The Structures of Conducting Polymers

What are the structures of conductive polymers, and what is the mechanism for electronic transport in the solid-state ... 

Tourillon and co-workers have also carried out extensive ex-situ studies on the structure of conducting polymers following the features around the carbon edge. ... 

Several attempts have been made to discover the structure of conducting polymers. There were few successes in describing either the structure of isolated polymer chains on graphite substrate [81] or the packing of short oligomers [82], both using STM. 

The diversity of conducting polymers is best illustrated by Krivoshei and Skorobogatov s book,15 although many more examples have since been reported. The most widely studied classes, from an electrochemical point of view, are the polypyrroles, polythiophenes, and polyanilines21 22 (Structures 2-4), and these are the focus of this chapter. A wide... 

All these data show that only small changes of the polymerization parameters may lead to characteristic differences in the resulting structures of conducting polymers. Structural properties - for example, regularity and homogeneity of chain structures, but also chain length play an important role in our understanding of the properties of such materials. Spectroscopic methods have proved particularly... 

The structures of crystalline polymer-salt complexes provide insight into the structure of the more conducting amorphous materials. To date, large single crystals of polymer-salt complexes have not been prepared, but it has been possible to obtain structural information from single crystal X-ray diffraction applied to stretched oriented fibres in the PEO NaI and PEOiNaSCN systems (Chatani and Okamura, 1987 Chatani, Fujii, Takayanagi and Honma, 1990). One of the most detailed studies is of (PEO)3 NaI, Fig. 5.11(a). The sodium ion in this structure is coordinated to both the polymer and to the iodide ion and the polymer is coiled in the form of an extended helix. 

As was shown, the planar conductivity of the film can be increased by immersing the substratum with the film in the ethanol-water (1 1) solution of LiNOs (0.1 mol/liter) for a short time. Then the film should be washed in water and allowed to dry. After such treatment the conductivity becomes 500 times greater and reaches the value 6x10 (Q/cm)". This increase may be due to the fact that in considering the second general model of the structure of this polymer it could be assumed that some additional quantity of Li cations might be absorbed into the ionic sphere of SO- groups, so that the total amount of Li in the electrolytic layers increases, and the conductivity then also increases. 

Chemical preparation methods for the synthesis of conducting polymers have been widely used [35]. It has become clear that to fully exploit the potential of the conducting polymer, better-defined soluble materials with a clear correlation between structure and properties need to be prepared. In this section, monosubstituted alkylthiophenes will be discussed as an example of how, via the development of well-documented systematic methods, ordered polymer layers can be obtained with improved conductive properties. 

Soliton — Solitons (solitary waves) are neutral or charged quasiparticles which were introduced in solid state physics in order to describe the electron-phonon coupling. In one-dimensional chainlike structures there is a strong coupling of the electronic states to conformational excitations (solitons), therefore, the concept of soliton (-> polaron, - bipolaron) became an essential tool to explain the behavior of - conducting polymers. While in traditional three-dimensional -> semiconductors due to their rigid structure the conventional concept of - electrons and -> holes as dominant excitations is considered, in the case of polymers the dominant electronic excitations are inherently coupled to chain distortions [i]. 

Fig. VII-1 shows a schematic of the structure of a polymer LED and a picture of a thin film flexible polymer LED seven-segment display. The bottom electrode of this display was made by spin-cas ting a layer of metallic polyaniline onto a flexible plastic substrate [69]. Polyaniline was chosen as the electrode material because it is flexible, conducts current, and is transparent to visible light. The emissive layer of the display was fo med by spin casting a layer of MEH-PPV over the polyaniline. The top electrodes were formed by evaporating calcium through a patterned shadow mask. Since the conductivity of undoped emissive polymers is relatively low, it was not necessary to pattern the polymer or the bottom electrode to prevent current spreading between neighboring pixels.

Depending on their structure, the polymers containing heterocycles have various applications. For example, poly(furfuryl alcohol) is used in composite materials with fillers such as sand and concrete, in copolymers with formaldehyde, etc. Some of the polymers from this group have special properties such as good electrical conductivity (after appropriate doping). Among these polymers are poly(thiophene-2,5-diyl) and particularly polypyrrole, CAS 109-97-7, (usually in carbon black doped with an organic acid anion). The structure of this polymer is shown below ... 

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