Langmuir-Blodgett thin films of electrically conductive surface-active polypyrroles

Thin Solid Films, 179 (1989) 215-220

215

L A N G M U I R - B L O D G E T T THIN FILMS OF ELECTRICALLY CONDUCTIVE SURFACE-ACTIVE POLYPYRROLES K. HONG AND M. F. RUBNER

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 (U.S.A.) (Received April 25, 1989; accepted May 25, 1989)

Electrically conductive polypyrroles have been synthesized at the air-water interface of a Langmuir-Blodgett film balance by spreading mixtures of pyrrole monomer and a surface-active pyrrole derivative (3-octadecyl pyrrole or 3octadecanoyl pyrrole) onto a subphase containing ferric chloride. The thickness and conductivity of the resultant polymers were found to depend strongly on the mole ratio of surface-active pyrrole to pyrrole monomer used in the dispersing solvent. By varying this mole ratio, it is possible to create films ranging from monolayer to multilayer thicknesses. The most highly conducting films were produced with mole ratios of pyrrole to substituted pyrrole approaching 5000:1. Multilayers of these films were fabricated successfully by both the horizontal and the vertical lifting methods. The electrical and optical properties of these new films are discussed.

1. INTRODUCTION Recently, there has been a great deal of interest focused on the fabrication of Langmuir-Blodgett (LB) films from electrically and optically active materials. In our laboratory, we have successfully fabricated LB monolayers and multilayer thin films comprising conjugated electroactive polymers which exhibit very interesting electrical and optical properties. To date, we have established two different methods to prepare multilayer thin films from electroactive polymers. One technique utilizes mixed monolayers comprising a non-surface-active poly(3-alkyl thiophene) and a surface-active compound to form highly ordered multilayer thin films ~. The second technique involves the direct synthesis of polypyrroles at the air-water interface followed by multilayer formation 2. In this paper, a discussion of our latest results concerning this unique interfacial polymerization of pyrrole will be presented. 2. EXPERIMENTAL SECTION The chemical structures of the substituted pyrrole monomers used in the polymerization reactions, 3-octadecyl pyrrole (3ODP) and 3-octadecanoyl pyrrole (3ODOP), are shown in Fig. 1. The synthesis of these materials has been described 0040-6090/89/$3.50

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K. HONG, M. F. RUBNER

CH 3 I (CH2) 1 6 I C=O

CH 3 I (CH2) 1 7

I

H

3ODP

I H

3ODOP

Fig. 1. Chemical structures of 3ODP and 3ODOP.

elsewhere 2. Polymerization was carried out on the water surface of a Lauda film balance at 20 °C by spreading a chloroform solution of pyrrole and substituted pyrrole onto a subphase containing 1 wt.% ferric chloride. Details of the polymerization conditions were reported earlier 2. Monolayers were transferred onto solid substrates as Y-type LB films at 2 5 m N m - I and 20°C. A dipping speed of 5 m m m i n -1 for the first dip and 10mmmin -1 for subsequent dips was used to transfer the films. For thickness measurements, the Sloan Dektak II instrument with a fine stylus was used. 3.

RESULTS AND DISCUSSION

In a previous publication 2, we demonstrated that electrically conductive polypyrrole films could be formed at the air-water interface of an LB trough by simply dispersing a solution containing a surface-active pyrrole monomer and a large excess of pyrrole onto a subphase containing ferric chloride (FeC13). The ferric chloride acts both to polymerize the mixture and simultaneously to oxidize the resultant polymer, thereby rendering it electrically conductive. Pyrrole monomer is needed to facilitate polymerization at the air-water interface as neither the surfaceactive pyrrole monomer nor pure pyrrole will polymerize independently under the conditions used to prepare the films. A large molar excess of pyrrole monomer is used because of the high degree of water solubility exhibited by this material. Our initial investigations focused on the use of 3ODP as the surface-active pyrrole monomer. This material forms very stable condensed monolayers at the air-water interface which can be readily transferred using a conventional vertical LB transfer technique into highly ordered Y-type multilayer thin films. X-ray diffraction analysis of these multilayer films indicates that the molecular layers stack with a bilayer repeating distance of about 55 ~. Near-edge X-ray absorption fine structure spectroscopy, furthermore, revealed a that the surface-active pyrrole molecules are well ordered within the multilayer films with their hydrocarbon tails aligned essentially normal to the plane of the substrate. This material is therefore well suited for a template polymerization scheme.

ELECTRICALLY CONDUCTIVE POLYPYRROLE LB FILMS

217

IR analysis and solvent extraction studies have revealed that the 3ODP monomer forms an insoluble copolymer with the added pyrrole when the mole ratio of pyrrole to 3ODP exceeds about 300:1. In addition, it appears that a fair amount of polypyrrole homopolymer is also formed at the air-water interface, particularly as the ratio of pyrrole to 3ODP increases beyond 500:1. It should be re-emphasized that pure pyrrole will not polymerize at the air-water interface without the presence of a surface-active pyrrole monomer. The net result in this case is the formation of a mixed monolayer comprising both 3ODP-pyrrole copolymer and polypyrrole homopolymer. The conductivities of the resultant mixtures, however, only reach respectable levels when the pyrrole-to-3ODP ratio is around 5000:1. With this high content of pyrrole, films transferred using the horizontal lifting method 4 exhibit conductivities between 10-3 and 10 -2 S cm-1. The horizontal lifting technique is needed to transfer these films because of their highly rigid nature. We have also found that the thickness contributed per monolayer to the transferred films varies from that expected for a monolayer of 3ODP (about 30/~ layer- 1) to much higher values (up to about 100/~, layer-1) as the ratio of pyrrole to 3ODP increases from 300:1 to 5000:1. Clearly the added thickness per layer at the higher mole ratios is due to the formation of a large amount of polypyrrole homopolymer. Figure 2 displays visible-near-IR absorption spectra of LB multilayer films (containing 10 layers) fabricated from monolayers formed at the air-water interface from spreading solutions containing a 300:1 (curve A) or 5000:1 (curve C) mole ratio of pyrrole to 3ODP. Also included in this figure for comparison is the spectrum of an electrically conductive (about 1 S cm-1) thin film of polypyrrole homopolymer (curve B) chemically synthesized using ferric chloride. As can be seen, the copolymer-homopolymer mixture formed with a mole ratio of 5000:1 and the chemically prepared polypyrrole both exhibit broad absorbances that extend deep into the near-IR region of the spectra. These broad bands are characteristic of electrically conductive polypyrroles and are generally attributed to absorption by free charge carriers (bipolarons) (see, for example, ref. 5). The copolymer formed with a mole ratio of 300:1 pyrrole:3ODP, in contrast, exhibits a much higher energy absorption band that only tails into the near-IR region, thus confirming the more insulating nature of this material. Although the above polymerization scheme can be utilized to form uniform thickness, electrically conductive thin films of polypyrroles at the air-water interface, the high degree of rigidity of the resultant films makes it very difficult to fabricate multilayers from these copolymers. To circumvent this problem, we have utilized 3ODOP as the surface-active monomer in conjunction with excess pyrrole to form electrically conductive monolayers. 3ODOP monomer also forms a stable condensed monolayer on the water surface of an LB trough which, in turn, can be readily transferred into well-ordered Y-type multilayer thin films. In contrast to 3ODP, however, this monomer does not react with ferric chloride, since the carbonyl group attached directly to the pyrrole ring deactivates the monomer to chemical oxidation. Thus, by utilizing this surface-active monomer, it is possible to limit the polymerization reaction solely to the homopolymerization of pyrrole monomer. In this case, the surface-active pyrrole provides an environment at the air-water interface suitable for the polymerization of pyrrole and enhances the

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K. HONG, M. F. RUBNER

z )-

I-

ra

A

A

3ODPIP,1:300

B

PP

C

300

700

1100

1500

3ODPIP,1:5000

1900

2300

2700

WAVELENGTH (nm)

Fig. 2. Optical absorption spectra of chemically prepared polypyrrole and the polymers formed by spreading 3ODP-pyrrole at the air-water interface.

spreading of the resultant electrically conductive polymer into uniform monolayer films. Figure 3 displays Fourier transform IR (FTIR) spectra obtained on films synthesized at the air-water interface from solutions containing a 5000:1 mole ratio of pyrrole-3ODOP. The top spectrum is that of the as-fabricated thin film, whereas the center spectrum represents the same film after solvent extraction with warm chloroform. The FTIR spectrum of the as-fabricated film displays absorption bands characteristic of both polypyrrole and 3 O D O P monomer. The FTIR spectrum of the film after extraction, however, changes significantly and is almost identical to that of a conducting polypyrrole homopolymer formed via the chemical oxidation route (bottom spectrum). These results clearly indicate that 3 O D O P has not reacted to form copolymer (it retains its high solubility in chloroform) and that only homopolymer polypyrrole is formed at the air-water interface. For this system, electrically conductive monolayers are also only obtained when the mole ratio of pyrrole to 3 O D O P is about 5000:1. Monolayers formed with this mole ratio of reactants are readily transferred into multilayer thin films using the vertical lifting method. The in-plane conductivities of the resultant multilayer thin films (60 layers) are between 10-1 and 10- 2 S cm - 1. In addition, preliminary results indicate that the average thickness per monolayer is about 30/~, the value expected

ELECTRICALLY

CONDUCTIVE

219

POLYPYRROLE LB FILMS

30DOPIP,

t15000

3ODOPIP. l/SO00 EXTRACTED

POLYPYRROLE

4ooo

WAVENUMBER

so0

Fig. 3. FTIR spectra of chemically prepared polypyrrole and the polymer formed by spreading 30DOP-pyrrole at the air-water interface (FTIR spectra before and after chloroform extraction).

for a monolayer of 30DOP. Thus, by inhibiting the copolymerization reaction it is possible to form flexible, electrically conductive monolayers comprising a polypyrrole homopolymer dispersed throughout a matrix of unreacted 30DOP. In this case, the thickness of the monolayer is essentially determined by the dimensions of the surface-active component. Preliminary measurements of the conductivity across the film thickness indicate that the multilayer thin films prepared from 30DOP-pyrrole are anisotropic with a conductivity anisotropy of at least 1000. Additional work is needed to determine the actual value for this parameter. 4.

CONCLUSIONS

Two polymerization

schemes have been identified that can be used to form

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K. HONG, M. F. RUBNER

electrically conductive polypyrroles at the air-water interface of an LB trough. Both methods are based on the spreading of a solution containing pyrrole m o n o m e r and a surface-active pyrrole derivative (mole ratio, 5000:1) onto a subphase containing a suitable oxidizing agent. In one case, the surface-active pyrrole derivative (3ODP) is capable of copolymerizing with the added pyrrole whereas, in the other, the surfaceactive pyrrole derivative (3ODOP) only serves to promote the formation of polypyrrole homopolymer. The latter system produces uniform monolayer films at the air-water interface that can be readily transferred into multilayers using a conventional vertical lifting technique. The conductivities of the resultant multilayer thin films reach values as high as i 0 - 1 S c m - 1. The ability to fabricate LB films from electrically conductive polypyrroles opens the door to the fabrication of a number of new thin film structures with novel molecular architectures and superstructures. In a future publication, we shall show that this technique can be used to create electrically anisotropic organic superlattices comprising layers of electrically conductive polypyrrole alternating with layers of an electrically insulating monomer. The conductivity anisotropy of these new superlattice films is greater than 108. ACKNOWLEDGMENTS The authors acknowledge Mr. Itsuo Watanabe and Mr. Robert Rosner for useful discussions and assistance. Financial support from the National Science Foundation and the M I T Center for Materials Science and Engineering is also acknowledged. REFERENCES 1 I. Watanabe, K. Hong and M. F. Rubner, J. Chem. Soc., Chem. Commun., (1989) 123. I. Watanabe, K. Hong, M. F. Rubner and I. H. Loh, Synth. Met., 28 (1989)C473. 2 K. Hong and M. F. Rubner, Thin Solid Films, 160 (1988) 187. 3 X.Q. Yang, J. Chen, P. D. Hale, T. Inagaki, T. A. Skotheim, Y. Okamoto, L. Samuelson, S. Tripathy, K. Hong, M. F. Rubner and M. L. den Boer, Synth. Met., 28 (1989) C251. 4 K. Fukuda, H. Nakahara, T. Kato, J. Colloidlnterface Sci., 54 (1976) 430. H. Nakahara and K. Fukuda, J. Colloidlnterface Sci., 69 (1979) 24. 5 T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. 1, Dekker, New York, 1986,p. 265.