Precursor monomer route: A novel concept for producing highly conductive polypyrrole films

Synthetic Metals, 55-57 (1993) 1085-1090

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PRECURSOR MONOMER ROUTE: A NOVEL CONCEPT FOR PRODUCING HIGHLY CONDUCTIVE POLYPYRROLE FILMS

HANS VAN DIJK, OLAV AAGAARD, and RONALD SCHELLEKENS* DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands.

ABSTRACT Heating of micro porous UHMW-PE films, impregnated with a solution of pyrrole-2carboxylic acid and FeCI3, results in tough and flexible films with conductivities up to 10 S/cm. Within the films a continuous network of polypyrrole is present as a coating layer on the micro fibrils of polyethylene. By triggering of the reaction via thermochemical, oxidative decarboxylation of the pyrrole2-carboxylic precursors, a controlled and efficient polymerisation is provided, resulting in a fast and continuous production process.

INTRODUCTION Intrinsic conducting polymers based on polyheterocycles (e.g. polypyrroles, polythiophenes) are usually non-processable which prevents production of conducting objects. Via modification of the monomers, e.g. N-alkylation or ring substituents, some processability has been obtained. However, this is frequently accompanied by a decrease of intrinsic conductivity [1,2]. We have developed a novel processing route in which precursor monomers are utilized [3,4]. Hence, highly conductive objects such as films, fibers, and coatings can be manufactured in an efficient way. Precursor monomers which can be used are e.g. pyrrole-2-carboxylic acid, or 3-methyl-thiophene-2-carboxylic acid. As an example the production of conductive films based on polypyrrole (PPy) will be presented in this paper.

EXPERIMENTAL Analysis Conductivity was determined using a four-probe technique. Mechanical properties were measured with a Zwick 1435 Tensile Tester at room temperature and a strain rate of 100 % 0379-6779/93/$6.00

© 1993- Elsevier Sequoia. All fights reserved

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min1. The storage modulus (E') was determined using a Rheometrics Solid Analyzer RSA-2 at a frequency of 0.2 Hz, a deformation of 0.1% and a heating rate of 5 °C min -t. Scanning electronic microscopy was performed using a Philips SEM 515. Film prep_ax~ti0n In 7 ml THF, 250 mg pyrrole-2-carboxylic acid (Aldrich) and 700 mg anhydrous iron trichloride (Merck) were dissolved at room temperature. A micro porous UHMW-PE film (porosity 85%) was impregnated with this solution followed by drying in air at ambient temperature. The film was heated in an oven at 100 °C at which stage polymerisation occurred. The resulting flexible film was extracted with acetone and dried. The bulk conductivity of the film was 1.1 S/cm. The volume fraction PPy in the film is 5 %.

RESULTS & DISCUSSION Mechanism In contrast to pyrrole, pyrrole-2-carboxylic acid cannot be oxidatively polymerized at room temperature due to the increased oxidation potential caused by the electron withdrawing carboxylic substituent (Figure 1). At high temperatures pyrrole-2-carboxylic acid is oxidatively decarboxylated leading to pyrrole radicals which initiate the PPy formation (Scheme I).

Oxidation Potential

®

~

COOH H FeCI 3

.........................................................

N

® H

H

H

H

Fig 1. Schematic representation of the oxidation potential of PPy monomers and oligomers compared to FeC13.

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The non-reactive state at ambient temperature is used to ideally mix oxidator and precursor monomer. Triggering of the reaction at any specific time can be obtained by simply heating of the impregnated film, resulting in an instantaneous production of PPy.

, '\± R ~0~COOH

~.+ I R ~COOH

N H

FeCI~

I.

R ~ .

N H

,CO

+ H+

N

H

H

H H H+

.

I

R

COOH H

H R ~

•

+

" ~ R

H . R

~

N H

-

-

R

H

n H+ 2FeCI

F ~ I ' n

Scheme I

General orooerties films In table 1, general characteristics of PPy/UHMW-PE films are listed. The mechanical properties are dominated by the initial UHMW-PE film. The composite films are tough and flexible. Bulk conductivities are, amongst others, determined by the initial concentration of pyrrole-2-carboxylic acid and FeC13, the stoichiometry of the mixture, and reaction temperature.

TABLE 1 General Properties of PPy/UHMW-PE films Conductivity, S/cm

10-6- 101

Thickness, /zm

10 - 150

E-Modulus, GPa

0.1 - 1.0

Tensile Strength, MPa

20 - 90

Elongation at break, %

10 - 40

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(A)

(B) Fig. 2. SEM pictures of cross sections of (A) the UHMW-PE film (sputtered with Au), (B) the composite PPy/UHMW-PE film (non-sputtered). Magnification is 655 and 1310 x, respectively.

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Morphology The following observations are illustrative for the presence of a continuous network of PPy as a coating layer on the micro fibrils of polyethylene: * An essential step in the production process is drying of the film after impregnation. This will result in a coverage of the PE fibrils with oxidator and precursor monomer. Therefore, polymerisation will preferably take place on the surface of the PE fibrils resulting in a PPy coating. * Scanning electron micrographs made of cross sections of the film (Figure 2) show that the porous, laminate structure of the UHMW-PE film is retained after PPy formation. * A relatively high conductance level with respect to the reaction temperature and the low volume fraction of PPy is obtained. * Uniaxial drawing of the PPy/UHMW-PE film at room temperature up to

26 % (just before

break) results in no significant decrease in conductivity (1.1 S/cm --, 0.9 S/cm). * In Figure 3, the storage modulus of the film is plotted as a function of temperature. The storage modulus of the initial UHMW-PE film falls to zero at its melting temperature (approximately 140 °C). Alternatively, the storage modulus of the PPy/UHMW-PE film levels off to a plateau value maintaining this level even up to 350 °C.

10

c~

I

I

I 120

I 100

I

I

I

I

I

I

I

I 80

I 60

I 40

I 20

I 0

I 20

I 40

I

I

I

I

I

I

I

I 60

I 80

I 100

I 120

I 140

I 160

I 180

10'

10'

10

!0 ~ 140

Temperature

200 G

Fig. 3. The storage modulus as a function of temperature of (A) the UHMW-PE film, (B) the composite PPy/UHMW-PE film.

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CONCLUSIONS By using the precursor monomer route, PPy/UHMW-PE films with excellent properties can be obtained. Heating of micro porous UHMW-PE films, impregnated with a solution of pyrrole-2-carboxylic acid and FeC13, results in tough and flexible films with conductivities up to 10 S/cm. It is suggested that a continuous network of PPy is present as a coating layer on the micro fibrils of polyethylene. By triggering of the reaction via thermochemical, oxidative decarboxylation of the pyrrole-2carboxylic precursors, a controlled and efficient polymerisation is provided, resulting in a fast and continuous production process.

REFERENCES 1 R. L. Elsenbaumer, K.Y. Jen, R. Oboodi, Synth. Met., 15 (1986) 169. 2 J.-E. Osterholm, J. Laasko, P. Nyholm, H. Isotalo, H. Stubb, O. Inganas, W.R. Salaneck, Synth, Met., 28 (1989) C435. 3 R. Schellekens, H. van Dijk (DSM/Stamicarbon), EP-A-495 549 (1992). 4 M. Bulters, H. van Dijk, R. Schellekens (DSM/Stamicarbon), EP-A-495550 (1992).