Polyacetylene doping mechanism

Polyaniline, polypyrrole, polyparaphenylene and polyacetylene (doped with SO , HSOj), added to the positive paste as powders or fibres, enhance the formation process and increase the capacity of the cells. The amount of polymers added to the paste should be within the range 0.8—2.0 wt%. Higher polymer loads impair the mechanical stability of PAM and hence the life of the battery is shortened dramatically. Polymers disintegrate on overcharge. Polyaniline has proved to be most stable on battery overcharge. 

Similar to Kivelson s model. Chance and co-workers proposed interchain hopping for spinless conductivity in doped polyacetylene, doped poly(p-phenylene) and other doped polymers [102]. The mechanism accounts for the observed dopant concentration dependence of the conductivity in rans-polyacetylene and the observation of anomalously... 

Values of the modulus and tensile strength versus draw ratio of polyacetylene doped to maximum conductivity are also plotted in Figures VII.3 and VII.4, for comparison with the corresponding properties of the undoped samples. Both the modulus and tenacity consistently decreased by about a factor of 4-5. It is not surprizing to find some degradation of the mechanical properties after doping this is to be expected when the mechanical properties... 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

Polyacetylene in the doped state is sensitive to air and moisture. Other polymers (e.g., those of pyrrole, thiophene, and benzene) are stable in air and/or toward humidity in their doped and undoped states. Generally, when stored in the doped state, the polymers lose doping level by mechanisms not fully understood in most cases the loss is reversible. 

In 1958, Natta and co-workers polymerized acetylene for the first time by using a Ti-based catalyst. This polymerization proceeds by the insertion mechanism like the polymerization of olefins. Because of the lack of processability and stability, early studies on polyacetylenes were motivated by only theoretical and spectroscopic interests. Thereafter, the discovery of the metallic conductivity of doped polyacetylene in 1977 stimulated research into the chemistry of polyacetylene, and now poly acetylene is recognized as one of the most important conjugated polymers. Many publications are now available about the chemistry and physics of polyacetylene itself. 

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. 

This combination of high electrical conductivity and outstanding mechanical properties has been demonstrated for doped polyacetylene [3-6,28-30]. Unfortunately, since doped polyacetylene is not a stable material, the achievement of stable high performance conducting polymers remains an important goal. 

Properties of representative conducting polymers. Doped conjugated polymers have generated substantial interest in view of possible applications such as lightweight batteries, antistatic equipment, and microelectronics to speculative concepts such as molecular electronic devices.37-38 These polymers include doped polyacetylene, polyaniline, polypyrrole, and other polyheterocycles (Figure 5). While the conduction mechanism of metals and inorganic semiconductors is well understood and utilized in microelectronics, this is not true to the same... 

The origin of the conduction mechanism has been a source of controversy ever since conducting polymers were first discovered. At first, doping was assumed to simply remove electrons from the top of the valence band (oxidation) or add electrons to the bottom of the conduction band (reduction). This model associates charge carriers with free spins (unpaired electrons). However, the measured conductivity in doped polyacetylene (and other conducting polymers such as polyphenylene and polypyrrole) is r greater than what can be accounted for on the basis of free spin alone. 

Doped polyacetylene conducts electrons via an intrinsic mechanism rather than by an extrinsic one. That is to say, conductivity of the polymer is due directly to electronic conductivity rather than to charge carrier motion. 

The unique properties of polymers such as polyacetylene, whose backbones consist of an alternating succession of single and double bonds, and most of which show extraordinary electrical, optical and magnetic properties including electrical conductivity when "doped" with electron donors or acceptors [35], are also outside the scope of this work. Sophisticated quantum mechanical treatments are required to predict these properties of such polymers. 

The mechanical and electrical properties of polyacetylene (PA) were modified by blending it with polybutadiene (PB). Further enhancement of the electrical conductivity of the blends was obtained by stretch elongation of the blends prior to doping. 

Blending of polyacetylene with polybutadiene provides an avenue for property enhancement as well as new approaches to structural studies. As the composition of the polyacetylene component is increased, an interpenetrating network of the polymer in the polybutadiene matrix evolves from a particulate distribution. The mechanical and electrical properties of these blends are very sensitive to the composition and the nature of the microstructure. The microstructure and the resulting electrical properties can be further influenced by stress induced ordering subsequent to doping. This effect is most dramatic for blends of intermediate composition. The properties of the blend both prior and subsequent to stretching are explained in terms of a proposed structural model. Direct evidence for this model has been provided in this paper based upon scanning and transmission electron microscopy. 

Radical-Cation Salts as Models for Conducting Polymers. Polymers that have an extended Tr-electron system in their backbones, for example, polyacetylene (PA) and poly(p-phenylene) (PPP), can be transformed by oxidation or reduction in the solid state (doping) to derivatives that exhibit metallike conductivity (24, 25). These materials are usually insoluble and infusible and exhibit a very complicated morphology that cannot be changed by subsequent treatment. The lack of knowledge about the structure and state of order is the cause of the current controversy about the conduction mechanism in doped polymers. 

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... 

---