Optical Anistropy of Partially Aligned Polyacetylene Film Polymerized Directly from Gaseous Acetylene

Recently, we reported that gaseous acetylene may be polymerized epitaxially on crystalline faces of biphenyl, (C6H5 )z, containing dissolved and/or adsorbed Shirakawa catalyst t o yield small regions of oriented polyacetylene film (thick- ness - 1000-3000 A) (1). The present study describes the polarized absorption and reflectance spectra of this partially oriented (CH), and compares the results obtained with (i) our previously reported reflectance spectra of (CH), partially oriented by mechanical stretching (2), and (ii) the absorption spectra of oriented (CH), polymerized by Meyer using mechanical graphoepitaxy techniques under shear-flow conditions (3).

Samples of partially oriented (CH), film were prepared as described previous- ly (1). For the present study, the oriented films were washed with toluene onto glass coverslips under an argon atmosphere. Isomerization to the trans isomer was carried out under vacuum at 170°C for 0.5 h. Some of the films were treated with AsFS vapor to obtain (CH), film doped to the metallic regime. The samples were then sealed under argon by carefully glueing a microscope cover- slip over the sample with epoxy cement so that a continuous seal was made be- tween the upper and lower coverslips. Care was taken to insure that no epoxy cement came in contact with the sample.

Polarized absorption and reflectance spectra were obtained with the use of a McPherson EU 700 series monochromator (tungsten lamp source) and an IR Industries Si-PbS twecolor detector using standard light chopping and lock-in amplifier techniques. The monochromatic beam was then passed through a single visible-near IR polarizer and into the inlet of a Leitz optical microscope set up either for reflectance or transmission studies. This permitted examination of the small partially oriented regions of the sample. The monochromator and lock-in amplifier were interfaced with an Analog Devices MacSym I1 minicompu- ter for data acquisition and analysis. Spectra were recorded over the region from 2.5 eV (visible) t o 0.6 eV (near-IR).

Figure 1 shows the polarized absorption spectrum obtained from a thin par- tially oriented region of trans-(CH), film. Spectrum (a) was obtained with the polarizer oriented in the beam to give maximum absorption by the sample. The absorption coefficient was normalized to 3 x lo5 cm-' at the maximum absorp- tion consistent with previous measurements (4). The spectrum obtained is char- acteristic of trans-(CH), showing the onset of the IT- r* interband transition at ca. 1.4 eV with a maximum at 2.0 eV (4). This shows that the material studied in this investigation is indeed (CH), and not, for example, some type of (CH), adduct involving crystalline biphenyl. Rotation of the polarizer by 90' produces spectrum (b) representing minimum absorption by the sample. The observation that the absorption coefficient of spectrum (a) is approximately twice that of spectrum (b) demonstrates the partially oriented nature of the (CH), film. The appearance of the interband transition at ca. 1.4 eV in spectrum (b) indicates that

Journal of Polymer Science: Polymer Letters Edition, VoL 22,119-124 (1984) 0 1984 John Wiley & Sons, Inc. CCC 0360-6384/84/020119-06304.00

120 POLYMER LETTERS EDITION

0 - v) m

I!

I I I I I I I I I

.

...** . ...-' ...* ,&#&::::.: 0.7 I 0.9 I 1.1 - ....**- 1.3 .*-:L-.-J I5 1.7 1.9 2 I 2 3

ENERGY ( e V )

Fig. 1. Polarized absorption spectrum of trans-(CH), polymerized epitaxially on crystalline biphenyl. Spectrum (a): polarizer oriented to give maximum absorption by the sample. Spectrum (b): polarizer rotated 90" from its position in spectrum (a).

the degree of orientation in the sample is not complete, or possibly that the transition moment is not precisely parallel to the chain axis direction.

Figure 2 shows the absorption spectrum obtained after heavy doping of a trans-(CH), sample with AsFS. Spectrum (a) was obtained with the polarizer oriented in the beam to give maximum absorption by the sample. The interband transition at ca. 1.4 eVhas disappeared and increasing absorption into the infrared characteristic of a metal is observed, consistent with what has been reported pre- viously for trans-(CH), upon doping with AsF5 (5). Rotation of the polarizer

POLYMER LETTERS EDITION 121

31

*..

W V .. .-. . .

.......a* *.* ....

lo 07 09 I I 1'3 1'5 1'7 1'9 2 1 1 23 2 5 ENERGY ( e V )

"F Fig. 3. Polarized reflectance spectrum of epitaxially grown trans-(CH), .

Spectrum (a): polarizer oriented t o give maximum reflectance by the sample. Spectrum (b): polarizer rotated 90" from its position in spectrum (a).

by 90" produces spectrum (b) representing minimum absorption by the sample. This spectrum shows a considerably smaller absorption whose increase in inten- sity on going t o lower energies is significantly less than that in spectrum (a). The observation that the absorption coefficient of spectrum (a) is approximately twice that of spectrum (b) demonstrates the oriented nature of this heavily doped, metallic (CH), film.

Figure 3 shows the reflectance spectrum obtained on a thicker sample of par- tially oriented (CH), film. Spectrum (a) is produced by aligning the polarizer in the beam t o give maximum reflectance by the sample. This curve is characteris- tic of the reflectance spectrum of trans-(CH), as reported by Tanaka, Watanabe, and Tanaka (6), showing increasing reflectance through the visible with a peak at 1.47 eV corresponding t o the interband transition in (CH),. The reflectance de- creases at lower energies characteristic of a semiconductor. Rotation of the polarizer by 90" produces the spectrum (b) in which the reflectance of the sam- ple is decreased by ca. a factor of 2. The actual anisotropy cannot be deter- mined exactly due t o contributions to the reflectance from the front glass cover- slip. The structure observed in both spectra a t energies below 1.1 eV is believed t o be the result of some air exposure (doping) of the sample during measurement of the spectra. The appearance of the peak at 1.47 eV in spectrum (b) is again consistent with reflection from those (CH), fibers which are not perfectly aligned.

Recently, we found that macroscopic regions o f partially oriented (CH), film can be prepared by a modification of our previous synthetic technique (1). This involves the use of a single crystal o f p-terphenyl (CbH5)(CbH4)(C6H5), cleaved so that the crystal consists of large (1 10) faces. p-Terphenyl (Eastman KodakCo.) is first purified in a Sloan-McGowan zone refiner t o obtain high-purity polycrys- talline starting material. Single crystals are then prepared from the melt in a Bridgeman furnace constructed in our laboratories (p-terphenyl m p 251 "C).

122 POLYMER LETTERS EDITION

Single crystals (0.25-0.50 cm') of p-terphenyl are obtained by cleaving the cylindrical single crystal with a sharp razor blade.

dipped for 1-2 s into a toluene solution containing Shirakawa catalyst at one- third the concentration usually used for the synthesis of freestanding (CH), films (1). The toluene is allowed to evaporate from the crystal faces in the argon atmosphere during 1-2 min to produce a thin, but visible, dry catalyst layer coating the faces. The dipping procedure may be repeated if necessary to ensure a visually uniform catalyst layer. If difficulty is encountered in obtaining a uni- form catalyst layer, the crystal should be covered with pure toluene for 1-2 h before treatment with catalyst. The toluene, in which the terphenyl is only slightly soluble, dissolves some material from the faces. This produces a surface which is more uniformly wettable by the catalyst solution.

Polymerization of gaseous acetylene is carried out on the single crystal faces under 70 cm of acetylene pressure at -78OC for 2-5 days during which time a thin cis-rich (1000-3000 A by optical absorption) film grows on the substrate sur- face. The film formed on the edges and on one of the faces of the crystal is scratched off with a razor blade in an argon atmosphere. The crystal is then placed on a glass microscope slide with the (CH), facing either upward or down- ward, depending on which side of the film is to be studied. The slide and the crystal is placed on the bottom of a small glass vial on a hot plate in an argon atmosphere and separate aliquots (ca. 3-5 mL) of toluene are carefully added and removed by a hypodermic syringe until the terphenyl and catalyst are com- pletely dissolved. Upon pumping, the film adheres to the glass slide on drying. Optical microscopy using crossed polarizers demonstrates that the (CH), film has grown in a partially oriented state on the crystal face. The average size of the oriented regions prepared by this technique is ca. 0.25-0.50 cm2.

Thicker films (> 3000 A) can be prepared by this method by increasing the amount of catalyst on the crystal face. However, in such cases only one side of the film shows partial orientation. Scanning electron micrographs of the edge of such films shows that the partially oriented layer in such samples is ca. 1000 A thick.

Our reflectance data may be compared with previously reported data on stretch-aligned (0, (2), and on oriented (0, prepared under shear-flow con- ditions (3). In our previously published polarized reflectance spectrum of stretch-aligned (CH), (2) having !?/no - 3 (where P is the length after stretching, Qo is the length before stretching), the optical anisotropy (change in total reflec- tance by rotation of the polarizer by 90') is approximately 4, somewhat greater than what we have observed for our epitaxially prepared films on biphenyl. It is not possible to compare the data from these two samples directly because of the contribution to the reflectance of the top coverslip over the epitaxially grown sample as noted above. The interband transition in the reflectance spectrum of the undoped partially oriented (CH), obtained in the present studies (1.47 eV) is identical in both the parallel and the perpendicular directions and is in good agreement with the value obtained by Tanaka, Watanabe, and Tanaka (6) (1.49

Under a dry, oxygen-free circulating argon atmosphere, the crystal is then

POLYMER LETTERS EDITION 123

eV) on nonaligned (CH),. However, it differs from our previous results on mech- anically partially stretch-aligned films which show somewhat broad peaks cen- tered at ca. 2.0 eV in the parallel direction and 1.7 eV in the perpendicular direction. The reasons for these differences are presently being investigated.

It has also been reported by Meyer (3) that (CH), film may be prepared in a highly oriented state by polymerization of gaseous acetylene under mechanical shear-flow techniques. Such polymerization results in small regions of partially oriented (0, (8). The published polarized Vis-near-IR absorption spectra of these films shows an optical anisotropy (i.e., change in absorption coefficient), upon rotation of the polarizer by 90°, of about 4. This value is greater than what has been observed in the present films. Very recently Chien et al. (9) have reported the synthesis of partially aligned (CH), “whiskers” by a somewhat dif- ferent mechanical shear-flow technique during polymerization. This material could not be compared with the partially aligned (CH), synthesized in the present study since optical spectra were not reported.

The observed absorption and reflectance spectra of undoped and doped (CH), synthesized in the present .study give optical anisotropies of ca. 2 and demonstrate that partially oriented polyacetylene film can be prepared readily by polymerization of gaseous acetylene on crystalline faces of biphenyl con- taining dissolved and/or adsorbed Shirakawa catalyst. This represents the first example of the synthesis of partially oriented (CH), by nonmechanicul methods. Preliminary studies indicate that it may be possible to synthesize significantly larger films of oriented (CH), by appropriate modification of the above method of synthesis. It is believed that judicious choice of substrate crystals and/or polymerization catalyst may result in even better orientation of the product polymer.

Acknowledgment

The synthesis and selected optical studies of the partially oriented (CH), (by T.W.) were supported by National Science Foundation Grant No. DMR80- 22870. Spectroscopic studies (by A.F.) were supported by the Office of Naval Research and the Defense Advanced Research Projects Agency by a grant moni- tored by ONR. The authors are greatly indebted to Dr. A. R. McGhie for the synthesis of the single crystals of terphenyl used in this study.

References

(1) T. Woerner, A. G. MacDiarmid, and A. J. Heeger, J. Polym. Sci. Polym.

(2) C. R. Fincher, Jr., D. L. Peebles, A. J. Heeger, M. A. Druy, Y . Matsumura, Lett. Ed., 20,305 (1982).

A. G. MacDiarmid, H. Shirakawa, and S. Ikeda, Solid State Commun., 27,489 (1 978).

(3) W. H. Meyer, Mol. Cryst. Liquid Cryst., 77, 137 (1981).

124 POLYMER LETTERS EDITION

(4) C. R. Fincher, Jr., D. L. Peebles, A. J. Heeger, M. A. Druy, Y . Matsumura, A. G . MacDiarmid, H. Shirakawa, and S. Ikeda, Solid State Commun., 27,489 (1 978).

(5) See, for example, (a) T. C. Clarke and C. B. Street, Synth. Met., 1, 119 (1979/80); (b) M. Tanaka, A. Watanabe, and J. Tanaka, Bull. Chem. SOC. Jpn., 53,645 (1980); (c) K. Seeger and W. D. Gill, Colloid Polym. Sci., 258,252 (1 980).

(1 980).

H. Shirakawa, S. Ikeda, J. Polym. Sci. Polym. Lett. Ed., 17,195 (1979).

147 (1981).

Chem. Rapid Commun., 4,s (1 983).

(6) M. Tanaka, A. Watanabe, and J. Tanaka, Bull, Chem. Soc. Jpn., 53,3430

(7) Y . W. Park, M. A. Druy, C. K. Chiang, A. C. MacDiarmid, A. J. Heeger,

(8) H. Kiess, D. Baeriswyl, and G. Harbeke, Mol. Cryst. Liquid Cryst., 77,

(9) J. C. W. Chien, F. E. Karasz, M. A. Schen, and Y. Yamoshita, Macromol.

T. Woerner A. G. MacDiarmid

Department of Chemistry University of Pennsylvania Philadelphia, Pennsylvania 19 104

A. Feldblum A. J. Heeger*

Department of Physics University of Pennsylvania Philadelphia, Pennsylvania 19 104

*Present address: Department of Physics, University of California, Santa Barbara, California 93 106.

Received November 30, 1982 Accepted September 22, 1983