Conducting polymers neutral state properties

These ideas developed by chemists resemble the bipolaron model, which presents the solid-state physicist s view of the electronic properties of doped conducting polymers [96]. The model was originally constructed to characterize defects in inorganic solids. In chemical terminology, bipolarons are equivalent to diionic states of a system (S = 0) after oxidation or reduction from the neutral state. The transition from the neutral state to the bipolaron takes place via the polaron state (= monoion, S = 1/2,... 

This list can be divided into three main classes based mainly on function and redox state. First, applications that utilize the conjugated polymer in its neutral state are often based around their semi-conducting properties, as in electronic devices such as field effect transistors or as the active materials in electroluminescent devices. Secondly, the conducting forms of the polymers can be used for electron transport, electrostatic charge dissipation, and as EMI-shielding mate-... 

It is the Peierl s instability that is believed to be responsible for the fact that most CPs in their neutral state are insulators or, at best, weak semiconductors. Hence, there is enough of an energy separation between the conduction and valence bands that thermal energy alone is insufficient to excite electrons across the band gap. To explain the conductive properties of these polymers, several concepts from band theory and solid state physics have been adopted. For electrical conductivity to occur, an electron must have a vacant place (a hole) to move to and occupy. When bands are completely filled or empty, conduction can not occur. Metals are highly conductive because they possess unfilled bands. Semiconductors possess an energy gap small enough that thermal excitation of electrons from the valence to the conduction bands is sufficient for conductivity however, the band gap in insulators is too large for thermal excitation of an electron accross the band gap. 

The metal-like property of these polymers is based on their chemical nature, which consists of chains of conjugated double bonds. If these polymers are oxidized, they become electrical conducting. In the neutral state they can have properties like an inorganic semiconductor. This has now become as important as the metal-like conductivity. One example is the development of an organic field effect transistor (OFET). 

Conducting properties can also be achieved by reduction of the neutral state. An example is the poly(dibutoxyphenylenevinylene). A platinum electrode coated with polymer in acetonitrile showed the transition between neutral and oxidized state at -r 1V versus a platinum quasi reference electrode and the transition between neutral and reduced state at-1.8 V. The cyclic voltaimnogram is shown in Figure 11.22b as an example to determine characteristic semiconductor properties from cyclic voltammograms. The reduced state has received much less attention than the oxidized state. 

Figure 11.17 Equivalent circuit of the conducting polymer fUm on an inert electrode surface. The model assumes a compact film on the metal surface with semiconducting properties in the neutral state (/ 5( and C q) and a porous film towards the electrolyte with electron resistance and ion resistance and a Faraday capacitance Cp. is the electrolyte resistance.

Composites of conducting polymers, e.g., polyaniline and PEDOT, with polyacids, e.g., poly (2-acrylamido-2-methyl-l-methyl-l-propanosulfonic add) (PAMPS), have been shown to be electro-chromic. The polyadd acts as a dopant for the polymer film with the optical properties of the composite being contributed by the conducting polymer. The composites are formed by dther chemical or electrochemical polymerization of the electrochromic component monomer in the presence of the polyacid. Films of polyaniline-PAMPS switch from yellow to green and finally to blue on oxidation [228,229]. Composite films of PEDOT and PAMPS show similar electrochromic properties to PEDOT with the films switching from dark blue in the neutral state to Kght sky blue in the oxidized state [140,230,231]. 

The redox properties of the conducting polymer film are the primary interest of the present chapter, because most of the important applications are associated with switching the electroactive polymer films from the neutral (reduced) state of the doped (oxidized) state. The voltage range in which representative polymers show electroactivity is shown in Figure 2.2, compared with inorganic materials. Li metal is chosen as the reference because of the interest in using the intercalation materials in lithium battery systems. 

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