Iodine doped polyacetylenes

In TGA studies on the decomposition of iodine-doped polyacetylene, at high heating rates (30°C/min), decomposition becomes explosive at the m.p. of iodine, 113°C. This was attributed to exothermic reaction of liquid iodine with polyacety-lene. 

In their studies of effects of oxidation of polyacetylene on its dopability, Pochan et al.545 reported that iodine-doped polymer loses its conductivity in vacuum and concluded that the I3 counter-ions are able to react with the polymer chain, leading to iodination. Huq and Farrington 5561 found that bromine- and iodine-doped polyacetylenes lose conductivity rapidly at temperatures below 60 °C, whereas samples doped with AsFs are very much more stable. 

Druy et al.562) showed that iodine- and perchlorate-doped samples lose conductivity quite rapidly in vacuum, due to reaction of the polymer with the counter-ion. Yang and Chien 56l) also observed the instability of these doped polymers and showed that the reaction of polyacetylene with perchlorate counter-ions can be explosive. They showed that doped samples are much more stable to oxygen than is the undoped material. Muller et al.565) also observed that the stability of polyacetylene in air depends on the extent of doping, as did Ohtsuka et al.566). Aldissi 5671 has suggested that iodine doped polyacetylene can be stabilized by phenolic antioxidants, although the effect was modest. 

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

Fig. VI-1 shows the correlation between the electrical conductivity (a) and the draw ratio ( ) for iodine doped polyacetylene films the conductivity increases approximately linearly with the draw ratio [4]. The slope of a versus is approximately a factor of two larger for the thinner films (evidently the details of polymerization and/or doping result in more homogeneous, higher quality material for the thinner films).

Figure VI-1 Electrical conductivity of iodine-doped polyacetylene (parallel to the draw ratio) vs. draw ratio solid points, thin films of thickness 3-5 pm open circles, films of thickness 25-30 pm. (Taken from ref. 4)...

The addition of N-bromosuccinimide (NBS) to pristine polyacetylene stabilized the undoped polymer (32). The enhanced stability was attributed to the reaction of NBS with the numerous free radicals found in polyacetylene, as evidenced by the decreased rate of oxygen uptake in treated samples and increased final conductivities for iodine-doped polyacetylene. The conductivity of the undoped polymer rose from 10 S/cm for untreated samples to greater than 10 S/cm for NBS-treated samples. A slight amount of Br was detected in treated samples this finding indicates that some doping accompanies the treatment. The stability of doped polymer samples was not significantly improved by this treatment. 

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. 

P. Robin, J.P. Pouget, R. Comes, H.W. Gibson, and A.J. Epstein, X-ray-diffraction studies of iodine-doped polyacetylene. Polymer, 24, 1558 (1983). 

A. Guiseppi-Ehe and G.E. Wnek, Stability of iodine-doped polyacetylene in aqueous environments, J. Physique, 3(6), 193-197 (1983). 

Conductivity of polyacetylene increases more than eight orders of magnitude when doped with iodine [1]. More interests have been paid to attention of a doped polyacetylene, since Naarmann and Theophilou synthesized highly conducting iodine doped polyacetylene [2]. 

Iodine-doped polyacetylene is one of the most extensively studied systems among... 

Figure 2.2 (a) Resistivity versus temperature for an iodine-doped polyacetylene sample aged from metallic B2 to insulating B6 (b) W versus T for the same data. The dotted lines... 

Figure 2.3 Conductivity (both parallel and perpendicular) versus temperature for iodine-doped polyacetylene samples at various stretching ratios (l/lg) Reproduced by permission from GO. Yoon, R. Menon, A.J. Heeger, E.B. Park, Y.W. Park, K. Akagi, and H. Shirakawa, Synthetic Metals, Elsevier Science SA, 1995, 69,1-3, 79)...

The problem which confined the use of polymers only in the area of insulators was overcome in 1977 with the discovery by McDiarmid and Shirakawa [1] that iodine-doped polyacetylene possesses metallic conductivity. 

Park, J.G., et al. 2001. Gating effect in the I—V characteristics of iodine doped polyacetylene nanofibers. Synth Met 119 469. 

Only two different UV-vis-NIR spectra were observed for solutions of HCSA fully doped PANI-ES in different solvents. The first type of spectrum (Fig. 3a), which was obtained from solutions in chloroform, NMP, DMF, and benzyl alcohol, had three distinctive absorption peaks at 360 nm (3.42 eV), 440 nm (2.80 eV), and 780 nm (1.58 eV), respectively. These polymer solutions ( 2% w/w) were green in color and were not viscous. Freestanding films cast from these solutions were very brittle with conductivities in the range 0.01-0.1 S/cm. The second type of spectrum (Fig. 3b), which was obtained for solutions in /n-cresol, p-cresol, 2-chlorophenol, 2-fluorophenol, and 3-ethylphenol, had an absorption peak at —440 nm (2.80 eV) and a steadily increasing free carrier tail from 10()0 nm to the IR region. Note that the free carrier tail in the IR region is characteristics of metallic conductive materials such as metals, iodine-doped polyacetylenes, and AsFj-doped poly(/>-... 

The conductivity of iodine-doped polyacetylene first reported by Shirakawa et al. [1] in 1977 was 30 S cm . Since then, the conductivity reported for doped polyacetylene has kept increasing, the highest conductivity obtained so far for an iodine-doped stretched polyacetylene film [17] being > 10 S cm, a value comparable with that of copper (6 x 10 S cm ). 

Figure 1.40. Temperature dependence of the d.c. conductivity of heavily (iodine) doped polyacetylene and fit of the model of fluctuation-induced tunnelling. Solid curve theoretical fit. (Reprinted with permission from ref 84)...

Figure 1.44. D.C. and microwave conductivity of iodine-doped polyacetylene and fit of extended-pair approximation O 30 Hz 9.9 GHz. (Reprinted with permission from ref 90)...

Figure 3.4, Scanning electron micrographs of iodine-doped polyacetylene, (a) Standard crosslinked polyacetylene (conductivity approximately 500 S cm ). (b) Stretched polyacetylene (conductivity > 100 000 S cm ). (Reprinted wth permission from ref 38)...

Functional polymers appeared in the second half of the twentieth century. Although polyaniline was first described in the mid-nineteenth century by Henry Letheby and polypyrrole derivatives were reported to be electrically conducting in 1963 by B.A. Bolto et al. (1963), substantial progress was not made with intrinsically conducting polymers until the pioneering work of Hideki Shirakawa, Alan J. Heeger, and Alan MacDiarmid who reported similar high conductivity in oxidized iodine-doped polyacetylene in 1977 (Shirakawa 1977). For this research, they were awarded the 2000 Nobel Prize in Chemistry for the discovery and development of conductive polymers. ... 

W. Ross et al. have used measurements of collinear magnetoresistance and coaxial Corbino resistance at 4.2 K to derive the effective carrier mobility in unoriented iodine-doped polyacetylene. Values obtained slightly decreased with increasing dopant levels and deoended upon doping method, but an observed value of 120 cm V s" was obtained even for a I/C ratio as high as 0.23. 

Fig. 129. Conductivity (both parallel and perpendicular) vs temperature for iodine-doped polyacetylene samples at various stretching ratios. Reproduced by permission of John Wiley Sons Limited from H. S. Nalwa, Ed., Handbook of Organic Conductive Molecules and Polymers, Vols. 1-4. Wiley, New York (1997). Copyright 1997, John Wiley Sons Limited.

---