Iodine-doped polyacetylene temperature

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. 

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)...

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 1.48. Magnetoresistance of iodine-doped polyacetylene at liquid-helium temperature (Reprinted with permission from ref 89)...

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.

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

Proton 1/Ti of heavily doped polyacetylene films with different dopants such as FSO3H, FICIO4, iodine, bromine and potassium was measured by Shimizu et al.113 and the behaviour expected for a quasi-ID metal was not found. Some of them showed a time dependence of 1/Ti. They have deduced the temperature dependence of resistivity from the 1/Ti and 1/Ti versus temperature shows T1 5 behaviour above 40 K and deviates from such behaviour below 40 K. Mizoguchi and Kuroda107 have given a comprehensive review of many investigations of NMR relaxation of both 1FI and 13C in undoped polyacetylene. 

The first work reported was done at the beginning of the eighties and dealt with polyacetylene. (CH)x has been characterised under different forms foam [32] iodine doped [33] AsFs doped [34] IrClg doped (which exhibits a giant dielectric constant) [35] encapsulated [36] cis and trans isomers [37] anisotropic [38]. The volution of the transport mechanism with doping level has been studied by measuring evolutions of Oj)c and (75.5 GHz with temperature [39]. An analysis based on the fibrillar structure of polyacetylene has been given. Recent works have been published, as in the case of n-doped polyacetylene [40]. 

Figure 13.23 Schematic variation of the conductivity of polyacetylene doped with iodine versus reciprocal temperature. Dopant concentration is approximately (a) 0.15 mol%, (b) 0.01 mol% and (c) 0.005 mol% of I2 per CH unit...

Doped polyacetylene has been the prototype system since the initial report of the achievement of a conductivity of a- 100S/cm[100 (Il cm) ] upon doping with iodine and other donors and acceptors [2]. Subsequently, (CH) was synthesized by alternate routes [63-69] that yielded higher conductivities upon doping. The room temperature... 

A positive microwave frequency dielectric constant is also found for modestly conducting iodine doped unstretched and modestly stretched Tsukamoto polyacetylene [75] and for unstretched PPy-TsO [74], Fig. 46.23(a) and (b). Using Eq. (46.9), the size of the low temperature metallic box, L, can be determined. Table 46.4 summarizes the low temperature microwave dielectric constant for typical modestly conducting doped polymers and the corresponding metallic box size calculated using Eq. (46.9). In each case L is approximately the size expected from x-ray diffraction studies of the structural coherence length,... 

Figure 1.54. ESR linewidth of iodine-doped cis- and trans-polyacetylene as a function of temperature. (Reprinted with permission from ref 117)...

The parameters obtained by numerical fitting to the equation are doped polyacetylenes. The resistance of 29 increases rapidly in the presence of oxygen or iodine. In contrast, the rhodium complex, 30, is an insulator (<7= 10 S cm at room temperature). 

The work of Kim et al. [110] focused on pofyacetylene fiber networks ranging in diameter from 60 to 80 nm. The dark red gel of low density foamlike polyacetylene doped with iodine showed a remarkably low temperature dependence of resistivity as compared to bulk polyacetylene film. Polyacetylene fiber networks were found to be more sensitive to air than the bulk polymer, because the resistance of the iodine-doped fiber network increased more rapidly than that of the bulk polyacetylene film on mqposure to air. 

Iodine-doped oriented polyacetylene has been studied extensively by several groups [17.125-128,130,131J. The maximum room temperature conductivity parallel to the chain axis for the best quality oriented l-(CH) is on the order of lO S/cm, and the anisotropy is greater than 100. 

Although resistivities as low as 10 /7cm have been reported for iodine-doped oriented polyacetylene parallel to the draw direction (chain axis), a positive temperature coefficient of the resistivity, typical of a metal, has not been observed. This indicates that defects or transport perpendicular to the chain axis limit the conductivity. We conclude, therefore, that the 500 A mean free path is not limited by phonon scattering. This implies that significantly higher room temperature conductivities will be achieved as the quality of the material is improved. The absence of any positive temperature coefficient implies that values at least an order of magnitude higher are to be expected i.e., the intrinsic conductivity at room temperature is greater than 10 S/cm. 

In Fig. 12 we compare the dc and microwave (9 GHz) conductivity of Iodine-doped trans-polyacetylene in the temperature range from 10 to 300 K /9/. (dc measurements were carried out hy a lock-in technique at 30 Hz to avoid ionic conductivity by migration of Iodine ions.)... 

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