electrooxidation of aromatics to polymer films

Makromol. Chem., Macromol. Symp. 8, 17-37 (1987) 17

ELECTROOXIDATION OF AROMATICS TO POLYMER FILMS

Jean-Christophe LaCroix A. F. Diaz * IBM Research, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099, USA

Abstract - This report describes the electrochemical preparation and properties of polyaniline films prepared on a bare platinum electrode and on electrodes precoated with a thin film of inert polymer. Films were prepared and studied in several aqueous acid electrolytes including sulfuric, hydrochloric, nitric, trifluoromethylsulfonic, hydrofluoric and trifluoroacetic acid. It is observed that both the concentration and nature of the acid influence the kinetics of the switching reaction. In the more acidic solutions, the reaction has a t dependence and depends on the movement of a front across the thickness of the film. In the less acidic solutions, the reaction has a t1I2 dependence and depends on the diffusion of ions in the film. The same response is observed for the oxidation and the reduction reaction. Polyaniline is also electroactive in other various nonaqueous solvents, such as, alcohols, acetic acid and acetonitrile, and voltammograms with well defined waves result when solution contains both organic salt and protic acid. Polyaniline was also prepared on a platinum electrode coated with an inert polymer film such as polymethylmethacrylate (PMMA). The resulting polymer blend is electroactive and the switching reaction is slower than for polyaniline formed on a bare platinum electrode. Finally, a brief description is given of electroactive polyaniline films which can be prepared by polymerization of aniline in a glow discharge.

INTRODUCTION

The electroactive properties of polyaromatics generated by electropolymerization is a subject

of investigation in numerous laboratories. These materials are of interest because of their

possible use in applications such as electrochromic displays (1.2). storage batteries (3), and

sensors (4). Convenient for these applications, is the fact that these materials can be used

directly as formed on the electrode. Most of the studies have been performed with the

polymers from the derivatives of pyrrole, thiophene and aniline. Since the latter has not been

a subject of recent review (5 ) . we have focused this manuscript which was written for the

International Workshop on "Electrochemistry of Polymer Layers," on the electrochemical

characteristics of polyanilie with the intent of providing a brief overview of the preparation

and electroactive properties of the composite films of polyaniline prepared by electrooxidative

polymerization of aniline. The films are composites of the oxidized polymer (ca. 50% by

18

weight) and the anion of the electrolyte. Also-described are blends which are films resulting

from the polymerization of aniline inside a structural film, such as PMMA. which has been

precoated on the electrode. Finally, a brief description is given of electroactive polyaniline

films which can be prepared by polymerization of aniline in a glow discharge.

ELECTROPOLYMERIZATION

The polyaniline films were prepared in an aqueous solution containing O.1M aniline plus 2M

acid, by sweeping the voltage applied to the platinum electrode between -100 and 900 mV at

50 mV/s (6). As the film begins to form, peaks for the redox reaction of polyaniline appear

a t 250 mV and the anodic voltage limit was gradually reduced to maintain the current densities

below 0.4 mA/cm2. This adjustment avoids the formation of hydrolysis product and other

by-products which oxidizes a t 500 mV (4.7-10). The amount of switching charge developed

per cycle is 0.1 mC/cm2. The ratio of the charge in the switching reaction of the polymer

(Q,) to the charge in the polymerization reaction (Q,) is 0.125. This value is constant during

the reaction and was determined from a plot of the cumulative charges for each successive

voltage sweep cycle.

SWITCHING REACTION

The voltammogram for polyaniline measured in 2M sulfuric acid containing no aniline shows

redox peaks at 120-250 mV as seen in Figure 1. A peak a t 900 mV is also present, although

the sweep was not extended into this peak during the initial analyses to avoid formation of

material with an oxidation peak at 500 mV. The peak heights scale linearly with the sweep

rate and the cathodic and anodic peak separation remains constant a t 130 mV. A

voltammogram with this simple form has been reported by Tamura by using a film prepared

galvanostatically (1,2). In the pH range -1 to 1, the peaks become sharper and shift linearly

to anodic values with increasing acidity a t the rate of 0.060 V/pH unit. This pH dependence

has been reported and ascribed to a one electron/one proton reaction (10).

19

Oxidation-reduction of the pi-system of the protonated polymer interconverts the benzenoid-

and quinoid-lie structures as shown in Equation 1 (2.9.11). The equation only shows the

protonated structures because they are relevant to the solution conditions in this study. These

structures have been detected spectroscopically with materials removed from the electrolyte

I

solution (12-15).

1000

800

600

400 a 3 200

0

-200

-400

I 1 I I I

0 200 400 600 800 E/mV

Fig. 1. Cyclic voltammogram of polyaniline (lOOOA) in 2M aqueous sulfuric acid (v = voltage scan rate) .

The reaction has a hysteresis as seen in Figure 1 or the corresponding plot with the current

integrated. This is observed with many of these polymers (2.6.16) and is ascribed to

conformational changes in the polymer accompanying the reaction (2,16,17). In this case, it

may result from the conformational differences between the benzenoid- to quinoid-like forms.

20

The switching/polymerization charge ratio, Q,/Q, = 0.125 and is calculated from the

integrated current from the voltammogram for the preparation reaction or the prepared film.

The fraction of the aniline units along the chain which is oxidized (f) is estimated from this

ratio which is related to the electrochemical stoichiometry (n) ratio for these reactions n,/np

(18) as shown in Equation 2. For the polymerization reaction, np = 2. and n, is the

electrochemical stoichiometry of the switching reaction. For the case where the reaction

generates only quinoid-like structures along the polymer chain, ns = 2, and f = 0.125. This

value is slightly smaller than the value 0.17-0.21 calculated from the report by Genies (19).

and the difference may reflect the use of different voltage limits.

Q,/Q, = fns/np

2.0

1.5

0 E 1.0 U* 1

0.5

0.0

~ I I I

I I I

50 100 150

tlms

Fig. 2. Plot of Q, versus time. Qs is obtained by integration of the chronoamperometric response resulting from a potential step from -100 mV to (1) 600, (2) 500, (3) 400 and (4) 300 mV for polyaniline (500A) in 2M aqueous sulfuric acid.

21

SWITCHING REACTION RATE

The chronocoulometric response of the film in 2M sulfuric acid was measured by stepping the

voltage applied to the electrode between -100 mV to an anodic voltage. A typical plot is

shown in Figure 2. The rate of switching is strongly dependent on the anodic voltage step, and

it does not have simple Fickian behavior. The response is initially linear in time and changes

to a tl/* dependence at longer times. The initial slope for the voltage step to 600 mV is 35

mC/s and is the same for films 500 to 1500 8, thick. The response is the same with 2M

sulfuric, hydrochloric or fluorosulfonic acid electrolyte. The same Q-t dependence is observed

for the reduction reaction with a small difference in the slope.

The linear Q-t dependence in the initial part of the reaction is observed when the sulfuric acid

concentration is greater than 1M. A similar behavior has been observed for the sorption of

solvent (21) and electrolyte (22) into polymer films and is referred to as Case I1 diffusion

(20). In these systems, the rate is controlled by the constant velocity of an advancing internal

boundary resulting from a chemical reaction or a phase transition. The general equations

required to quantify Case I1 diffusion across a polymer film on an electrode can be obtained

from the models developed from Fick’s law to describe Case I1 diffusion in glassy polymers.

Perterlin developed such a model and rewrote the diffusion equation to incorporate a term

describing the movement of a front with a constant velocity, Y (20),

dC/dt = d[DdC/dx - vC]/dx . (3)

For the case of a film on an electrode, C is the concentration of electroactive centers in the

film, x is the distance from the surface of the metal electrode, and D is the coefficient for

Fickian diffusion across the film. The equation for the electrochemical reaction of a film on

an electrode follows with the conditions that C(x,t) = C, for all values of x and t < 0, and

C(x = 0,t) = C(E) for x = 0 and t > 0, and considering that after a short time frontal

diffusion is the controlling process and that dx = vdt.

22

dQ/dt = nFAv[C, - C(E)] . (4)

In this equation, C, is the bulk concentration, in moles/cm3, of electroactive centers in the

film, and dx is the incremental change in the thickness of the moving front moving with a

velocity Y during dt. The other parameters have the usual definitions.

As seen in Figure 2, dQ/dt is sensitive to the magnitude of the voltage step. It also scales

linearly with the area of the electrode but is not sensitive to the thickness of the film. The

values of Y were calculated for the different anodic potential steps by defining the parameter,

t, which is the intercept from the extraplotation of the initially linear slope and the zero slope

line at very long times. This parameter defines the time required to switch the film completely

by the moving front mechanism, and provides Q, = nFAI(C, - C(E)), where 1 is the thickness

of the film. Combining this expression with Equation 4 provides 1 = vt,, and v results directly

from &/I. The Y values for a film SOOA thick are in the range 0.2-1.0 x cm/s andas can

be seen in Figure 3, Y is linear with the potential when the anodic step voltage is greater than

100 mV of E,, and falls below the extrapolated line when the step is smaller. The linear

response of v to the applied voltage shows that the controlling step of the reaction is driven

by the voltage gradient and is not a thermal process.

Reducing the solution acidity changes the kinetics of the reaction. The switching times are

almost identical for 1-2M sulfuric acid electrolyte and ca. 3 times longer with 0.1M acid

electrolyte. In addition, the time dependence of the reaction changes and already with the

0.1M acid electrolyte, it is linear with t1/2 in the region 20 to 80% reaction. Thus, ion

diffusion is now important in the kinetics of the reaction. In the solutions containing 0.001

to 0.1M sulfuric acid, D is in the range 1.4-1.9 x cm2/s (500A film), respectively.

23

1 .a

0.8 - m

3 5 0.6 d ?

0.4

0.2

0.c

c

100 200 300 400 500 600 700 800 E/mV

Fig. 3. Plot of Y values versus the anodic voltage step for a polyaniline film (500A) in 2M aqueous sulfuric acid.

These values were calculated by combining the total amount of charge in the reaction, Qs, with

the slope of the plot, and using Equations 5 and 6.

Q, = nFAl(C, - C(E)) (5)

Q/t”2 = ~~FAC,[D/W]’’~ .

The reduction reaction is even more complicated in these dilute acid solutions and is neither

linear with t or t112. For comparison, Oyama (23) reports a t112 dependence and

D = 1.2 x 10-lo cm2/s using a 0.36 micron thick film in pH 1 solution.

In describing the process associated with this observation, we must consider that in the acidic

solutions the polymer is highly protonated and the film is saturated with electrolyte.

24

Therefore, mobility of the anion is not a limitation in the early part of the reaction. In

addition, if only the partially protonated polymer segments are oxidized because the fully

protonated polymer, which lacks extended conjugation along the chain, oxidizes a t much

higher potentials, then the reaction may be limited by the rate of deprotonation of the

polymer. It is this chemical reaction which may generate the appearance of a front that

traverses the film during the early part of the switching reaction. After the initial period, the

kinetics of the process change to a t1I2 dependence suggesting that the electrolyte environment

in the film has changed drastically and the process has become associated with anion mobility.

Over all, the switching times for polyaniline are in tens of milliseconds and are significantly

less than for polypyrrole (hundreds of milliseconds) (16).

EFFECT OF THE ANION ON THE SWITCHING REACTION

Unlike the reactions of polypyrrole and polythiophene which are very sensitive to the anion,

the reaction for polyaniline is little influenced by the anion. Polyaniline films have been

generated and studied in hydrochloric (7,10), nitric (7). and perchloric acid (7) electrolyte.

The voltammograms for films prepared and tested in sulfuric, hydrochloric, nitric and

fluorosulfonic acid electrolytes are effectively superimposable. The E, values are listed in

Table 1. With trifluoromethylsulfonic, hydrofluoric and trifluoroacetic acid electrolytes, the

oxidation peak for the polymer is shifted cathodically by 40 mV. Overall, the reaction isquite

insensitive to the nature of the anion even when the anion is polymeric and has limited

mobility. For example, the polyaniline/polyvinylsulfonate polymer blend was prepared by

polymerizing aniline in an aqueous solution containing polyvinylsulfonic acid, pH 1. The

voltammogram for the resulting film measured in polyvinylsulfonic acid solution containing

no aniline was very similar to the one for polyaniline in aqueous sulfuric acid solution.

25

TABLE 1. Summary of E valuesa for polyaniline measured in aqueous acid electrolyte at 50 mVA scan rate.

Ep I mV Acid, 2M Anodic Cathodic

H2S04 260 140 HCl 250 110 HN03 250 130

CF3S03H 210 130 CF3COOH 210 130 HF 190 50

FSO3H 250 120

(CH2CHSO,H), 150 20 (CH2CHSO,H), 250 120

Walues versus NaSCE. bFilm preparation in polyvinylsulfonic acid and analysis in I12S0,.

The switching rate is also comparable for the film in both solutions. The voltammogram for

the polymer blend measured in aqueous sulfuric acid solution was identical to that for

polyaniline prepared and measured in sulfuric acid solution. The film had a fairly even

distribution of polyaniline and appeared more plasticized than polyaniline. The tga of the

blend did not show the weight loss due to decomposition at 190°C (see p. 13). but instead

appeared fairly stable up to 36OoC as is observed with the polyvinylsulfonic acid.

NONAQUEOUS SOLVENTS

The majority of the electrochemical studies with polyaniline have been performed in aqueous

solutions (1-11). although the polymer is also electroactive in other solvents, such as

propylene carbonate and anhydrous ammonium fluoride solutions (9,19). However, unlike

polypyrrole (16) and polythiophene (24) which are electroactive in neutral aprotic solutions,

voltammograms with well-defined waves are observed with polyaniline only when both

organic salt and protic acid are present in the electrolyte solution. The voltammograms

measured in acetic acid are shown in Figure 4 and the E, and i,/@.v)values with the various

solvents are listed in Table 2.

26

1 ooa

800

600

400

Q z 200 h

0

-200

-400

-600

I I I

mV/s

I I I I I I

a 200 400 600 800 1000 EImV

Fig. 4. Cyclic voltammogram of polyaniline (lOOOA) in 2M sulfuric acid in acetic acid solution.

The position and the width of the peaks are also sensitive to the solvent/electrolyte

combination of the solution, and are consistently sharper with sulfuric acid than with the

salt/protic acid electrolyte. The peak widths at half heights are 90 and 120-140 mV with the

sulfuric acid and trifluoroacetic acid electrolyte, respectively.

The potential for the switching reaction of the polyaniline film shows a rather large solvent

effect. In fact, the shift in the E, and E, values appear to respond to both the polarity and

the basicity of the solvents. The shifts correlate best with the % values, a solvent polarity

parameter developed by Dimroth and collaborators (25) from spectroscopic measurements of

absorption spectra of phenylpyridinium salts in various solvents. As seen in Figure 5 , a good

linear correlation is observed with both anodic and cathodic peak potentials and the solvent

sensitivity is comparable.

27

TABLE 2. Cyclic voltammetry data for polyaniline in several electrolyte solutions

Anodic Cathodic Solvent Electrolyte Epa ipfi4’ Epa ‘&4* Water 2M H$O4 250 13.5 120 8.5

Ethanol 2M H2SO4 380 13 250 12.5 1M CH3COOH/ 0.1M B u ~ N B F ~ 170 4.5 80 6

2,2,2-Trifluoro- 2M H2SO4 370 12 230 7.5 e t h a t X 2 l 1M CF3COOH/

0.1M B u ~ N B F ~ 500 8 260 7

Acetic Acid 2M HzSO4 420 15 320 12.5 1M CH3COOH/ O.1M B u ~ N B F ~ 380 4.8 200 5.5

Acetonitrile 2M H2S04 450 13.5 360 12 1M CF3COOH/ 0.1M B u ~ N B F ~ 540 5.5 260 6.5

40 50 60 ET/( kcal/rnol)

Fig. 5. Plot of % values for polyaniline in various solvents versus values from reference 25.

28

500 I I I I I I I I I I

-A

'A Anodic 400 -

Ep, 300 200 mV - - C2H50H cH''N\\ C2H50H Cathodic \ - -

0 O\ 100 - H 2 0 H 2 0 -

I 1 I I I I I 1 I o 1

The Ep values also correlate with the solvent basicity as measured by the pK, values (26) and

the log of the proton transfer quotients (27) of the solvents. The poorest correlation is

obtained with the pKa values where the value in water is off the line (Figure 6). Therefore,

the general characteristics of the solvents, polarity and proton transfer, are important in the

redox reaction of polyaniline.

Fig. 6. Plot of E, values for polyaniline in various solvents versus pKa and log Q, values from references 26 and 27.

FILM STABILITY

The films are quite stable to storage in the ambient conditions for several weeks and remain

electroactive and showing only a small anodic shift in the switching potential. The films are

less stable to storage at 80"C, and show a 15% loss in the coulombic capacity after five days.

The films are also stable to storage in 1M sulfuric acid solution under open circuit conditions

at room temperature, but show a 12% loss in coulombic capacity after 4 hours at 8OoC. The

change is most likely due to hydrolysis (28) . The films are also stable to the switching reaction

as evidenced by the continuous cycling experiments. For example, Genies finds that

electropolymerized polyaniline can be switched up to 100 to 2000 cycles with coulombic yield

of 1.0 (9). while chemically polymerized polyaniline maintains coulombic capacity of 9346%

during 200 sweep cycles with low current density (29.30).

29

A more direct measure of the thermal stability of polyaniline was determined by thermal

gravimetric analysis of films prepared on a thin gold wire. Polyaniline from sulfuric,

fluorosulfonic and hydrochloric acid solutions decomposes in the temperature range,

185-19SoC. This temperature is comparable to the decomposition temperatures for

polypyrrole (16) and polythiophene (24). All these films contain 18-40% moisture which is

lost at 60-80°C, and the content is higher with the films which were polarized at 480 mV.

The films prepared in trifluoromethylsulfonic acid are different from the rest. They contain

little water, less than 1%, and decompose at 110'C. Mass spectral analysis of the volaties

showed the presence of moisture only.

SURFACE

Literature reports from different laboratories describe the polyaniline deposit as a film and

as a powder. The first deposit is a continuous film which has a surface roughness on the

submicron scale (6). These films are adherent, continuous, and covered the 0.5 cm2 electrode

area fairly even with no visible uncoated areas. No difference was detected between films

prepared with sulfuric and hydrochloric acid electrolyte. Extending beyond this film, a fibril

structure forms (9,18) which has been described as a powder (7). The fibers have a diameter

of ca. 0.1 microns and extend ca. one micron between branch points. Fiber formation occurred

with all the electrolytes and there is a variation in the degree of formation with each acid. In

particular the film prepared in trifluoroacetic acid produced extensive fibril structure giving

the film the appearance of a powder. The continuous film and the fibers are electrochemically

equivalent, as are the pressed pellets of the powders (7) and of polyaniline from a chemical

preparation (29) have nearly the same electroactive behavior as the continuous film.

POLYMER BLENDS

Polymer blends consist of a mixture of the electropolymerized composite, described above.

and an inert "host" polymer. In general, the blend consists of regions of the the electroactive

polymer plus anion interfacing with regions of the "host" polymer. The interest in these

materials stems from the desire to modify the mechanical properties of the composite film,

30

and several papers have recently appeared which describe the electrochemical preparation and

properties of polymer blends on electrodes. In particular, polypyrrole and polythiophene have

been prepared electrochemically in polyvinylchloride an inert "host" polymer (31-34) and in

ionic "host" polymers having ion exchange membrane characteristics such as Nafion (35) and

Nafion in impregnated Gore-Tex (36.37). The polymerization reaction occurs at the electrode

surface since all of the films which coat the electrode are sufficiently porous to permit the

transport of electrolyte across the film. This is true even with highly crosslinked nonswelling

polymer films which can be generated by plasma polymerization (38). This approach to new

materials was first demonstrated with the incorporation of a conducting charge transfer salt,

lTF, inside the polymer Nafion (39-41) and is proving to be quite general for the preparation

of polymer blends.

POLYANILINE/POLYMETHYLMETHACRYLATE

The polyaniline/polymethylmethacrylate (PMMA)/platinum electrodes were prepared by

polymerizing aniline (0.2M aniline plus 2M sulfuric acid solution) as described above and

using a PMMA precoated platinum electrode. The PMMA film (2000A) is sufficiently porous

to allow aniline plus electrolyte to diffuse across the film and react at the platinum electrode.

During the preparation, the wave for the oxidation of aniline was small and had the step-shape

as is observed for reactions limited by slow diffusion in the region of the electrode surface

(42). The amount of charge consumed corresponded to a 500A tilm on a clean electrode.

Visual inspection of the polyaniline film reveals an uneven distribution of the color across the

electrode surface. Microdomains of "pure" polyaniline are dispersed in the PMMA. As can

be seen in the SEM in Figure 7, the polyaniline has traversed the thickness of the PMMA film

and is growing in fibril form. There is a wide variation in the size of the microdomains where

the larger ones are ca. 10 microns and the smaller ones are ca. two microns in diameter.

Removal of the 2000A film from the platinum surface by peeling did not remove all of the

polyaniline from the electrode.

31

Fig. 7. SEM of the film-solution face of a polyaniline/PMMA film. (a) Low magnification. (b) Small polyaniline deposit not visible in a. (c) Large polyaniline deposit appearing as dark spots in a.

The cyclic voltammogram of a polymer blend which was rinsed and mounted in a cell

containing 2M sulfuric acid solution is shown in Figure 8. The anodic peak is narrower and

the cathodic side of the peak rises abruptly as compared to the peak for polyaniline prepared

on a bare platinum. The reaction is coulombic reversible and the hysteresis is preserved. The

Q,/Q, ratio is 0.115 and is similar to the value for the film on a bare electrode. The anodic

and cathodic current peak heights vary linearly with the sweep rate. The same results were

obtained when thinner PMMA films (700A) were used. These results are similar to those with

electroactive polypyrrole produced on polyvinylchloride coated electrodes (31,32).

The chronocoulometric response of the reaction for a voltage step from -100 to 600 and back

to -100 mV is also different than for a polyaniline formed on a bare platinum electrode. With

the PMMA blend, the reaction is slower, where the film switches completely in ca. 370

milliseconds instead of 50 milliseconds for polyaniline on bare platinum. The reaction now

shows a t1l2 dependence indicating that the reaction rate depends on the diffusion of ions, and

the slope of the plot is 7.7 mC/(s1/2 cm2). The origin of the reduced rate and change in the

reaction mechanism is not clear, and may result from steric or pH buffering effects imposed

by the PMMA matrix.

32

I I 1 I I I

600

500 -

400 -

300 - a 3 200 -

100 -

0 -

-100 -

mV/s

-200 0 200 400 600 800 ElmV

Fig. 8. Cyclic voltammogram of a SOOA polyaniline/20006; PMMA/platinum electrode in 2M aqueous sulfuric acid.

The polyaniline film blend electrode is electroactive in acetonitrile solution containing 1M

trifluoroacetic acid and 0.1M tetraethylammonium tetrafluoroborate. In this solution,

PMMA is solubilized in a few minutes leaving behind the exposed polyaniline film which

shows a voltammogram resembling the one for polyaniline prepared on bare platinum. The

electrochemical results with the PMMA film are not unique to this material and were observed

with polybutylmethacrylate and polycarbonate "host" films.

POLYANIL.INE/POLYACRYLIC ACID/POLYMETHYLMETHACRYLATE.

Polymer electrodes containing two structural polymers were also prepared by first coating the

platinum electrode with a 2000 A polyacrylic acid film and then with 2000A

polymethylmethacrylate. This double-coated electrode was mounted in an electrochemical

cell containing aniline in aqueous sulfuric acid electrolyte and the polymer film was grown as

33

described above. Polyaniline in a polyaniline/polyacrylic/PMMA/platinum electrode

generates a cyclic voltammogram which is distinctly different from the polyanilinekPMMA

electrode. As seen in Figure 9, the redox peaks are very broad and widely separated, most

likely reflecting a large IR drop across the PMMA film.

30

20

10

Q 3 0

-10

-20

-30

I I l l I I I I mV/s 100

80

60

40 20

I I l l I I I 1 ,300 -100 0 100 300 500 700 900

E/mV

Fig. 9. C clic voltammogram of a polyaniline/2000A polyacrylic acid/2000i PMMA/platinum electrode in 2M aqueous sulfuric acid solution.

This is the case. in both 2M aqueous sulfuric acid and in acetonitrile containing 0.1M

tetraethylammonium tetrafluoroborate plus 1M trifluroacetic acid as seen in Figure 9. The

reaction is still coulombically reversible and Q, equals 0.6 mC/cm2. As with the

polyaniline/PMMA blend, the chronocoulometric plot resulting from a potential step from

-300 to 800 mV is linear with t1l2, however the time required to switch the film is ca. 50 times

longer. The slope of the chronocoulometric plot is approximately 0.15 mC/(s1/2 cm2).

Removal of the PMMA layer by dissolution left a film on the electrode which is electroactive

in acetonitrile. but unlike the polyaniline/PMMA electrode, the film readily dissolved in

34

aqueous electrolyte. Thus, the polyaniline is generated in the polyacrylic acid layer and its

redox reaction responds only to the polyacryclic acid environment and not to the bulk

electrolyte.

PLASMA POLYMERIZED POLYANILINE

The strong interest generated by the electropolymerized films encouraged us to explore the

gas phase oxidation route to polymer films (44). The availability of such a process could prove

to be a useful complement to the electrochemical route, since some of the limitations such as

the oxidation of compounds with very high oxidation potentials and solubilities are not an

issue. The films of aniline polymer were prepared by introducing aniline into an argon RF

plasma. The chamber used had two parallel plates spaced by 2 to 3 cm and the platinum

electrode was placed on the plate which was electrically grounded. In these experiments, the

total pressure of the chamber was maintained at 50-100 microns and the RF power level was

kept low, 150-300 mW/cm2. Polymer films which were ca. 5OA thick were formed in

approximately ten minutes. After the deposition the electrode was removed from the chamber

and mounted in an electrochemical cell. The plasma films were electroactive and produce

broad redox waves in the voltammogram which appear at the same voltage as

electropolymerized polyaniline. ESCA and IR analyses indicates that the film is composed

of aniline structures which survive the plasma conditions without fragmenting. Thin passive

films which provide excellent protection to the underlying electrode could also be prepared

by this procedure when using higher RF power levels, 300-450 mW/cm2 (44). These

passivating films reduce the approach of ions to the underlying metal electrode by a factor of

105.

EXPERIMENTAL

The materials, equipment (43) and experimental procedures relevant to this study have been

previously described (6,18).

electrode cell using a gold counter electrode and a sodium chloride saturated calomel

electrode. The measurements were made with IR compensation.

All the voltammetric measurements were made in a three-

35

ACKNOWLEDGMENTS.

The authors wish to acknowledge the technical assistance of

R. Siemens, E. Hadziioannou, and J. Duran. This work was carried out

while one of us (JCL) was under sponsorship from Elf-Aquitaine. JCL

wishes to thank Professor 0. Kahn for making the necessary

arrangements for this sponsorship.

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1.

2.

3.

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