Polyacetylenes conductivity range

The conductivity of polyacetylene is also increased by dopants that are electron donors. For example, the polymer can be doped with alkali metals to give, for example, [Li5 (CH) ln. The wide range of conductivities produced by these two forms of doping is illustrated in Figure 6.3. 

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

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. 

Figure 1. Conductivity range of polyacetylenes. Reproduced with permission from Ref. 10.

Polyacetylene has been the subject of intense research activity for many decades. A critical review of its history and evolution reveals that synthetic methods can be used to improve upon the processability of PA, albeit at a cost in the electrical conductivity. Although polyacetylene and its analogs exhibit a wide range of useful properties, none of these polymers are widely used in commercial applications. This observation suggests that the chemistry and physics of PA and PA analogs will continue to be a heavily researched topic for many more years to come. 

This chapter presents some examples of the application of the ab initio crystal-orbital method described in Chapter 1. Though these applications range from the field of plastics (polyethylene and its fluoro derivatives) through highly conducting polymers [polyacetylenes and polydiacetylenes, (SN) c, TCNQ and TTF stacks] to biopolymers (homopolynucleotides and homopolypeptides), they are only illustrative. No attempt has been made to review the numerous other applications performed by the Namur group and by other researchers, as this would increase unduly the size of this book. 

The electrical conductivity of polyacetylene has been much improved since 1960 as shown in Fig. 7.1 [12]. Data reported by earlier investigators were obtained with nondoped compressed pellets prepared from the intractable black powder [13-15] therefore, the values were between 10 and 10 S/cm depending on the processing and manipulation of the specimens. The conductivities of as-prepared free-standing films were in the same range as those of the compressed pellets, and it become clear that values changed almost linearly with cis-trans content. For example, the values of 76 and 15% trans films were 1.3 x 10 and 6.0 x 10 S/cm, respectively [16]. At the first report of doping in 1977, the values were 0.5 and 38 S/cm for films doped with bromine and iodine [2], respectively, and 560 S/cm for films doped with AsF.< [3]. 

It was also observed that, with the exception of polyacetylene, all important conducting polymers can be electrochemically produced by anodic oxidation moreover, in contrast to chemical methoconducting films are formed directly on the electrode. This stimulated research teams in the field of electrochemistry to study the electrosynthesis of these materials. Most recently, new fields of application, ranging from anti-corrosives through modified electrodes to microelectronic devices, have aroused electrochemists interest in this class of compounds... 

Since the discovery of doped polyacetylene, a range of polymer-intense semiconductor devices have been studied including normal transistors and field-effect transistors (FETs), and photodiodes and light-emitting diodes (LEDs). Like conductive polymers, these materials obtain their properties due to their electronic nature, specifically the presence of conjugated pi-bonding systems. 

The e. s. r. and conductivity of polymers such as the polyacene/qui-none radical polymers (PAQR polymers), polyacetylenes and polybenzimidazoles have been investigated (59, 60, 61). The term eka conjugated has been coined to decribe their properties which are very similar to those previously described. The e. s. r. signal is without structure, the activation energy which ranges from 0.2—2.0 ev is considerably... 

As discussed earlier, substitution onto the polyacetylene chain invariably has a deleterious effect on dopability and conduction properties. At the same time the stability tends to improve. Masuda et al.583) studied a large range of substituted polyacetylenes and found that stability increased with the number and bulkiness of the substituents, so that the polymers of aromatic disubstituted acetylenes were very stable, showing no reaction with air after 20 h at 160 °C. Unfortunately, none of these polymers is conducting. Deitz et al.584) studied copolymers of acetylene and phenylacetylene they found that poly(phenylacetylene) degrades even more rapidly than does polyacetylene and that the behaviour of copolymers is intermediate. Encapsulation of the iodine-doped polymers had little effect on the degradation, which is presumably at least in part due to iodination of the chain. 

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