Polyacetylene material properties

It is somewhat disappointing to find, after 20 years of activity, that there is no clear connection between electrical conductivity and crystallinity, nor is there one between conductivity and orientation per se. In spite of the deficient knowledge, the conductivity obtained in doped polyacetylene has steadily risen over the years and values of over 10 S cm have been reported. Obviously, we face the task of controlling and reducing the disorder, which exists at various levels and the nature of which is ill-understood, if we want to understand and optimize the material properties of polyacetylene. 

Molecular Conductivity, A wide variety of poly-triacetylenic materials have been synthesized (84) (see example in Figure 6). The basic material properties resemble those of polyacetylene. It is interesting to observe oligomer units of this material in conjunction with U02+ . The U02+ forms a complex with the oligomer unit at positions where the R oups branch from the main polymer chain (electrical neutrality is assured with additional ions present). The chain thus becomes a very heavy unit, with a sequence of U02 units positioned along the chain. It is possible to observe the individu polymer chains with Transmission Electron Microscopy (TEM) when these uranium dioxide ions are complexed with the chain. 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

Even with improvement in properties of polyacetylenes prepared from acetylene, the materials remained intractable. To avoid this problem, soluble precursor polymer methods for the production of polyacetylene have been developed. The most highly studied system utilizing this method, the Durham technique, is shown in equation 2. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

All commodity polymers (that is those manufactured and sold in high volume) act as insulators because they have no free electrons to conduct electricity. Some low-volume polymers, such as polyacetylene, are conductive or semi-conductive, but their applications are specialized and their use limited. In this section, we shall concentrate on the properties of commodity polymers, because these materials represent the vast majority of polymers used in electrical applications. 

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. 

An observant smdent has also heard of many new and old technologies that seemed powerful and promising, but have not found much application. The buckminster-fullerene and the related carbon tubules seem like marvelous materials with unmatched and fascinating properties, but they are not used in the marketplace. Neither are the electrically conducting polyacetylenes, which hold the promise of a moldable conductor that can be made at low temperature. Almost every professor of chemical engineering and chemistry has numerous research results that are not used in the marketplace today. A visit to their offices and discussions wifh fheir research sfaff may resulf in several suggestions that are worth further investigations. 

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

Coated materials are evaluated in S-SBR and in 50 50 blends of S-SBR and EPDM rubbers. In blends, the partitioning of fillers and curatives over the phases depends on differences in surface polarity. In S-SBR, polythiophene-modified silica has a strong positive effect on the mechanical properties because of a synergistic reaction of the sulfur-moieties in the polythiophene coating with the sulfur cure system. In S-SBR/EPDM blends, a coating of polyacetylene is most effective because of the chemical similarity of polyacetylene with EPDM. The effect of... 

For silica in SBR, a polyacetylene coating gives the lowest filler-filler interaction, a good filler-polymer interaction, and the best dispersion compared to untreated and the other plasma-treated samples. However, for the stress-strain properties, the polythiophene-treated sample gives the best results. This shows the importance of sulfur moieties on the surface of the filler, which form a secondary network in the cured materials. In the blend of S-SBR and EPDM rubbers, the situation is less conclusive. The Payne effect, the bound rubber, and... 

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