Poly doped polymer conductivity

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. 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

As discussed earlier, substitution onto the polyacetylene chain invariably has a deleterious effect on dopability and conduction properties. At the same time the stability tends to improve. Masuda et al.583) studied a large range of substituted polyacetylenes and found that stability increased with the number and bulkiness of the substituents, so that the polymers of aromatic disubstituted acetylenes were very stable, showing no reaction with air after 20 h at 160 °C. Unfortunately, none of these polymers is conducting. Deitz et al.584) studied copolymers of acetylene and phenylacetylene they found that poly(phenylacetylene) degrades even more rapidly than does polyacetylene and that the behaviour of copolymers is intermediate. Encapsulation of the iodine-doped polymers had little effect on the degradation, which is presumably at least in part due to iodination of the chain. 

Conducting polymers, such as poly (aniline), poly(pyrrole) or poly(thiophene) (Figure 7.1) have a conjugated system of delocalized it -orbitals, which allows conduction to occur in the oxidized, or doped polymer. 

Organic photoreceptors can be prepared in either a flexible web or drum format. Webs are usually prepared on polymer substrates, polyethylene tere-phthalate being the most common. The substrates are between 100 to 200 pm in thickness and coated with a conducting surface layer. The substrates often contain layers on the reverse side for reduced curl, static discharge prevention, and control of frictional characteristics. The web configuration is also widely used for laboratory studies. For drums, the substrate is a metal cylinder, usually Al. Recently, however, drums of a poly(phenylene sulfide) resin doped with conductive C black have been developed (Kawata and Hikima, 1996). Drums are widely used in low- and mid-volume applications. Drums, however, are not well suited for research purposes. Thus, the preparation and characterization of drum photoreceptors is usually related to a specific application. 

The related fully sulfonated, self-doped polymer poly(2-methoxyaniline-5-sulfonic acid) (PMAS 9) may be prepared under normal atmospheric pressure by the oxidation of 2-methoxyaniline-5-sulfonic acid (MAS) monomer with aqueous (NH4)2S208 in the presence of ammonia or pyridine (to permit dissolution of the MAS monomer).141 The polymerization pH was therefore >3.5. Subsequent studies showed that the product consisted of two fractions a major fraction with Mw of ca. 10,000 Da whose electrical conductivity and spectroscopic and redox switching properties were consistent with a PAn emeraldine salt, as well as a nonconducting, electroinactive oligomer (Mw ca. 2,000 Da).143 144 Pure samples of each of these materials can be obtained using cross-flow dialysis.145... 

Radical-Cation Salts as Models for Conducting Polymers. Polymers that have an extended Tr-electron system in their backbones, for example, polyacetylene (PA) and poly(p-phenylene) (PPP), can be transformed by oxidation or reduction in the solid state (doping) to derivatives that exhibit metallike conductivity (24, 25). These materials are usually insoluble and infusible and exhibit a very complicated morphology that cannot be changed by subsequent treatment. The lack of knowledge about the structure and state of order is the cause of the current controversy about the conduction mechanism in doped polymers. 

Poly-yne polymers composed of phthalocyanine (Pc) complexes (42) with silicon were synthesized (equation 40) by Mitulla and Hanack. The conductivities of the polymers were up to 10 S cm", and increased to 10 S cm by I2 doping. 

Poly-ene polymers including metal moieties, for example selenium (43), were prepared (equation 41) from 1,4-benzenediselenol and diethynylbenzene in the presence of a radical initiator or by UV-light irradiation . The polymer was insoluble in organic solvents and the conductivity after I2 doping was 10 S cm k... 

For conducting polymers, neutron diffraction may indeed provide very complementary data, since the dopants tliat are used to make polymers conductive often contain heavy elements. In the case of x-rays, the atomic scattering is proportional to the square of the number of clecfrons (atomic number). Dopants containing heavy atoms will therefore dominate the x-ray pattern, and the infonnation about the structure of the polymer is reduced. In alkali-metal doping of poly-... 

PANI has been doped with poly(amic acid) in order to obtain an all-polymer conducting material, by Angelo-poulos el a . [337]. Its structure has not been investigated, but the authors present x-ray diffraction results for cured specimens, in which polyimide formation has taken place at the expense of the carboxylic acid groups. The diffraction is structureless there is no indication that the elimination of the ionic interaction leads to phase separation of the polymers. 

At least in heavily doped polymers, the temperature-independent susceptibility is obsci-vcd in PT and P3AT (poly(3-alkylthiophenc)) doped with AsF(, [255,279, 282] PFc," [282] SO.iCF.r [295] CIO - [282,296] and BFj" [258], except for PT/P3AT-1T [255,297]. Possible reasons why it is not obseiwed in PT/P3AT-Ij arc (1) the ESR linewidth from the Pauli spins is too broad to observe by ESR and (2) the charge transfer from iodine to polymer is insufficient to fill a band gap. The situation with these systems is similar to the case of PPy on the metallic electronic states the electrical conductivity reaches 500 S/cm [298-300], the small... 

Wellinghoff et al. [215] reported synthesis of nitrobenzene soluble poly(N-methyl-3,3 -carbazole) on doping with iodine in the ratio of one iodine per carbazole monomer unit. The presence of I3 and 15 counter-ions in the doped polymer were confirmed by Raman spectroscopy. The black shiny material showed an electrical conductivity in the range of 1-lOohm cm" and long term stability in air at room temperature for many months as shown in Figure 16.41. 

Formerly, Bradley and Hammes [152] studied dc conductivity phenomma on a number of metal-free and some metal-containing plasma polymers. They foimd plots of Igdc resistivity versus 1/T to be generally linear and concluded that a single activation process is responsible, with activation energies of 1.36 eV for metal-free polymers and < 1 eV for metal-doped polymers. For evaporated Cu/poly-ethylene films [129] the Ig sheet resistance revealed a linear dependence on The conduction process was explained in terms of hopping of electrons between traps in localized states dose to the Fermi level. 

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