Mobility, charge carrier conductive polymers

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

This significant low-field conductivity demonstrates that conjugated polymers can be encapsulated in nanometer channels and still support mobile charge carriers. In contrast to the experiments with polypyrrole in zeolite Y and mordenite (see above), the channels in the MCM host provide more space and apparently allow some important interchain contact to occur. 

Polyaniline filaments within the mesoporous channel host (aluminosilicate) have significant conductivity, and this demonstration of conjugated polymer with mobile charge carriers in nanometer channels represents a step toward the design of nanometer electronic devices. These composites have potential as stable molecular wires, which can be applied in the design of batteries and systems to accumulate electric charge. SBA-15 with polyaniline inside the pore channels was used as a dispersed phase in electro-rheological (ER) fluids. 

Attempts to obtain transport number information by various methods such as pulsed field gradient NMR [62], radio tracer diffusion [77], and potentiostat-ic polarization technique [46] have suggested that both cation and anion mobilities are important for the total ionic conductivity seen. In general, however, the nature of charge carriers in polymer electrolytes is quite complex and ion aggregates such as triple ions have been implicated in conductivity [78-79]. 

Reported studies deal with measurements of the electrode potential of zero charge Epzc [698, 699], double layer investigations [700-702] and studies of electrode reaction mechanisms [703, 704] for an overview, see also [693, 697], Numerous studies (beyond many conducted ex situ) deal with intrinsically conducting polymers, particularly photogenerated mobile charge carriers [705-707]. 

This concept - mobile charge carriers in the valence (HOMO) or conduction (LUMO) band of the polymer, with the counter-charges on ionic species - provides the fundamental framework for the subsequent development of polymeric conductors. 

Conducting polymers are unusual in that they do not conduct electrons via the same mechanisms used to describe classical semiconductors and hence their electronic properties cannot be explained well by standard band theory. The electronic conductivity of conducting polymers results from mobile charge carriers introduced into the conjugated -system... 

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