Polyacetylene n-doped

FIGURE 57. Coupled soliton modes in n-doped polyacetylene. Reprinted with permission from Reference 66. Copyright (1990) American Chemical Society... 

If the principles, so far outlined, are valid then it is to be expected that n-type doping of polyacetylene would lead to a decrease in stability towards oxidation, and this is indeed so 578). However, the introduction of electrons into the chain can also give a new instability in that the oxidation potential can fall to the point where the polymer is able to reduce water and it becomes hydrolytically unstable. Thus n-type doped polyacetylene reacts rapidly with water and with alcohols, with partial hydrogenation of the chain and a rapid decrease in conductivity 579,580,581). Whitney and Wnek 582) have used the reaction of n-doped polyacetylene with alkyl halides and other reagents to prepare functionalized poly acetylene films. 

The first rechargeable cells described in 198were based on a doped polyacetylene film serving as a positive electrode coupled with the negative lithium electrode in a nonaqueous electrolyte. Later versions included cells constructed of p- and n-doped polyacetylene films. Since then, several other systems have also been studied for potential battery applications. They include doped... 

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]. 

The partially reduced polyacetylene films formed are gold and have become good electrical conductors (16). It should be noted that in addition to the heavier alkaline earth metals, all the alkali metals (Li, Na, K, Rb and Cs) dissolve in liquid ammonia and they can be incorporated into polyacetylene as n-dopants using this method. An earlier preliminary report to our work had suggested the use of sodium-ammonia solutions for n-doping polyacetylene (17). 

Table 1. Conductivity and giavimetiy data for n-doped polyacetylenes chemically synthesized in metal-ammonia solutions. 

Conductivities of all the n-doped polyacetylene are stable over time if the films are kept in an inert environment. Great care needs to be taken since exposure to moisture or oxygen will rapidly diminish their conductivity. 

Jow and Shacklette (108) have interpreted their results on n-doped polyacetylene in terms of a charge-transfer resistance and a diffusion process. (See Section 3.5). The measured double-layer capacitance, however, does not scale with thickness of the films, and is far smaller than would be expected for a material with the observed surface area (50 -150 mVg)- This suggests, perhaps, that the observed high-frequency process is controlled by the porosity rather than a charge-transfer reaction, as in the case of RuOj. Specific capacitances for n-doping are voltage dependent and range from 95 F/g at 1.1 volts vs. Na (NaB 4 in THF) to 320 F/g at 0.6 volts. 

In Fig. 1 one polymer is included which is very close to the classical metals (CHIq 2 n doped polyacetylene. In fact,... 

If an electron acceptor is added, it takes electrons from the lower n bonding band. The doped polyacetylene now has holes in its valence band and, like p-type semiconductors, has a higher conductivity than the undoped material. Electron donor dopants add electrons to the upper n band, making this partly full, and so producing an n-type semiconductor. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

All three compounds (WF6 and MoF6 are best) will bring about a reaction (not a fluorination) that may have synthetic utility at 0CC in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) or chloroform they will cleave N,TV-dimethyl- and N-tosylhydrazones and oximes back to the parent carbonyl compounds12,14 (UF6 converts any first-formed aldehydes into acid fluorides1213). All three hexafluorides will convert1215 tertiary amines into carbonyl compounds and carboxylic acids into acid fluorides.16 They also dope polyacetylene to the metallic regime.17... 

Nechtschein et al.106/110 have carried out a detailed study of 1/Ti with frequency and temperature, both in undoped and doped polyacetylene (PA). Their data analysis shows that PA is quasi-ID system throughout the temperature range of study, except at very low temperatures. Their analysis further, showed that, the intra-chain diffusion follows power law behaviour, T n = 0.65 above 50 K, and T" n = 1.5 below 50 K. These arguments along with Sach s model111 may result in an empirical model for 1/T1 versus temperature data in a limited range of temperature. 

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. 

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