Conductivity doped polymers

A positive microwave frequency dielectric constant is also found for modestly conducting iodine doped unstretched and modestly stretched Tsukamoto polyacetylene [75] and for unstretched PPy-TsO [74], Fig. 46.23(a) and (b). Using Eq. (46.9), the size of the low temperature metallic box, L, can be determined. Table 46.4 summarizes the low temperature microwave dielectric constant for typical modestly conducting doped polymers and the corresponding metallic box size calculated using Eq. (46.9). In each case L is approximately the size expected from x-ray diffraction studies of the structural coherence length,... 

FIGURE 46.23. emw(6.5 x 10 Hz) versus temperature for modestly conducting doped polymers, (a) I3 doped Tsula-moto polyacetylene (unstretched end and modestly stretched neck portions of sample) (from Refs. [75,160]), (b) unstretched PPy-TsO (from Ref. [74]), and (c) intermediate crosslinked 3.5 times stretched HCI doped PAN-ES (from Ref. [73]). 

Conductivities of polymers of technological interest such as polypyrrole and polythiophene are typically 1000 cm in the doped state, and the conductivity can be tuned by reversibly doping and undoping the polymer. Derivatives of these and other polymers have achieved even higher conductivities. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Electrogenerated conducting polymer films incorporate ions from the electrolyte medium for charge compensation (182). Electrochemical cycling in an electrolyte solution results in sequential doping and undoping of the polymer film. In the case of a -doped polymer, oxidation of the film results in the... 

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... 

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

Although cyclic voltammetry in a variety of electrolyte systems, and with a variety of doped polymers, has shown strong effects due to ion transport, it has provided little understanding. In fact, one of the important uses of ion transport data from the techniques discussed in the preceding subsections is that they help to provide an understanding of the cyclic voltammetry behavior of conducting polymer films. Their importance will... 

Heeger AJ, McDiarmld AG (1980) Conducting Organic Polymers Doped Polyacetylene. In Alcacer L (ed) The Physics and Chemistry of Low-Dimensional Solids. Reidel, Dordrecht, p 353... 

Conduction in the raw doped polymer can be improved by devices such as compression and stretching and judicious after-synthesis dopant ion exchange. Matrix-guided electropolymerization also yields superior products. 

Conducting polymers with p-phenylene groups in the backbone are generated by metathesis (Equation 7). Doping of these polymers with acceptors such as I2, Br2 or AsF5 increases the conductivity. Similar polymers have been prepared from 1,3- and 1,4-ClSC6H4SCl 294... 

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

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