Polyacetylene Highly Oriented

A particularly interesting property of Durham polyacetylene is that it can be stretched to draw ratios of up to 20 during the transformation, to yield a polyacetylene sample with high levels of orientation. This effect was reported by Bott et al. 378) for thin films in the electron microscope and then by Leising et al. 379), who drew single fibres of polyacetylene to a highly oriented /rani-state with a density of 1.06 g cm-3. 

Oyanagi H, Tokumoto M, Ishiguro T, Shirakawa H, Nemoto H, Matsusita T, Ito M, Kuroda H (1984) Polarized X-ray absorption spectra of bromine doped polyacetylene evidence for highly-oriented polybromine ions. J Phys Soc Japan 53 4044-4053 Pauling L (1960) The Nature of the chemical bond. Cornell Univ. Press, Ithaca, NY Perera RCC, Barth J, LaVilla RE, Deslattes RD, Henins A (1985) Multivacancy effects in the X-ray spectra of CH3C1. Phys Rev A 32 1489-1494... 

As described in Section II.B.l above, doping causes a drastic change in the electrical properties of polyacetylene. The initial values of electrical conductivity were of the order of 10 S cm" for unoriented materials d24-i30 when doped by iodine and AsFs, were enhanced to the order of 10 S cm, which was obtained in the parallel direction of the doped films oriented by mechanical stretching 31 Improvements in polymerization methods and in the catalyst systems also enhanced the electrical conductivity. Highly oriented films prepared in liquid crystal solvents (Section II.A.l.d.iii) exhibited a conductivity higher than 10 S cm, as did also a well stretch-oriented film prepared by Ti(OBu)4-EtsAl dissolved in silicon oil and aged at 120°C. In further studies Naarmann and Theophilou and Tsukamoto and coworkers attained a conductivity of ca 10 S cm k... 

Billaud et al. [99] have also recently proposed a distorted hexagonal structure (actually monoclinic) for highly oriented polyacetylene chemically doped with lithium to y = 0.16. 

An x-ray pattern of a doped, highly oriented film of Akagi polyacetylene is shown in Figure 1.16. For this... 

Recent diffraction data for highly oriented Tsuka-inoto polyacetylene [22,136] suggest stronger correlations between neighbouring polyiodide columns in this new, highly conductive (cr > 10 S cm ) material. Tsukamoto and Takahashi [136] report a monoclinic structure with a = 7.81 A, 6 = 15.62 A, c = 51.2 A and ) = 120 for their fully-iodine-doped polyacetylene. 

It has been suggested that the three orders of magnitude differences in conductivity between some highly oriented polyacetylene types (e.g. Tsukamoto polyacetylene versus Durham polyacetylene) are related to differences in the tilt angle of the polyiodide ions (Pouget et al. [137]). Dopants may have a crucial bridging function, providing the necessary interchain conduction pathways [138],... 

Some of the results obtained over the years are collected in Tables 1.5 and 1.6. The first of these lists the value of the intercalation period, the second the meridional repeat distance of the ion sublattice (along the chain axis). More reflections have been observed to result from doping, notably at 3.3-3.4 A and at 4.5-4.7 A. The values of the c-repeat of the ion sublatticc show considerable variation, but those obtained from highly oriented polyacetylene are more reliable [102,138,147]... 

The polyacetylene shown in Fig. 1 was prepared by the new BASF technique. It is linear (no defects) and is therefore highly orientable (up to 660 %) with a conductivity of more than 100,000 s/cm. 

On the other hand, orientation is one of the most important factors to enhance the conductivity of one dimensional pi-conjugated polymers like polyacetylene. Several attempts were made to obtain an oriented polyacetylene [3-5]. Akagi and Shirakawa applied magnetic force into a polymerization system consisting of nematic liquid crystals and obtained a highly oriented polyacetylene. Kahlert and Leising synthesized an oriented polyacetylene film by pyrolyzing prepolymers. Ozaki polymerized acetylene on the surface of benzene crystals and prepared an oriented polyacetylene. 

Fig. 4. Infrared spectrum in polarized light of a highly oriented film of trans-polyacetylene (Assoreni) electric vector perpenthcular (dotted) and parallel (fuU line) to the stretching direction...

Orientation of the polymer chains can be obtained by different methods. Highly oriented films of poly acetylene have been made by performing polymerization in a liquid crystal 11,2]. It has also been shown that it is possible to attain highly oriented films of polyacetylene 13] and poly(p-phenylene vinylene) [4] by stretching their precursors. Kaneto et al. 15] have oriented polythiophene by stretching a film, prepared electrochemically on In-Sn oxide (ITO)-coated poly(ethyleneterephthalate) films. The processability of the recently developed poly(3-alkylthiophenes) [6-8] makes this class of polymers suitable for stretching. Yoshino et al. [9] have shown that it is possible to orient poly(3-alkylthiophene), both as free-standing films and in a blend with an elastomer. 

The polarized reflectivity, in the energy range between 0.06 and 0.6 eV, is shown in Fig.l. The measurements on highly oriented (draw ratio 7) cis -polyacetylene, (polymerized with a procedure described elsewhere [5]), were performed by a Bruker 113v Fourier transform spectrometer, equipped with a I S-5 polarizer. 

G. Masetti, E. Campani, G. Gorini, R. Tubino, P. Piaggio and G. Dellepiane, "Polarized Raman scattering of highly oriented trans-polyacetylene", Chem. Phvs. 108 141 (1986). 

D. Comoretto, A. Borghesi, G. Dellepiane, G. Lanzani, G. F. Musso, A. Piaggi and R. Tubino, "Optical properties of highly oriented fibrous polyacetylene", to be published. 

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