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Chemical oxidation and polymerization of indole
ELSEVIER
Synthetic
Metals
69 (1995)
571-572
Chemical oxidation and polymerization D. Billauda, E.B. Maaroufb aUniversit6
Henri Poincare bUniversite
Nancy I, Laboratoire
of indole
and E. HarmecartC
de Chimie Minerale Appliquee, France
BP 239, 54506 Vandoeuvre
Mohammed ler, Laboratoire d’Electrochimie Appliqute, Faculte des Sciences, ‘Societe Solvay et Cie, 310, rue de Ransbeek, 1120 Bruxelles, Belgium
les Nancy Gdex,
Oujda, Morocco
Abstract Indole, CgNH7. can be polymerized in selected oxidizing solutions such as FeC13, CuC12 or KIO3 dissolved in acetonitrile, CH3CN. The polymerization can also occur in FeC13 solution of CH3N02, CH2C12, C6H5OH. C2H5OH and CH30H. Doped polyindole is obtained in the form of an air stable conducting powder, green in colour. The influence of the experimental synthesis conditions on the values of the electrical conductivity and on the morphology of polyindole is discussed.
1. INTRODUCTION Electrochemical polymerization of indole, CgNH7, can be achieved in aprotic solutions (l-31. The polymer, which grows at the surface of a positive electrode (gold, platinum or nickel), exhibits electrochromic properties : green in the doped state, it becomes yellow in the reduced form. The experimental conditions for obtaining polyindole have been recently extensively studied [3-51. The room temperature electrical conductivity of the doped polymer, close to 10-l S.cm-l, is found stable during long air exposure times. However, a loss of this conductivity appears for potentials higher than 1 V.vs SCE where an irreversible polymer degradation occurs. In this work, we will show that polyindole can be obtained by chemical oxidative techniques. The effects of the experimental polymerization conditions on the properties and the morphology of chemically polymerized indole will be examined.
2. EXPERIMENTAL Oxidizing agents like FeC13 (Aldrich), CuC12 (Aldrich) and KI03 (Fluka) were used as received. The polymerization was achieved in different solvents (CH3CN. CH2Cl2, CH3NO2, C6H5OH. C2H5OH and CH30H) used without purification. The electrical measurements were carried out on pressed pellets of polyindole powder using the method of Van der Pauw [6].
3.
RESULTS
Indole polymerization depends on different parameters : the redox potential of the oxidizing (dopant) species, the nature of the solvent, the concentration of the reacting agents and the temperature. It was found previously that the dopingdedoping of electrochemically polymerized indole is equal to 0.56 V vs SCE and that irreversible degradation of the polymer occurs significantly for potentials higher than 1 V vs SCE. In case of a chemical polymerization, the redox potential of the dopant must be situated between these values 0379-6779/95/$09.50Q 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02573-H
to obtain a conducting material. We found that the chemical polymerization of indole occurs in solutions of FeC13 dissolved in CH3CN. CH2C12, CH3N02, C6H5OH. C2H5OH and CH30H. In acetonitrile solutions, FeC13. CuC12 were found as effective dopants while only small amounts of polymer were produced with KI03. In case of polymerization with FeC13 as a dopant, two techniques were used. In the first one, when a FeC13-CH3CN solution is poured into a CH3CNion indole solution, thin particles are formed and are in suspension in the reacting medium. The polymer is collected either by centrifugation at lo4 rpm or by precipitation by large water dilution. In the second method, polyindole precipitation is obtained when FeClg-water solution is poured into a mixture of water containing indole and a small amount of CH3CN necessary to solubilize the monomer. Filtration or centrifugation allows to collect the polymer, green in colour. Similar experiments were carried out with CuC12. In any case, the polymerization is accompanied with gaseous HCl evolving. Chemical analyses performed on these chemically obtained polyindoles show that the counterion is Cl‘. The doping level y in [CgNH5Cly]x is equal to 0.33 when the dopant is CuC12, a value similar to that was found in electrochemically synthesized polyindole [4] ; with FeC13 in acetonitrile, lower y values can be found (0.13 I y 5 0.16). Moreover, the presence of variable amounts of Fe can be due either to traces and/or side reduction products of FeC13 trapped in the polymer bulk or to the formation of FeC4‘ acting as a counterion. Extra experiments are needed to precise this point. The electrical conductivity Q of polyindole is strongly dependent on the experimental conditions. It is always lower than the value, (T = lo- 1 S.cm- l, found for the electrochemically synthesized polyindole. Typical values of electrical conductivities are presented in Table I. The texture of polyindole can be controlled by varying the experimental conditions and by addition of emulsifying agents. Figures 1A. 1B and 1C present SEM micrographs of polyindole obtained with FeC13 as a dopant. In figure 1A. it appears that polyindole is made of aggregates 100 pm in average diameter and of variable shapes. The control of the reaction temperature and the addition of emulsifying agent such as Cl2H25SOqNa result in drastic modifications of the morphology of polyindole. Figures 1B and 1C present SEM
D. Billaud et al. / Synthetic Metals 69 (1995) 571-572
572
Table 1 Conductivity
values for polyindole
synthesized
in different
media
Solvent
CH3CN
CH3CN
CH3CN
CH2C12
CH3N02
CH30H
C2H5OH
Dopant
cuc12
FeC13
m03
FeC13
FeC13
FeC13
FeC13
o(S.cm-1)
10-2
10-3
10-6
10-5
10-3
10-4
10-4
micrographs of polyindole synthesized with Cl2H25SO4Na (0.42 g.ml-l) at respectively -7’C and -15’C. Very homogeneous polyindole latex can thus be obtained.
4.
CONCLUSION
The obtention of an air stable doped polyindole can be achieved in various oxidizing media. The doping levels in these materials, correlated to the oxidizine strength of the reaction medium, are lower than those found in polyindole ; this explains synthesized by electrochemical methods probably the low values of the electrical conductivity by comparison with materials obtained by electrochemical techniques. The morphology of polyindole can be easily controlled by varying the experimental polymerization procedures. Works are now in progress to obtain materials exhibiting higher conductivity values especially by adjusting the redox potentials of the reaction media.
Acknowledgements : The financial support of Solvay S.A., Central Laboratory (Bruxelles) is gratefully acknowledged.
Research
REFERENCES 1. G. Tourillon and F. Gamier, J. Electroanal. Chem., 135 (1982) 173 2. R. Waltman, A. Diaz and J. Bargon, I. Phys. Chem., 88 (1984) 4343 3. E.B. Maarouf, Ph. D. Thesis, Nancy, France, 1989. 4. D. Billaud, E.B. Maarouf and E. Hannecart, Polymer, Vol. 35, No 9 (1994) 2010. 5. E.B. Maarouf, D. Billaud and E. Hannecart, Mat. Res.Bull., in press. 6. L.J. Van der Pauw, Philips Rev. Report, 13 (1958) 1 and 16 (1961) 167.
Figure 1. SEM micrographs of polyindole obtained in FeC13CH3CN medium at room temperature (1A) and in the presence of the emulsifying Cl2H25SOqNa agent at -7’C (1B) and at -15°C (1C)
Synthetic
Metals
69 (1995)
571-572
Chemical oxidation and polymerization D. Billauda, E.B. Maaroufb aUniversit6
Henri Poincare bUniversite
Nancy I, Laboratoire
of indole
and E. HarmecartC
de Chimie Minerale Appliquee, France
BP 239, 54506 Vandoeuvre
Mohammed ler, Laboratoire d’Electrochimie Appliqute, Faculte des Sciences, ‘Societe Solvay et Cie, 310, rue de Ransbeek, 1120 Bruxelles, Belgium
les Nancy Gdex,
Oujda, Morocco
Abstract Indole, CgNH7. can be polymerized in selected oxidizing solutions such as FeC13, CuC12 or KIO3 dissolved in acetonitrile, CH3CN. The polymerization can also occur in FeC13 solution of CH3N02, CH2C12, C6H5OH. C2H5OH and CH30H. Doped polyindole is obtained in the form of an air stable conducting powder, green in colour. The influence of the experimental synthesis conditions on the values of the electrical conductivity and on the morphology of polyindole is discussed.
1. INTRODUCTION Electrochemical polymerization of indole, CgNH7, can be achieved in aprotic solutions (l-31. The polymer, which grows at the surface of a positive electrode (gold, platinum or nickel), exhibits electrochromic properties : green in the doped state, it becomes yellow in the reduced form. The experimental conditions for obtaining polyindole have been recently extensively studied [3-51. The room temperature electrical conductivity of the doped polymer, close to 10-l S.cm-l, is found stable during long air exposure times. However, a loss of this conductivity appears for potentials higher than 1 V.vs SCE where an irreversible polymer degradation occurs. In this work, we will show that polyindole can be obtained by chemical oxidative techniques. The effects of the experimental polymerization conditions on the properties and the morphology of chemically polymerized indole will be examined.
2. EXPERIMENTAL Oxidizing agents like FeC13 (Aldrich), CuC12 (Aldrich) and KI03 (Fluka) were used as received. The polymerization was achieved in different solvents (CH3CN. CH2Cl2, CH3NO2, C6H5OH. C2H5OH and CH30H) used without purification. The electrical measurements were carried out on pressed pellets of polyindole powder using the method of Van der Pauw [6].
3.
RESULTS
Indole polymerization depends on different parameters : the redox potential of the oxidizing (dopant) species, the nature of the solvent, the concentration of the reacting agents and the temperature. It was found previously that the dopingdedoping of electrochemically polymerized indole is equal to 0.56 V vs SCE and that irreversible degradation of the polymer occurs significantly for potentials higher than 1 V vs SCE. In case of a chemical polymerization, the redox potential of the dopant must be situated between these values 0379-6779/95/$09.50Q 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02573-H
to obtain a conducting material. We found that the chemical polymerization of indole occurs in solutions of FeC13 dissolved in CH3CN. CH2C12, CH3N02, C6H5OH. C2H5OH and CH30H. In acetonitrile solutions, FeC13. CuC12 were found as effective dopants while only small amounts of polymer were produced with KI03. In case of polymerization with FeC13 as a dopant, two techniques were used. In the first one, when a FeC13-CH3CN solution is poured into a CH3CNion indole solution, thin particles are formed and are in suspension in the reacting medium. The polymer is collected either by centrifugation at lo4 rpm or by precipitation by large water dilution. In the second method, polyindole precipitation is obtained when FeClg-water solution is poured into a mixture of water containing indole and a small amount of CH3CN necessary to solubilize the monomer. Filtration or centrifugation allows to collect the polymer, green in colour. Similar experiments were carried out with CuC12. In any case, the polymerization is accompanied with gaseous HCl evolving. Chemical analyses performed on these chemically obtained polyindoles show that the counterion is Cl‘. The doping level y in [CgNH5Cly]x is equal to 0.33 when the dopant is CuC12, a value similar to that was found in electrochemically synthesized polyindole [4] ; with FeC13 in acetonitrile, lower y values can be found (0.13 I y 5 0.16). Moreover, the presence of variable amounts of Fe can be due either to traces and/or side reduction products of FeC13 trapped in the polymer bulk or to the formation of FeC4‘ acting as a counterion. Extra experiments are needed to precise this point. The electrical conductivity Q of polyindole is strongly dependent on the experimental conditions. It is always lower than the value, (T = lo- 1 S.cm- l, found for the electrochemically synthesized polyindole. Typical values of electrical conductivities are presented in Table I. The texture of polyindole can be controlled by varying the experimental conditions and by addition of emulsifying agents. Figures 1A. 1B and 1C present SEM micrographs of polyindole obtained with FeC13 as a dopant. In figure 1A. it appears that polyindole is made of aggregates 100 pm in average diameter and of variable shapes. The control of the reaction temperature and the addition of emulsifying agent such as Cl2H25SOqNa result in drastic modifications of the morphology of polyindole. Figures 1B and 1C present SEM
D. Billaud et al. / Synthetic Metals 69 (1995) 571-572
572
Table 1 Conductivity
values for polyindole
synthesized
in different
media
Solvent
CH3CN
CH3CN
CH3CN
CH2C12
CH3N02
CH30H
C2H5OH
Dopant
cuc12
FeC13
m03
FeC13
FeC13
FeC13
FeC13
o(S.cm-1)
10-2
10-3
10-6
10-5
10-3
10-4
10-4
micrographs of polyindole synthesized with Cl2H25SO4Na (0.42 g.ml-l) at respectively -7’C and -15’C. Very homogeneous polyindole latex can thus be obtained.
4.
CONCLUSION
The obtention of an air stable doped polyindole can be achieved in various oxidizing media. The doping levels in these materials, correlated to the oxidizine strength of the reaction medium, are lower than those found in polyindole ; this explains synthesized by electrochemical methods probably the low values of the electrical conductivity by comparison with materials obtained by electrochemical techniques. The morphology of polyindole can be easily controlled by varying the experimental polymerization procedures. Works are now in progress to obtain materials exhibiting higher conductivity values especially by adjusting the redox potentials of the reaction media.
Acknowledgements : The financial support of Solvay S.A., Central Laboratory (Bruxelles) is gratefully acknowledged.
Research
REFERENCES 1. G. Tourillon and F. Gamier, J. Electroanal. Chem., 135 (1982) 173 2. R. Waltman, A. Diaz and J. Bargon, I. Phys. Chem., 88 (1984) 4343 3. E.B. Maarouf, Ph. D. Thesis, Nancy, France, 1989. 4. D. Billaud, E.B. Maarouf and E. Hannecart, Polymer, Vol. 35, No 9 (1994) 2010. 5. E.B. Maarouf, D. Billaud and E. Hannecart, Mat. Res.Bull., in press. 6. L.J. Van der Pauw, Philips Rev. Report, 13 (1958) 1 and 16 (1961) 167.
Figure 1. SEM micrographs of polyindole obtained in FeC13CH3CN medium at room temperature (1A) and in the presence of the emulsifying Cl2H25SOqNa agent at -7’C (1B) and at -15°C (1C)