Doping polyparaphenylene

Another polymer composite that is of interest is the more complex system SbCls-doped polyparaphenylene/polyparaphenylene sulphide (PPP/PPS) sintered materials (Rueda etal, 1988). 

In polyparaphenylene, soliton waves are impossible because the two phases (aromatic and quinoid, as shown in Fig. 9.12c) differ in energy (low-energy aromatic phase and high-energy quinoid phase). However, when the polymer is doped, a charged double defect bipolaron. Fig. 9.12c) may form, and the defect may travel when an electric field is applied. Hence, the doped polyparaphenylene, similarly to the doped polyacetylene, is an organic metal. 

FIGURE 5.8. Structures of some common doped conductive polymers, each projected on a plane normal to the direction of the polymer chains. Cross-hatched circles indicate dopant ions, (a) The square channel structure of potassium-doped polyacetylene (other alkali-doped PA lattices show similar structures),(b) lithium-doped polyparaphenylene, (c) potassium-doped ppp,(ii3) AsFs-doped PPR In (d) the dopant species, represented by x, are probably... 

FIGURE 5.16. Response of host and dopant sublattices to various displacements z(Li) of a Li ion (labeled Li ) from its perfect-lattice site in Li-doped polyparaphenylene. Dopant ions are aligned between two PPP chains. (From Ref 107 by permission of the publishers.)... 

Diffraction studies on the structures of doped polyparaphenylene show that although the polymer chains retain their nonzero setting angles in the presence of alkali metal cations, polyatomic ions such as AsF6 may well impose an all-parallel arrangement on polymers [see Fig. 5.8(d) and Stamm and Hocker s comments on the most favored structure of AsFs-doped polyparaphenylene]. The latter structure is rather similar to that foimd in the simulated doped PPy and PTh lattices, with the ions occupying sites that are coplanar with the chains. 

Figure 14 also explains why polypyrrole and polyparaphenylene are not as environmentally stable as polythiophene. Undoped poly pyrrole is susceptible to oxidation by oxygen in air, since the potential for undoped polypyrrole is below the threshold for oxygen reduction. On the other hand, /7-doped polyparaphenylene has an oxidation potential greater than that for water and is unstable toward moisture. F-doped polyparaphenylene is reduced to a less doped, less conducting state. Although oxidized and neutral polyacetylene appear to be within the stability window, experimental results show that polyacetylene is not stable [293]. The chemical reactivity of oxygen toward the carbon-carbon double bonds or defect sites in polyacetylene may make polyacetylene unstable in air. 

Table 5.3 Examples of electronically conducting polymers, y is the level of electrochemical doping and k is the maximum electrical conductivity. Except for poly acetylene and polyparaphenylene, only p-doping is considered... 

Solitons are considered to be important defect states in these conjugated polymers (see Fig. 6.48). It has however been shown that correlation energy is the more important factor in giving rise to the energy gap in (CH) (Soos Ramasesha, 1983). Other polymers related to polyacetylene are polythiophene, polypyrrole, poly-phenylenesulphide, and polyparaphenylene (Section 3.3). Extensive measurements on doped polyacetylenes have been reported in the last five years and these materials, unlike other conducting polymers such as (SN), seem to have good technological potential. 

The application of Langmuir-Blodgett films as rectifiers and/or switches have been also proposed. Peterson [106] investigated two semiconductors polyparaphenylene 178 and polyphenylenevinylene 179 with these purposes in mind. These systems with the chain lengths of at least 20 units could also be used as photovoltaic devices since their electroluminescence should be readily detectable. The doping of such materials may be necessary but at present it is not clear whether they will form Langmuir-Blodgett films when doped. 

The synthesis and characterization of polyacetylene (PA) provided new incentive for understanding 7r-electronic spectra, electron-phonon interactions and electronic correlations[l, 2, 14]. The electrical conductivity of chemically doped PA rivals that of metals. Families in Fig. 2 such as polydiacetylenes (PDAs), poly thiophenes (PTs), cr-conjugated polysilane (PSs) and polyparaphenylene vinylene (PPVs), among... 

Polymeric tubes for clad lead-acid battery plates have been made from intrinsically conductive polymers [19] by weaving fibres of polypyrrole, poly(phenylene vinylene), or polythiophene of several hundred microns in thickness. Polyaniline [20] as well as polypyrrole, polyparaphenylene, and polyacetylene doped with anions (C104, FeCl, AsFj, sol", and HSO4) have also been added as 1.15pm powders or fibres to positive pastes to improve capacity [21]. 

By contrast, the ECP must have conjugated rigid-rod macromolecules. Several such polymers show high electrical conductivity (usually after doping), viz. polyacetylene (PAc), polyaniline (PANI), polypyrrole (PPy), polyparaphenylenes (PPP), or poly-3-octyl thiophene (POT). The resins are expensive, difficult to process, brittle and affected by ambient moisture, thus blending is desirable. For uniaxially stretched fibers the percolation threshold is 1.8 vol%, hence low concentration of ECP (usually 5-6 vol%) provides sufficient phase co-continuity to ascertain conductivity similar to that of copper wires (see Table 1.79). 

Finally an LDA calculation was also performed for a polyparaphenylene (PPP) helix.100 This conducting polymer (if doped) is quite interesting for its applications in light emitting diodes101 (especially because it emits blue light102), in field-effect transistors103 and in non-linear optical devices.104... 

BT), they also act as models of the conjugated organic polymers polyparaphenylene (PPP) and polythiophene (PT), respectively. As is discussed in detail elsewhere [96], the results for the electronic excited states of the cation and anion of BP can be used for interpreting the polaron states in doped PPP. The main features of the spectra are reviewed briefly below. 

Polyaniline, polypyrrole, polyparaphenylene and polyacetylene (doped with SO , HSOj), added to the positive paste as powders or fibres, enhance the formation process and increase the capacity of the cells. The amount of polymers added to the paste should be within the range 0.8—2.0 wt%. Higher polymer loads impair the mechanical stability of PAM and hence the life of the battery is shortened dramatically. Polymers disintegrate on overcharge. Polyaniline has proved to be most stable on battery overcharge. 

J.L. Bredas, R.R. Chance, and R. Silbey, Comparative theoretical study of the doping of conjugated pol3Tners Polarons in polyacetylene and polyparaphenylene, Phys. Rev. B. Condens. Matter., 26, 5843 5854 (1982). 

The crystal structures of several doped species of Li have been determined by diffraction methods. While uncertainty shrouds the structure of lithium-doped PAc, it has been shown that (probably because of the small size of the Li ion) its accommodation in the polyparaphenylene (PPP) lattice occurs with very little modification to the position of the polymer chains. The Li" ions occupy sites on the monoclinic (nearly orthorhombic) a and b axes, but although they thereby experience a channel-like environment, the walls of the channels are rhombic rather than tetragonal. 

J. L. Bredas, R. R. Chance, R. Silbey, Theoretical-studies of charged defect states in doped polyacetylene and polyparaphenylene, Molecular Crystals and Liquid Crystals 1981, 77, 319. 

J. E. Bredas, B. Themans, J. G. Fripiat, J. M. Andre, R. R. Chance, Highly conducting polyparaphenylene, polypyrrole and polythiophene chains - an ah initio study of the geometry and electronic structure modifications upon doping, Physical Review B 1984, 29, 6761. 

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