Conductivity in polymers

In solid state materials, single-step electron transport between dopant species is well known. For example, electron-hole recombination accounts for luminescence in some materials [H]. Multistep hopping is also well known. Models for single and multistep transport are enjoying renewed interest in tlie context of DNA electron transfer [12, 13, 14 and 15]. Indeed, tliere are strong links between tire ET literature and tire literature of hopping conductivity in polymers [16]. 

Table 2 shows the present state-of-the-art for the electrical conductivity of doped conjugated polymers. The magnitude of the electrical conductivity in polymers is a complex property determined by many stmctural aspects of the system. These include main-chain stmcture and TT-ovedap, molecular... 

It is assumed that conduction in polymer films is ionic —it is difficult to see how it could be otherwise — and the factors which break down this resistance, or render it ineffective, will now be considered. 

Kilbride BE, Coleman JN, Foumet P, Cadek A, Hutzler S, Roth S, Blau WJ (2002). Experimental observation of scaling laws for alternating current and direct current conductivity in polymer-carbon nanotube composite thin films. J. Appl. Phys. 92 4024—4030. 

An irradiation-induced expansion could conceivably be caused by the ions, formed as precursors of the radicals, or by thermalized electrons trapped within the polymer. Irradiation induces electrical conductivity in polymers, and this conductivity decays after irradiation is ceased (4, 5). The decay process is accelerated by increased temperature or plasticity of the specimen, presumably by facilitating leakage of the trapped electrons or ions to ground. One might speculate that the sample expands upon irradiation because of the local mutual electrical repulsions of like charges which are trapped in the polymer matrix, and that both increased temperature and plasticizer content diminish this expansion because of charge leakage out of the specimen. It is difficult to prove or disprove this hypothesis. 

Polaron Pair State. There are a number of experimental observations which can be interpreted neither by invoking charged excitations injected or photo-generated in the polymer, nor by excitons. However, it may happen that the singlet exciton is broken, as described above, and a pair of charges, negative P and positive P+ polarons, are separated onto adjacent chains, but still bound by the Coulomb attraction. These pairs will be referred to as polaron pairs. Polaron pairs are intermediate states between electronic molecular excitations and free charge carriers. They are formed by excitation of the photo-conductivity in polymers and other molecular solids, as well as... 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

Miyamoto, T. and Shibayama, W. D. Free volume model for ionic conductivity in polymers. J. Appl. Phys. 44 5372, 1973. 

To provide a framework for discussing conduction in polymers and why polymers are normally classified as insulators, some of the basic ideas of band theory in solids should be reviewed. Cowan and Wlygul (23), in their review of the organic solid state, discussed this topic from the organic viewpoint and provided a useful framework within which chemists can appreciate the salient features of the theory. 

The electrical property of a material is determined by its electronic structure, and the relevant theory that explains the electronic structure in the solid state is band theory. This theory, however, does not fully explain conductivity in polymers. It is noteworthy that the energy spacing between the highest occupied and lowest unoccupied bands is called the bandgap the highest occupied band is called the valence band and the lowest unoccupied band is called the conduction band. The bandgaps of insulators and semiconductors are wide and narrow, respectively there are no band-gaps in metals. 

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