aniline included its contributions to both the autocatalysis and the external catalysis. Addition of PPDA with an appropriate concentration resulted a polymer with high polymerization degree and higher conductivity, which were closely related to the contribution of PPDA to the autocatalysis in the polymerization of aniline. Memory effects in and else properties of the catalytically electrosynthesized polyaniline were also studied. Key words: aniline, p-phenylenediamine, electropolymerization, 1. INTRODUCTION

Polyaniline (PAn) has attracted much attention in the field of conductive polymers. It is readily prepared from aqueous acid solutions, and its conducting form exhibits excellent environmental stability[ 11, moderately high electrical conductivity[2], and potential applications in secondary batteries[3, 41, in ion exchange[S], in corrosion inhibitor[6, 71, in electrochromic displaysC8, 91, in a microelectronic device[ lo], in gas-separating membranes[ 1I], in and electrocatalytic reduction[ 121 and oxidation[ 13, 141. Many studies on it have been devoted to electrochemistry of the polymerization process[15-181, while much less has been studied about its electrocatalytic synthesis though the autocatalysis in electropolymerization of aniline has been known quite well[19, 201. Zinger[21] studied effects of tiron (4,5-dihydroxy-1, 3-benzene-disulfonic acid) as a catalyst on electrochemical polymerization of pyrrole and aniline in aqueous solutions. In the presence of tiron the anodic deposition potential of aniline was shifted in the negative direction by 140-200mV, though tiron exhibited much better catalytic effects onto the deposition of polypyrrole. Hyodo et al. reported that small additions of polymer electrolytes promoted the growth rate of PAn, depending both on the nature of the polymer electrolytes[22, 231 and on the molecular weight for polymer electrolyte with same chemical structure[24]. A preliminary study of Mailhe-Randolph and Desilvestrol[25] on copolymerization of aniline (0.2M) and para-phenylenediamine (PPDA) in HCl demonstrated that small additions of PPDA to aniline solutions could impart a “network-like” mor* Author to whom correspondence should be addressed.

catalytic, memory effect

phology to the PAn films and also increased the growth rate of the polymer. For concentrations of PPDA greater than 1 mM in the starting aniline solution ([PPDA]/[aniline] > l/200), polymer growth occurred rapidly and the resulting polymer film was poorly adherent. Recently Yang and Wen[26] investigated electrochemical copolymerization of aniline (cu. 0.2M) and PPDA on IrO,coated titanium electrode in 0.5 H2S04, which showed that the growth rate was strongly dependent on the concentration of PPDA in the growth solution. It was not surprising that the copolymer exhibited different properties from those of PAn because the concentration of PPDA was considerably high ([PPDA]/[aniline] > l/38). Further information about growth rate and the properties of the PPDA-modified PAn would be needed for its practical application. Thus, a more elaborate study of catalytic effects of PPDA on electrosynthesis of PAn was reported in the present work. 2. EXPERIMENTAL All chemicals used were special reagent grade or better, and were used as received. A typical threeelectrode cell was used consisting of a 0.44cm2 Pt electrode (or a 2cm2 Pt electrode for preparation of bulky samples), a flat Pt counterelectrode, and a saturated calomel reference electrode (see). All the electrochemical experiments were performed at 25°C under an inert atmosphere, with using a HA-301 Potentiostat (Hokuto Denko Ltd), equipped with a HB-104 Function Generator (Hokuto Denko Ltd). Voltammograms were plotted with a YEW Type 3086 X-Y Recorder (Yokogawa Electric Works, Ltd). PAn films were obtained in solutions of 1.3 M aniline and 2.3 M HCIO,, referred to as the base

849

H. TANG

850

solution, with or without presence of PPDA by applying sequential linear potential sweeps at SOmVs-’ between -0.2 and f0.8 V. For cyclic voltammetric measurement in 0.1 M H,SO, or 0.2 M HC104 at 50 mV s-i, the films were prepared galvanostatically until a given amount of the total anodic charge Q. had passed to obtain a film with suitable thickness, while the bulky samples for measurement of the molecular weight and the electric conductivity were prepared potentiostatically at 0.8 V. The bulky samples were removed from the cell after being held at 0.45 V (in the emeraldine salt) for 4min, and then were powdered after a well rinsing procedure and a vacuum drying at 30°C for about 24 h[2]. Prior to the cu measurement of the PAn film, the film electrode was rinsed by dipping it into distilled water to remove the excess acid and oligomers produced. In order to compare the first cyclic voltammogram and the multicycle voltammogram, the film electrode was held at a waiting potential E, for a waiting time t, prior to potential scanning. Usually E, = -0.2V and tw = 1 min, unless specified elsewhere. A relative loss of the cathodic charge of the film in the cyclic voltammetric responses after being oxidized at a given condition was measured to evaluated the chemical stability of the film. After the bulky sample was dissolved in DMF, its molecular weight distribution was evaluated with GPC, calibrated with polystyrenes of known mass. The elemental analysis of the polymer samples for C, H, and N was carried out. The conductivity was measured with the conventional four-probe method after the powdered samples were made pellets. ESCA measurement, including the survey spectra measurement and the multiplex measurement, was conducted using the ESCA PHI 5400 (Perkin-Elmer Co.) system. The vacuum level during the measurement was lower than 10e9Torr. The X-ray power of the Mg-K, radiation was about 400 W. 3. RESULTS

AND DISCUSSIONS

It has been showed that the kinetics of thick (> 1Opm) films has features quite different with respect to that of thin (< 1 pm) films[15]. The concentration of aniline used in the present study is so high that it is quite easy to grow a film with a thickness up to lo-20mm. Hence different growth features are expected for the resulting films. Several parameters may be used to evaluate the growth of PAn films, such as, the current of the first oxidation peak (I,), the weight of the film, and the polymerization current at the upper potential limit (I,,,). It is impossible to weigh films much beyond a lprn thickness due to the peeling and flaking problems encountered during the drying stage. And the I r,,,lymay be not so effective to completely apply to film growth. Therefore, the dependencies of I, on the growth cycle number (N) was used to evaluate the film growth rate, which was validly used to monitor the rates of polymer depositionC27-J.

et al.

3.1. Eflects 0fPPDA on thefihn growth Cyclic voltammograms of the Pt electrode in the base solution containing PPDA of various concentrations were measured, parts of which were given in Fig. 1. PPDA of a low concentration (up to about 0.15mM) affected little the growth of PAn. At moderate concentrations (from 0.5 to lOmM), PPDA increased the growth of the polymer without marked changing of the shapes of the cyclic voltammograms. It was evident that PPDA exhibited a catalytic effect on the polymerization of aniline. The higher the concentration, the stronger the catalytic effect. However, if the concentration of PPDA was too high (50mM or higher), the cv shape during the polymerization changed due to the relatively faster increasing of the current for the middle peak (at ca. 0.55V) with respect to the increase of the current of the first oxidation peak. Thus, the cyclic voltammogram with presence of 1OOmM PPDA looked quite different from the others; in this case, Ip,, was very high but I, was surprisingly low, which was possibly because the resulting polymer was more soluble. The measurement of uv-vis spectra of the resulting polymers indicated that the solubility of the polymer obtained with addition of 1OOmM PPDA is much greater than that of the PAn obtained with absence of PPDA. The existence of middle peaks in cyclic voltammograms during the polymerization has been well known[28-301. Usually, it is related to the redox response of some intermediates formed during the polymerization. For example, Genies et al.[30] have suggested that it is related to the presence of phenazine rings resulting from the formation of crosslinked PAn chains by a direct reaction of the aniline nitrenium cation (CBHSNH+) or a reaction between the PAn chain itself, through the substitution of a nitrenium cation in another PAn chain. With increase of the PPDA concentration, a middle anodic peak (at about 0.55 V) appeared in the cyclic voltammograms [Fig. l(c) and (d)], which was similar to the observation of Mailhe-Randolph and Desilvestrol[25], and and Wen[26]. Mailhe-Randolph and Yang Desilvestro[25] suggested that the middle peak may be attributed to (a) a crosslinking site induced by PPDA during the polymerization and (b) the oxidation product of PPDA, eg, a partial hydrolysis to a quinone. Yang and Wen[26] assigned the middle benzoquinonepeaks respectively to the para-aminophenolhydroquinone the and benzoquinone-imine reactions with the later being the predominant process controlling the observed cu behavior. The insertion of PPDA units into the PAn main-chains would favor the formation of the intermediates. It was easily found from Fig. 1 that the current for the middle peak increased markedly with increase of the PPDA concentration. This additional peak might correspond to the para-aminophenolbenzoquinone imine redox couple and crosslinking sites as suggested by Yang and Wen[26]. The catalytic effects of PPDA on the polymerization of aniline were more directly found from the dependence of I, of N showed in Fig. 2, which were well linear at a suitable interval of N. PPDA gave the best catalytic effects at about 1OmM based

Electropolymerization

8.51

of aniline N = 20 A

N = 25

‘9

(a)

-2wtw -a.*

1

1

,

0

0.2

_

/

0.4

,

,

0.6

,

0.8

E O-9

200f

-o.?--

0

0.2

Fig. t. Cyclic volt~rnu~~rn~

on the dependence

0.6

0.8

(d)

-o.T-+-

0

0.2

___I_” 0.4 0.6 0.8 . E 0’) during the ~~rne~~t~oa of aniline from the base sofution containing {a) 4 (b) 1, (c)SO and (d) 1OOmM PPDA.

of the slope on the PPDA conmonomers (E,,) shift negatively. Thus, the shift of centration. E due to presence of PPDA, AE,, = E,,,Ot _!YEpfk), was possibly used to evaluate another part The autocatalysis has been a well-knob fact in ~i~rneri~~on of aniIine[f6, 19, 243. It is possibly of the catalytic effect (excluding the auro~ta~ysis~ evaluated, though not strictly, by the difference of exerted by PPDA, which might be referred to as the polymerization potentials for the first (E,,) and external catalysis compared to the autocatalysis, second (E,,) cycles, that is, A&,, 2 = E,, - EDZr Hence, the catalytic effects of PPDA consisted of the which will be influenced by the presence of PPDA. contributions to both the auto- and the externalFurthermore, as a conventional catalyst, added catalysis during polymerization of aniline. It was eviPPDA made the ~~~rneri~at~on potential of the dently found from Table 1 that, with the increasing

H. TANGet al.

852 250

IOOmM ASOmM 10mM ZOOo5mM +I mM A

n

4 150- 0 0.5mM x 0.15 mM E l OmM 3 IOO-

50-

0

.

.

.

. 0

. A0 .AO

. l. Cl 0

0

+ +o

;/

‘An

9 azF:&:

.,@ np .

= *

0 I

5 CyZe

numbe:5(N)

I 20

25

Fig. 2. Dependence of I, on N During the polymerization of aniline with presence of PPDA.

of the PPDA concentration, its effects on the external catalysis became stronger, while its effects on the autocatalysis showed a maximum at about 5mM.

The different trends in the influences of PPDA on the autocatalysis and the external catalysis reflected the complexity in the catalytic polymerization of aniline. PPDA is more easily oxidizable than aniline with formation of a rather stable cation radical or quinonic dication. This is beneficial to the initiating and stabilizing of the radical cations and dications rising from the aniline monomer, which has been thought as the prerequisite for the growth of PAn. Moreover, the observation of polymer morphology suggested that PPDA function as a crosslinking agent in the polymerization reaction[Z, 261. More branching and porous polymer films were obtained as the PPDA content was increased in the growth solution. The increase of the active surface area in the polymer film was of course one of the reasons accounting for the enhanced growth rate. Furthermore, the oxidation of the polymer mainchain, being necessary for its further polymerization, may lead to its hydrolysis and degradation rather than further polymerization. Stilwell and Park[31] suggested that PPDA was possibly involved in the hydrolysis and degradation process of PAn as a component of the products. It was reasonable that addition of PPDA would be beneficial to further polymerization of the polymer chain due to the shifting of the related equilibrium state. All of these mentioned above made contributions to the catalysis effects of PPDA. Each of them

would lead to increasing in the polymerization current and the growth rate. Hence, the polymerization current (Fig. 3) and the polymer growth rate for PPDA concentration not higher than ca. 1OmM (Fig. 2) increased with increase of the PPDA concentration. For PPDA concentrations higher than about 50mM ([PPDA]/[aniline] 2 l/26), the growth rate decreased rapidly with increase of PPDA concentration (Fig. 2). Similar depression of the growth rate at higher PPDA concentrations U’pDAl/ [aniline]) > -l/9) was also observed by Yang and Wen[26], who assigned it to another dramatic change in polymer morphology, ie being absence of fibers. This explanation seemed invalid in the present study because the polymerization current at + 0.80 V was high enough though the peak current of the first oxidation peak (the growth rate) became much lower [Fig. l(d)]. During the polymerization at higher concentrations of PPDA, many more colored substances were observed being diffusing from the electrode surface. The results of GPC measurements showed also that the polymers with much lower molecular weight were obtained

at high PPDA concentrations

(Fig. 4). The measurement of uu-vis spectra of the resulting polymers indicated that the solubility of the polymer obtained with a presence of 100mM PPDA is much greater than that of the PAn obtained absence of PPDA.

Cycle number(N)

Fig. 3. The polymerization currents at +0.8 V for the first five cycles during the polymerization.

Table 1. Dependence of polymerization potential shifts on PPDA concentration* cppnA/mV

E,,IV

E,,IV

0 0.15 0.5 1 5 10 50 100

0.740 0.740 0.730 0.730 0.680 0.620 0.570 0.550

0.710 0.710 0.650 0.630 0.570 0.530 0.520 0.520

with

AE,,/mV 0 0 10 10 60 120 170 190

AE,,, JmV

ZAEJmV

30 30 80 100 110 90 50 30

* A=%,= E,,(c) - E,,(c = 0); AE,,. 2 =E,,-EpZ;~AEp=AEp,+AEp,,*

30 30 90 110 170 210 220 220

Electro~lyme~zation

1 mM PPDA

853

of aniline

100 mM PPDA

61000

4

4 7

*

R, (mini’

t2

6

8

10

12

14

R, (min)

l4

30000

6400

4000 / 1200

28300 50 mM PPDA

0.5 mM PPDA

m

i4

6

8 30000

10

6

12

14

4

6

8

OmMPPDA

8

\

----_L

IO

12

14

10

12

14

R, (min)

R, (min)

6~~

4

\

-j

10

12

10mM~~~oo

14

4

6

8

R, (min)

R, (min)

Fig. 4. The molecular weight distribution of PAns obtained with the presence of PPDA.

In addition, during the polymerization in 2.3M HCIO, containing 0.1 M PPDA without aniline monomer, the polymerization current was much lower and no extensive polymer growth was observed on the electrode surface except of the initial formation of a thin polymer layer, though quite a lot of colored species was formed in the solution. Therefore, it could be concluded that, it was the formation of rather soluble oligomers and polymer with lower polymerization degree rather than another change in the morphology resulted from the PPDA of high concentrations that made the accumulation of the polymer on the electrode surface became much more difficult. Thus, the film growth rate (I,) in the cases of higher PPDA concentration was lower than that expected from the view of the high polymerization current [Fig. l(c, d)]. 3.2, Properties of PPDA-modified

PAn

Evident effects of PPDA concentration on the molecular weight distribution of the polymers were easily found from the data measured with GPC showed in Fig. 4. With increasing of PPDA concentration, formation of polymers with higher polymerization degree was somewhat favored initially due to the crosslinking effect, and then much more soluble polymers with lower molecular weight was obtained because of too many reactive centers resulted from high PPDA concentration. Eb. 4Q:Z.I

Figure 5 shows the ESCA survey spectrum of the PAn obtained with presence of 50mM PPDA. The core level of the carbon Is, nitrogen Is, and oxygen 1s appeared around 286, 402 and 534eV, respectively. In the deconvoluted ESCA nitrogen multiplex spectrum, the second peak of higher binding energy is assigned to the positively charged nitrogen atoms in the main-chains. Hyodo et al.[23, 24] reported that the strength of the second peak depended on the added polymer electrolyte, but we could not observe similar influence of PPDA on the second peak. It was reasonable because no new function group was introduced into the structure of PPDA-modified PAn. Elemental analysis were carried out by a few of groups[6, 32-341. The chemical composition of the polymers derived from the data of the elemental was similar to those we reported analysis previously[32], with a total C, H, N content of 6468.5% for the PAn salts. In consideration of the uncertainty in explanation of the elemental analysis data due to the effects of the doping and drying processes[W], only the atomic ratio of nitrogen to carbon was checked here. The N-C ratio is independent of the doping level and the drying process, with an idea1 value of unity. If the chemical composition of PAn is formulated as C,H,N, . m(dopant) . nH,O, y = 1 for the ideal case, and usually y < 1 because of partial degradation of PAn. Our measurements indicated that y took a value of 1.01, 1.01, 1.04 and 1.07

H. TANGet al. 10

8

0 1000

800

600

400

200

0

Binding energylev Fig. 5. ESCA survey spectrum of PAn synthesized with presence of SOmM PPDA.

corresponding to the presence of PPDA with concentration of 0, 10, 50 and 1OOmM. This suggested insertion of PPDA into the PAn mainchains because of its crosslinking effects. Cyclic voltammograms of the resulting polymers were obtained in both 0.1 M HzS04 and 0.2M HClO,. The CDbehaviors of the polymer prepared from the base solution were similar to those reported in the literature. A given PAn showed similar ce, behaviors both in 01. M H,SO, and 0.2 M HCIO,, except that the corresponding peaks in 0.2 M HCIOo shifted negatively by about 5OmV compared to those in 0.1 M H,SO, (Fig. 6). The cyclic voltammograms of the polymer prepared with presence of low concentrations of PPDA (up to cu. 0.5 mM) kept

nearly the same as the pure PAn. However, if the PPDA concentration was increased up to about 5mM or higher, an additional oxidation peak appeared at about +0.34V (Fig. 7), which implied that new oxidizable intermediate had been formed. The peaks in the cyclic voltammograms broadened and emerged into only one very broad one with the consecutive potential cycling. The stability of the resulting polymers was evaluated with the relative loss in the couiombic capacity (Q,) during potential cycling in 0.1 M H$O,. After a cycling of 150 cycles, the relative loss in the coulombit capacity was 21, 12 and 19% for the polymer

t .0-

0

’

0.2

w”.-0.6

0.8

E (voj” Fig. 6. The first (dotted) and multicycle (solid) volt&mmograms of PAa in 0.1 M H,SO, and 0.2 M HCIO, . The polymerization conditions were: cppDA= 0.5 mM, ip = 1 mAcra_‘, and Qp = lOOmCcm-“.

Fig. 7. The first (solid) and multicycle (dotted) voltammograms of PAn in 0.1 M H,SO,. The polymerization conditions were: c,, = 0.5mM, i = 1mAcm’*, aad Q, = IOOmCnn-‘.

Electropolymerization prepared from the base solution containing PPDA of 0, 0.5 and 50mM, respectively. These results suggested the presence of PPDA with a moderate concentration favored to the stability of the resulting polymer. The total conductivity, o,, of doped PAn is a function of its intrachain (intramolecular) conductivity, ga and interchain (intermolecular) conductivity, (r,,[35], which would be affected many parameters including the presence of PPDA. The conductivities of the polymers obtained with presence of 0, 0.5, 5, 50 and 1OOmM PPDA were 12.5, 38.7,43.3, 12.5 and 6.4 S cm- i, respectively. PPDA (with an u~~ropriafe concentration) favored the formation of longer PAn mainchains and hence extension of the conjugated a system in the mainchain, leading to increase of fhe conducfiuity. A denser structure of the polymer was

also beneficial to the increase of the conductivity. However, PPDA of a high concentration provided too many active sites and hence shortened the length of the PAn mainchains, resulting in decreasing of conductivity. It seemed that both the molecular weight distribution and the conductivity of the polymers were closely related to the autocatalysis due to the presence of PPDA: a maximum of AE,,, L at 5mM PPDA was accompanied by a maximum of the conductivity at 5 mM PPDA. 3.3. Memory e#ect in PPDA-modi~ed PAn Doping is a prerequisite to enable PAn being electrochemical active, however, the doping level achieved and the resulting redox response depends on the history of the applied potential during the doping. This history dependence of the doping response, is the so-called “first cycle effect”[36] or “memory effect”[37]. The shape of the first voltammogram recorded after waiting at the reduced state, is different from that observed during continuous cycling. The initial oxidation (the first cycle) evidently requires a substantially greater overpotential than on the subsequent cycle[36], and yields higher peak current[36, 371 and redox charge[36]. In addition, the memory effects have also been observed in other properties, such as ion exchange[38], the spin susceptibilityC371 and the optical absorbance[39]. Figure 6 indicated that with a 1min holding at a reduced potential (-0.2 V), the initial oxidation peak, or the “relaxed” peak (because “something” is relaxing during the waiting time) was shifted towards more positive potential and was higher than the peak observed in steady state conditions, which was similar to the facts reported in the literature~36-391. The first voltammogram and the multicycle voltammograms were respectively similar to each other in 0.1 M H,SO, and 0.2 M HCIO, except that the peaks shifted negativeiy about 5OmV correspondingly in 0.2 M HCIO, compared to those in 0.1 M H,SO,. These results were nearly the same for the polymers obtained from the solution containing PPDA with a con~ntration up to about 0.5 mM. When the concentration of PPDA was increased to 1mM, the first voltammogram of the resulting polymer showed an additional peak (at about 0.35 V), which disappeared in the second voltammogram (Fig. 7). It was still true that there was only the first cycle memory, that is, the second voltam-

855

of aniline

mogram coincided nearly exactly with the multicycle voltammograms. This additional peak appeared still in the first voltammogram for higher PPDA concentrations, and shifted towards more anodic potentials with increasing of the PPDA con~ntration. When the PPDA concentration was ca. 5mM or higher (Fig. S), the additional oxidation peak appeared in the first voltammogram (at about 0.45 V in the case of IOmM PPDA), decreased in the second voltammogram and disappeared in the multicycle voltammograms; at the same time, another new additional peak appeared at a less anodic potential (at about 0.35 V in the case of I OmM PPDA) in the second voltammogram and increased to a higher value in the third and multicycle voltammograms, as well the first oxidation peak recovered a little in the third and multicycle voltammograms compared to its height in the second voltammogram. With consideration of the first cycle effect reported in the literature[36-391, we might refer to the fact observed here as the second cycle effect. The memory effects in PAn were influenced by many other factors, such as the wait-time, the waitpotential, and the scan rate. Here the following parameters were used to evaluate the degree of the memory effects: the peak potential shift A.Eij = Ei - Ej, the peak current increment Al, = li - Ij, and the relative current increment Al,+,, where i = 1 for the first cycle, and j took a value of 2 for the second cycle and 3 for multicycle. The strong influence of the PPDA concentration on the memory effects in the resulting polymers was also readily found from Table 2. The peak potential shift AE,, increased with increase of the PDDA concentration, and the second cycle effect appeared when it became about 5mM or higher. The relaxing process of the polymer at E, = -0.2 V was very fast.

.o0

0.2

0.6

0.8

E (v”;”

Fig. 8. The first (I), second (2) and multieycle (3) voltamrno~~s of the polymer in 0.1 M H,SO,. The p&ymeiization conditions were: c,, = lOmM, ip = 2mAemma, and Q, = 100mCem-2.

H. TANGet al.

856

Table 2. Dependence of the peak potential shift (BE, mV) on the PPDA concentration (cppoA, mM). Preparation conditions: i, = 2mAcm-’ and Q, = lOOmCcm-* %‘DA

A%, AE,,

0 10

0.15 15

0.5 20

1 25

5 30 20

10 40 20

50 40 25

100 50 40

Thus, no evident effects of t, from 1 to 20min were observed on all the cyclic voltammograms. With t, = 1 min, more negative waiting potentials than -0.2 V exerted little effect on the second and multicycle voltammograms, but somewhat influenced the first voltammogram. For instance, when E, = -0.4V, the potential of the first oxidation peak shifted further positively by about 1OmV and the oxidation currents in the whole potential window increased little compared to those in the case of E, = -0.2V. Odin and Nechstein[39] reported that the thickness exerted little effect on the memory effects. However, we observed that the Qp, hence the film thickness influenced the potential shift of the first oxidation peak. For instance, for the first anodic peak at about 0.15V in the polymer obtained with 1OmM PPDA at i, = 2 mAcm_‘, the potential shifts AE,, = 25, 35 and 40mV and AE,3 = 15, 20 and 25mV corresponding to Q, = 50, 100 and 2OOmCcm- ‘. As for the peak current, with a few of exceptions, it was general that with increase of the thickness, AI 1j increased but AI lJIj decreased because of the increase of the proper Zj. The polymerization current density also influenced somewhat the memory effects: higher i, resulted usually higher peak current for the first anodic peak compared to other peaks in the first voltammogram. The reason accounting for the memory effects in polyaniline was yet not definite. About the first cycle effects, one of the explanations for it is based on the dependence of conductivity on the polymer redox state. The conductivity decreases with reduction. A complete reduction is achieved by maintaining the film at negative potentials for some time. A higher overpotential is therefore necessary to oxidize the film. During continuous cycling the film is never totally reduced and the multicycle voltammogram shape is different. However, it was surprising that the effect of the temperature was very weak though it should be strong for relaxation phenomena being generally activated processes[39]. In addition, this explanation seemed to not be so reasonable for the secondary memory effects in the polymer. An alternative explanation might be an alteration of the exchange of ions and/or solvent coupled to the redox process. The ion exchange of PAn, and hence the memory effect, was not only affected by the pH but also by the ionic strength of the solution[39]. The related ion species included usually H+ and anions. It was difficult to explain why there was no essential difference between the memory effects exhibited by the polymer in 0.1 M H,SO, and those in 0.2M HCIO,, though their anions were well considered as typical of two quite different groups[15]. The disorder nature of the polymer may play an important role, especially in the second cycle effect.

4. CONCLUSIONS The presence of PPDA, primarily as a crosslinking agent, exhibited significantly catalytic effects on the electropolymerization of aniline in acid solutions. The polymerization current increased with increase in the PPDA concentration, while the deposition rate of the polymer onto the electrode surface showed a maximum with the increase of the PPDA concentration because of less efficient deposition of more soluble polymer in the case of high PPDA concentrations. There were differences in the properties of the polymers synthesized catalytically from those of PAn synthesized conventionally. The catalytic effects exerted by PPDA involved its contributions to both the autocatalysis and the external catalysis. The promoted growth rate and the improved electrical conductivity of the resulting polymers were much more dependent on its contribution to the autocatalysis. The memory effects in PPDA-modified PAn were investigated, which were strongly dependent on the experimental conditions. The presence of high enough concentrations of PPDA in the growth solutions resulted in occurring of the second cycle effect, which was rather different from the conventional first cycle effect. Further investigations would be carried out in our laboratory. REFERENCES 1. E. M. Genies, M. Lapkowski and C. Tsintavis, New J. Chem. 12, 181 (1988). 2. K. Yoshikawa, K. Yoshioka, A. Kitani and K. Sasaki, J. electroad Chem. 270,42 1 (1989). 3. A. Kitani, M. Kaya and K. Sasaki, J. electrochem. Sot. 133, 1069 (1986). 4. T. Osaka, S. Ogano and K. Naoi, J. electrochem. Sot. 136, 306 (1989). 5. N. Oyama, T. Ohsaka and T. Shimizu, Arm\. Chem. 57, 1526 (1985). 6. G. Mengoli, M. T. Munari, P. Biacco and M. M. Musiani, J. appl. Polym. Sci. 26,4247 (1981). 7. R. Noufi, A. J. Nozik, J. White and L. F. Warren, J. electrochem. Sot. 129,226l (1982). 8. S. Gottesfeld, A. Redondo and S. W. Feldberg, J. electrochem. Sot. 134, 271 (1987). 9. K. Hyodo, Electrochim. Acta 39, 265 (1994). 10. E. W. Paul, A. J. Ricco and M. S. Wrighton, J. phys. Chem. 89, 1441 (1985). 11. M. R. Anderson, B. R. Mattes, H. Reiss and R. B. Kaner, Science 252, 14 I2 ( I99 1). 12. G. Mengoli and M. M. Musiani, J. electronnal. Chem. 269,99 (1989). 13. L. Doubova, M. Fabrizio, G. Mengoli and S. Valcher, Electrochim.

14.

Acta 35, 1425 (1990).

V. E. Kazarinov,

V. N. Andreev, M. A. Spitsyn and A. P. Mayorov, Electrochim. Actn 35, 1459 (1990). 15. P. Nunziante and G. Pistoia, Electrochim. Acta 34, 223 (1989). 16. D. E. Stilwell and S.-M. Park, J. electrochem. Sot. 135, 2254 (1988).

17. Y.-B..Shim,

M.-S. Won and S-M.

Park, J. electrochem.

Sot. 137, 538 (1990). 18. J. F. Wolf, C. E. Forbes, S. Gould and L. W. Shacklette, J. electrochem. Sot. 136, 2887 (1989).

19. K. Sasaki. M. Kava, J. Yano, A. Kitani J. electround. Chek 215,401 (1986). 20. G. Zotti, S. Cattarin and N. Comisso, Chem. 239,387 (1988).

and A. Kunai. J. electroanal.

Electropolymerization 21. B. Zinger, J. electroad Chem. 244, 115 (1988). 22. K. Hyodo and M. Nozaki, Electrochim. Acta 33, 165 (1988). 23. K. Hyodo, M. Omae and Y. Kagami, Electrochim. Acta 36,87 (1991). 24. K. Hyodo, M. Omae and Y. Kagami, Elecrrochim. Acta 36,357 (1991). 25. C. Mailhe-Randolph and J. Desilvestro, J. electroanal. Gem. 262,289 (1989). 26. C.-H. Yang and T.-C. Wen, J. appl. Electrochem. 24, 166 (1994). 27. Y. Wei, Y. Sun and X. Tang, J. phys. Gem. 93, 4878 (1989). 28. T. Kobayashi, H. Honeyama and H. Tamura, J. electroanal. Chem. 177, 28 1(1984). 29. A. Kitani, M. Kaya, J. Yano, K. Yoshikawa and K. Sasaki, Synth. Met. 18, 341 (1987). 30. E. M. Genies, M. Lapkowski and J. F. Penneau, J. electroanat. Chem. 249,97 (1988).

of aniline

857

31. D. E. Stilwell and S.-M. Park, J. electrochem. Sot. 135, 2497 (1988). 32. A. Kitani, 3. Izumi, J. Yano, H. Hiromoto and K. Sasaki, Bult. them. See. Jap. 57,2254 (1984). 33. A. G. MacDiarmid. S. L. Mu. N. L. Somasiri and W. Wu, Mot. Cryst. liq. Cry& 121; 187 (1985). 34. D. E. Stilwell and S.-M. Park, 1. elecrrochem. Sot. 135, 2491 (1988). 35. E. J. Oh, Y. Min, J. M. Wiesinger, S. K. Manohar, E. M. Scherr , P. J. Prest, A. J. MacDiarmid and A. J. Epstein, Synth. Met. 5557,977 (1993). 36. M. Kalaji, L. M. Peter, L. M. Abrantes and J. C. Mesquita, J. e~e~fro~~u~.Chem. 274,289 (1989). 37. C. Odin and M. Nechstein, Synth. Met. 41-43, 2943 (1991). 38. C. Barbero, R. KGtz, M. Kalaji, L. Nyholm and L. M. Peter, Synth. Met. 55-57, 1545 (1993). 39. C. Odin and M. Nechstein, Synth. Met. 55-57, 1281 (1993).