ELSEVIER 1998 PPy Composite

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*Corresponding author.

Present address: School of Applied Science, Griffith University, Gold Coast Campus, PMB 50, Gold

Coast Mail Centre, Queensland 9726, Australia

Polymer Gels and Networks 6 (1998) 233245

Polypyrrole/poly(2-methoxyaniline-5-sulfonicacid) polymer composite

Huijun Zhao, Gordon George Wallace*

Intelligent Polymer Research Institute, University of Wollongong, Northfields Avenue, Wollongong,

NSW 2522 Australia

Received 8 June 1998; received in revised form 9 July 1998; accepted 9 July 1998

Abstract

The incorporation of an electronically conducting polyelectrolyte (poly(2-methoxyaniline-5-

sulfonic acid)[PMAS]) as the dopant in polypyrrole results in the formation of a new class of

materials. The resultant polymer structure has high water content ('90%), reasonable elec-

tronic conductivity (58 S cm\) and multiple switching capabilities. The latter arises from the

fact that both the polypyrrole and the PMAS are redox active. 1998 Elsevier Science Ltd. All

rights reserved.

1. Introduction

It is well known that conducting polymers such as polypyrrole are readily syn-

thesised electrochemically according to

(1)

0966-7822/98/$ see front matter 1998 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 6 - 7 8 2 2 ( 9 8 ) 0 0 0 1 6 - 1


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The counteranion incorporated during synthesis may be a simple anion [1] such as

chloride or nitrate or may itself contain some functionality, e.g. metal complexing

agents [25], enzymes [69], antibodies [10, 11] and even living cells have been

incorporated [1214].

Recently both ourselves [14, 15] and others [16] have advocated the use of polyelec-trolytes (PEs) as the counteranion in electronically conducting polymer systems. This

approach results in production of more chemically stable systems since it is difficult for

large (polyelectrolyte) counterions to leach out of the polymer, even when the latter is

reduced. The incorporation of immobile dopants during synthesis ensures that cation

exchange processes predominate upon reduction/oxidation (Eq. (2)):

(2)

Furthermore, the introduction of polyelectrolytes introduces some unique properties

to the resultant conducting polymer structure including a high water content, some-

times as much as 90% (w/w). These conducting polymerpolyelectrolyte composites

can be dehydrated and rehydrated making them electronically conducting hydrogels.

Finally, the use of polyelectrolytes as dopants provides a convenient route for

introducing predetermined functional groups that are part of the polyelectrolyte

chain. This can be used to introduce redox activity (e.g. ferrocene groups) [19] metal

complexing groups [20] or groups that render the resultant material biocompatible

[21, 22]. The use of appropriate polyelectrolytes can also result in improved mechan-

ical properties [23, 24].

Previously reported work in this area has considered the introduction of non(electronically) conductive polyelectrolytes into the polymer networks. We report here

the incorporation of conductive electroactive polyelectrolytes as the counterion sys-

tem. In the course of this work, an electrically conductive polyelectrolyte, POLY(2-

METHOXYANILINE 5-SULFONIC ACID) was incorporated into polypyrrole as

the dopant during growth.

The effect of incorporating such a dopant on the electropolymerisation process as

well as on the properties of the polymer produced has been investigated.

2. Experimental

2.1. Chemicals

All reagents used were of AR grade purity unless otherwise stated. Pyrrole was

obtained from Merck and was distilled prior to use. Poly(2-methoxyaniline-5-sulfonic

acid) [PMAS] and p-toluenesulfonate sodium salt (PTS) were obtaianed from

234 H. Zhao, G.G. Wallace /Polymer Gels and Networks 6 (1998) 233245


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Table 1

Conductivity and tensile strength data for PP/PMAS films

Sample number Composition of polymerisation Conductivity

Tensile strength

solution (S cm\) (MPa)

1 0.5 M Py#0.8% PMAS 5.4$2.3 89$35

2 0.2 M Py#0.5% PMAS 7.9$1.8 97$41

3 0.5 M Py#0.1% PMAS 8.5#1.7 111$47

Note: All films were prepared galvanostatically at the charge density of 4.8C cm\. The current density

used for samples 1 and 2 was 2.0 mA cm\ and, for sample 3 was 0.4 mA cm\.

NITTO Chemical Industry Co., LTD and Merck respectively. All solutions were

prepared with deionized Milli-Q water (18 M cm).

2.2. Instrumentation

A princeton applied research (PAR) model 363 Potentiostat/Galvanostat was

employed for the electropolymerisation. When the PAR-363 was used for application

of pulsed potentials, it was combined with a home made signal generator (Science

Faculty workshop in University of Wollongong, Australia). Cyclic voltammetric

experiments were performed using a BAS CV-27 (Bioanalytical System Inc, USA).

All electrochemical data were recorded using a MacLab interfaced with a Macintosh

computer. All electrochemical experiments were carried out using a single com-

partment cell with a three electrode system. In the case of film synthesis, a stain-less steel plate (10.0;6.0 cm, mirror finish surface), a platinum coated PVDF

membrane (0.22 m, d"6.5 cm) or an ITO-coated glass slide (4.0;1.0;0.5 cm)

were employed as the working electrodes and a reticulated vitreous carbon plate

(10;7;1 cm) auxiliary electrode was used. All electrode potentials were measured

relative to a Ag/AgCl (BAS, 3 M NaCl) reference electrode. A standard four-

point probe (made at the University of Technology, Sydney, Australia) was used

to obtain the DC-conductivities of films. Studies of the mechanical tensile strength

of the membranes were carried out using an Instron Dynamic Analyser model 4303.

2.3. Polymer film preparation

All films used for physical property tests were prepared electrochemically using

either galvanostatic or potentiostatic polymerisation techniques. The composition of

the polymerisation solutions and the electrochemical conditions used for film

preparation are shown in Tables 14. The films for conductivity and tensile strength

measurements were deposited onto a stainless-steel substrate electrode. For water

content measurements films were deposited on platinum-coated PVDF substrate. Forcapacitance/surface area measurements, the films were electrochemically deposited on

a platinum disc electrode. Films used for measuring optical properties were electro-

chemically deposited onto ITO glass unless otherwise stated. A solution containing

0.2 M pyrrole and 0.5%. PAS and a current density of 4.0 mA cm\ was employed.

H. Zhao, G.G. Wallace /Polymer Gels and Networks 6 (1998) 233245 235


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Table 2

Elemental analysis data of PP/PMAS polymer composite

Specifications (%) Amount N/S ratio PP/PMAS ratio

C 46.61H 3.91 3.69 1.84

N 11.09

S 6.87

Note: Polymerisation was carried out in a solution containing 0.2 M Py and 0.5% PMAS at current

density of 2 mA cm\ for 4 h.

Table 3

The change of dimension of the polymer composite during dehydration

Specifications Wet polymer Dried polymer

Diameter (cm) 4.0 2.1

Thickness (cm) 0.35 0.08

Volume (cm ) 4.4 0.3

Note: Polymerisation was carried out in a solution containing 0.2 M Py and 0.5% PMAS at currentdensity of 2 mA cm\ for 4 h.

Volume"/4D2 h, where D"diameter, h"thickness.

Table 4

Surface area data of PP/PMAS film

SPECIFICATIONS PP/PTS Film PP/PMAS film

C&

(f cm\) 20

Capacitance (f ) 70 000 670 000

Surface area (cm) 3 500 33 500

Note: Polymerisation was carried out in a 0.5 M Pyrrole solution containing 0.5% PMAS or 0.1 M PTS at

current density of 10 mA cm\ for 30 s. The measurements were carried out eloectrochemically using

a triangular potential waveform with a potentil range 0.150.20 V (vs Ag/AgCl) and scan rate of

0.1 mV s\.



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Fig. 1. Cyclic voltammogram of 0.2 M pyrrole in 0.5% PMAS at a Pt disc electrode. Scan

rate"20 mV s\.

3. Results and discussion

3.1. Electropolymerisation of pyrrole in the presence of PMAS

The polymerisation of pyrrole in the presence of PMAS (1) shown below wasinvestigated:

The polymerisation process can be described, according to:

(4)

Initially, polymerisation was carried out using a potentiodynamic method by applica-

tion of a potential sweep (Fig. 1). It was found that the current increased with



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Fig. 2. Chronoamperometric responses at different potentials for oxidation of 0.2 M pyrrole in 0.5%PMAS at a Pt disc electrode.

increasing potential cycles indicating polymer deposition. The current response ob-

served within the negative potential range (#0.2 V to!0.6 V) was due to the reduc-

tion and re-oxidation of the polymer formed and deposited on the electrode surface.

The large increase in current in this potential range suggests high polymer deposition



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Schematic 1. Schematic representation of the conducting polymer composite formation.

efficiency and/or formation of a polymer with a large electroactive surface area. The

results also indicate that the polymer can be formed at potentials as low as#0.40 V

vs Ag/AgCl.

Results obtained from potentiostatic polymerisation show an increasing current

transient at potentials as low as #0.45 V vs AgAgCl (Fig. 2) confirming that thispolymer can be formed and deposited at much lower potentials than polypyrroles

containing other dopants.

The low polymer deposition potential and high deposition efficiency can be

explained by considering the unique role of an electronically conductive poly-

electrolyte such as PMAS. Upon application of a positive potential, oxidation of

the pyrrole monomer will occur. Polypyrrole will then deposit on the electrode

incorporating PMAS as dopant. Since this counterion system is itseslf a conduct-

ing polymer, electron transfer during subsequent polymerisation can be achievednot just through the polypyrrole network but also via the PMAS chain (Schematic 1).

Since the effective electrode surface area continues to increase with polymer depos-

ition time the rate of polymerisation is accelerated.

Application of this positive potential is also sufficient to oxidise PMAS [25]

creating potential electrocatalytic sites for subsequent pyrrole oxidation.

Chronopotentiograms recorded during growth using current densities in the range

0.42.0 mA cm\ showed a potential that was constant (varying between#0.40 and

#0.45 V depending on current density) throughout the polymer growth period

indicating that a conducting polymer was deposited.



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Fig. 3. AFM imgae of dried PP/PMAS polymer.

3.2. Physical properties of the PP/PMAS polymer composite

Conductivities of approximately 5 S cm\ and tensile strengths of about 100 Mpa

were obtained for stand alone films grown galvanostatically using current densities in

the range of 0.42.0 mA cm\.

The effect of the polymerisation solution composition on both the conductivity andtensile strength was investigated (see Table 1). The results indicate that films with

higher conductivity were obtained when a lower concentration of PMAS solution was

employed. In contrast, an increase in the PMAS concentration up to 0.8% (w/w)

results in an increase in the tensile strength.

The surface image of a dried polymer composite was obtained using AFM (Fig. 3).

It reveals a nodular morphology with the largest nodular size less than 100 nm.

Note that the polymer sample used in this study was 4 mm thick (when grown wet).

Drying of the polymer resulted in shrinkage of more than 90% of its volume anda very smooth surface was obtained. Attempts to compare surface morphology of the

wet polymer composite and the dried polymer failed because high quality surface

images of the wet polymer could not be obtained.

Table 2 shows the results of elemental analysis. It was found that the ratio between

the pyrrole and MSA units was 1.8 1.0. This ratio is much lower than polypyrroles

prepared using conventional counterions where this value is normally between 3 and

4. This result indicates that excess sulfonate groups are present in the polymer



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Fig. 4. Dehydration of PP/PMAS as a function of time.

material, a result consistent with the high-water contents observed. The water content

of the polymer composites formed were found to be greater than 90%. It was found

that variations of the polymerisation had no marked effect on the water content of the

resultant polymer composite.

It should be noted that this polymer composite can be quite easily dehydrated in air

but cannot be rehydrated if totally dried. Fig. 4 shows the change in mass due to

dehydration of the polymer composite as a function of time. The polymer compositeloses 70% of its weight in the first 10 h (drying in air, at 25C). The dehydration of the

polymer composite was accompanied by dimensional changes (see Table 3 and Fig. 5).

The sample volume changed between the wet and dried state by a factor of ap-

proxiamately 15.

The solubility of the polymer composite in other solvents was investigated. Immer-

sion experiments revealed that only trace amounts of the polymer composite could be

dissolved in water. The polymer was insoluble in all common organic solvents such as

alcohol, DMF and ether.The optical properties of the polymer composite film were characterised using

UVVis spectrophotometry (Fig. 6). Samples were prepared by electrodeposition of

PP/PMAS film onto a conducting ITO glass plate. The results indicate that a doped

(conducting) polymer was obtained. This is evidenced by the strong absorbance band

observed at wavelengths greater than 620 nm.

The surface area and capacitance of the polymer composite were measured electro-

chemically. It was found that the surface area/capacitance of the PP/PMAS polymer



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Fig. 5. A photograph of wet and dried PP/PMAS polymers. The original sample sizes of wet and dried

polymer were 4.0 cm\ and 2.1 cm in diameter, respectively.

Fig. 6. UV/Vis spectrum of a PP/PAMS film electrochemically deposited on a ITO glass plate.



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Fig. 7. Cyclic voltammogram of (a) PP/PMAS and (b) PP/PTS-coated Pt disc electrode in a solution

containing 0.2 M KCl. Scan rate"20 mV s\.

composite obtained was about 10 times larger than that obtained for PP/PTS

(Table 4).

3.3. Electrochemical properties of the PP/PMAS polymer composite

Cyclic voltammograms recorded in a 0.2 M KCl solution reveal that the polymer

obtained was electroactive (Fig. 7a) and similar to a polypyrrole coated electrode

prepared using normal counterions such as Cl\, NO\

or PTS (e.g. Fig. 7b). When

a larger cation (Mg>) was employed, the cation incorporation/expulsion responses

observed were even more pronounced than that observed in K> case (see Fig. 8).



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Fig. 8. Cyclic voltammogram of a PP/PMAS coated Pt disc electrode in a solution containing

0.2 M MgCl

. Scan rate"20 mV s\.

Fig. 9. Cyclic voltammogram of a PP/PMAS coated Pt disc electrode in a solution containing0.2 M NaCOOC

H

. Scan rate"20 mV s\.

When a large organic anion was used, the cyclic voltammogram clearly shows a pair

of redox peak at about#0.3 V. This response is presumably due to the aniline based

polyelectrolyte (see Fig. 9).

4. Conclusions

A novel conducting polymer polyelectrolyte composite has been developed. The

structure has good conductivity and mechanical properties. It also has high water

content and an open porous, high surface area, structure in the wet state. Multiple

electrochemical switches are available but apparently only in the presence of selected

species. These properties should be usseful for membrane transport, controlled release



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and sensing applications. Perhaps most facinating is the apparent electrocatalytic

growth of this polymer that occurs at low potentials. This will assist in incorporation

of species such as proteins or living cells that are denatured at more extreme

potentials.

Acknowledgements

Professor Gordon Wallace acknowledges the continued support of the Australian

Research Council. We are grateful to Nitto Chemicals for the supply of PMAS.

References

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[3] Barisci JN, Murray P, Small CJ, Wallace GG, Electroanalysis 1996; 8 : 330.

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[9] Adeloju SB, Shaw SJ, Wallace GG. Anal Chim Acta 1993; 281 : 621.[10] Barnett D, Sadik OA, John MJ, Wallace GG. Analyst 1994; 119: 1997.

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