SOLUBLE POLYACETYLENE G. Baker, F. Bates

To cite this version: G. Baker, F. Bates. SOLUBLE POLYACETYLENE. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-11-C3-16. .

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Colloque C3, suppl6ment au n06, Tome 44, juin 1983

page C3-11

SOLUBLE POLYACETYLENE G.L.

Baker and F.S.

Bates

Bell Laboratories, Murray HiZZ, New Jersey 07974, U.S.A.

Re'sumh-Les syntheses de copolym6res solubles contenant du polyacitylene ont it6 rialisies. Dans ces solutions les unitis polyacityliniques ont id isoltes. Leurs caractiristiques par spectroscopies infrarouge, visible, et de rgsonance magngtique nucliaire sont dicrites dans ce papier. Abstract-Soluble block copolymers containing polyacetylene have been prepared in which the polyacetylene segment is isolated in solution. The characterization of this material by infrared, visible, and NMR spectroscopy is described.

The advances in research on the chemistry and physics of conducting polymers have been preceded by corresponding advances in the synthesis of new materials. When polyacetylene was known only as a black intractable p o ~ d e r , little ' ~ ~ research effort was devoted to this semiconducting polymer. The advent of polyacetylene as silvery free-standing films?x4 convenient for physical measurements led to an explosion of research interest which has lasted for over ten years. Free-standing films appear to the eye to be homogeneous, but a closer examination reveals a complex morphology of randomly oriented fibrils, characteristically 200-400 angstroms in diameter. Each fibril is composed of polyacetylene chains oriented parallel to the fiber

The interpretation of physical

measurements of polyacetylene can be complex since the properties measured reflect contributions from both chains within fibrils, and chains at the junction of fibrils. Indeed, it has not been possible to measure the properties of single chains of polyacetylene. Unfortunately the morphology of polyacetylene is not easily modified since polyacetylene neither melts without decomposition, nor dissolves in

solvent^.^

The polymer-polymer interactions are greater than the

polymer-solvent interactions which leads to the precipitation of the polymer during or soon after its preparation. Despite variations in the catalyst used to polymerize acetylene, the fibrillar morphology is a common feature of the product. In such systems, crosslinking need not be invoked to explain the insolubility of polyacetylene. We have found that these unfavorable polymer-solvent interactions can be overcome be attaching a suitable solubilizing group to polyacetylene. Although in principle this group could have various chemical forms, one of the simplest methods of solubilizing polyacetylene is to covalently bond to the polyacetylene chain a polymer which has the desired polymer-solvent

interaction^.^

The benefit of using a polymer as a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983302

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solubilizing group is twofold. The grafting reaction can be carried out using standard chemical techniques, and as little as one polymer chain need be bound to the polyacetylene. Although the grafting site can occur at any position on the carrier polymer, it is preferable that the grafting site on the polyacetylene segment be one of the ends of the polymer. This preserves the high symmetry and the desirable electronic properties of the polyacetylene segment. We have prepared graft block copolymers of polyacetylene with polystyrene or polyisoprene as the solubilizing block, imparting the solubility characteristics of the homopolymers to polyacetylene. The polyacetylene segment of the block retains the electronic and spectroscopic properties of the homopolymer, but now is isolated from other polyacetylene chains in solution, effectively removing the chain-chain and interfibril pertubations of the single chain properties of polyacetylene. It is extremely important to adequately analyze samples of polyacetylene thought to be soluble in order to verify the integrity of the polyacetylene segments, and also to assure that the chains are isolated in solution. A clear colored solution is the necessary result for soluble material, but a colored solution can also result from a suspension of finely divided particles. In the remainder of this text, we will focus on the questions of polyacetylene characterization and solubility. The simplest characterization method available to verify that the presence of polyacetylene segments in these block copolymers is infrared spectroscopy. When the spectrum of the polyisoprene-polyacetylene block copolymer was measured, it was found that it was merely the linear combination of the spectra for polyisoprene and that for trans polyacetylene. No bands were observed that do not appear either in the polyisoprene or polyacetylene spectrum. The visible spectra of the block copolymers in solution are very similar to spectra obtained from thin films of solid polyacetylene prepared by the method of Shirakawa et. a1.334Absorptions from the carrier polymers occur at significantly higher energies, and do not interfere. Thespectrum of the predominately cis copolymer is nearly identical to that of a cis film, with the peak of the absorption envelope at the

same energy. The spectrum of the trans isomer is similar, but a slight shift of the peak to higher energy is visible, along with a decrease in the slope of the absorption edge near the band gap. Although a vibronic band at 1.53 eV complicates the analysis, it appears that both the thin solid films and the soluble material have similar band gaps. From this information, it is clear that by spectroscopic methods, polyacetylene is present in the block copolymers. We have been unable to directly measure the molecular weight of the polyacetylene segment due to a unique adsorption phenomenon. Soluble polyacetylene adsorbs strongly to a variety of surfaces, particularly metals, making normal chromatographic analyses impossible. However this adsorption phenomena in conjunction with infrared spectroscopy was used to reliably estimate the number average molecular weight to be between 3 x lo4 and 6 x lo4 g/mole.

This compares favorably to previous

estimates for the molecular weight of polyacetylene prepared by the method of Shirakawa.1°

WRVENUMBERS

Figure 1.

Infrared spectrum of a, polyisoprene, and b, polyacetylene/polyisoprene block copolymer.

nm

Figure 2.

Visible spectrum of a, cis polyacetylene prepared by the method of Shirakawa, b, cis polyacetylene/polyisoprene block copolymer in toluene.

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Figure 3.

Visible spectrum of a, trans polyacetylene prepared by the method of Shirakawa, and isomerized a t 190°C,b, trans polyacetylene/polyisoprene block copolymer in toluene, isomerized at room temperature.

To prove that the polyacetylene is indeed soluble is more difficult. Several observations suggest that it is. The cis isomer, prepared a t low temperatures, was found to rapidly isomerize in solution at room temperature, giving all trans polyacetylene in less than 60 hours.

Varying solvents produced a

corresponding shift in the visible spectra, consistent with the variation in solvent polarity and dielectric constant. The block copolymers were very susceptible to oxidation when dissolved, and unlike solid state polyacetylene, bromine adds quantitatively to the double bonds of polyacetylene rather than acting as a dopant. Finally, the broadening of the visible absorption envelope can be interpreted as arising from the change from an ordered state in solid state polyacetylene, to a disordered state in solution. Although these observations are consistent with the notion of polyacetylene chains in solution, the most convincing data was obtained by NMR spectroscopy. Until now, NMR studies of polyacetylene were hindered by the broad line spectra obtained when solid state samples are employed. By contrast, dilute solutions of soluble polymers give narrow line spectra. For the block copolymers, a doublet

ppm vs TMS

Ppm vs TMS

Figure 4.

a 'H and b

13cNMR spectra of trans polyacetylene/polyisoprene block copolymer in CDCI,, and CCI, (inset).

centered at 128.8 pprn is observed in the

I3c NMR

spectrum with linewidths comparable to the

resonances due to the carrier polymer, indicating that indeed the polyacetylene segments are in solution. The narrow line widths also suggest that polyacetylene does not exist as rods in solution, but rather adopts a nonlinear conformation. A rigid rod of molecular weight 3 x lo4 would be unable to tumble at a sufficient rate to eliminate field inhornogeneities, leading to a broad line spectrum. Future efforts will be directed at investigating the single chain properties of polyacetylene, and applying the new technique of polyacetylene solubilization towards the development of new materials.

References

1. Gutmann, F.; Lyons, L. E. "Organic Semiconductors", Wilely; NY 1967. 2.

Berets, D. J.; Smith, D. S. Trans. Faraday Soc. 1968, 64, 823.

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3. Shirakawa, H.; Ikeda, S. Polym. J. 1971, 2, 231 4.

Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci., Polym. Chem. Ed., 1974, 12, 1 1 .

5.

Fincher, C. R., Jr.; Chen, C.-E.; Heeger, A. J.; MacDiarmid, A. G.; Hastings, J. B. Phys. Rev. Lett. 1982, 48, 100.

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Chien, J. C. W.; Karasz, F. E.; Shimamura, K. Macromolecules 1982, 15, 1012.

8. Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Sci., Polym. Chem. Ed. 1974, 13, 1943. 9. Cohen, R. E.; Ramos, A. R. Macromolecules 1979, 12, 131.

10. Wnek, G. E.; Capistran, J.; Chien, J. C. W.; Dickinson, L. C.; Gable, R.; Gooding, R.; Gourley, K.; Karasz, F. E.; Lillya, C. P.; Yao, K. D. in "Conductive Polymers", Seymour, R. B., Ed., Plenum: NY 1981, p. 183.