Polymer conducting after "doping

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

All electronic and electro-optic applications of poly-conjugated systems require the preparation of polymers with high chemical and structural homogeneity. Several optical and electrical properties of conjugated polymers such as their quantum efficiency of electroluminescence or maximum conductivity after doping can be correlated with the concentration of conjugation breaking defects introduced to the polymer upon its preparation. 

PPS and PPO are polymers which behave differently due to the nature of the atom (S or O) between the rings and the geometric parameters. The inter-ring bondii s are not linear and the bond length C-S (1.75 A) is very different from that of C-O (1.36 A). In addition, conductivity after doping with AsFs is found to be s 2-3 S/cm for PPS and = 10 S/cm for PPO. A quinoid structure can be expected in these two cases, but its local extension is limited to one ring. 

The synthesis of conducting polymer materials is aimed at forming a structure with extended Tr-electron conjugation that exhibits high conductivity after doping. 

Bis(2-thienyl)pyridine and 2,6-bis(2-thienyl)pyr-idine were electrochemically polymerized [12]. The resulting polymers showed poor electrical conductivity after doping with anions. Poly[2,6-bis(2-thienyl)pyri-dine] was obtained in high-quality films which had a very smooth surface and was composed of fine fibrils (Figure 7.7). 

The parent polythieno[3,4-Z ]pyrazine (19, R = H) has recently been prepared by reaction of 2,3-pyrazine-dicarboxylic anhydride (92) with P4S10 (Scheme 8) as a black powder whose dedoped conductivity was 10 -10 S/cm. Doping with NO BF4, I2. or FeCU gave conductivities of (5-7) x 10 S/cm [22]. A similar reaction using 2,3-pyridinedicarboxylic anhydride (93) produced polythieno[3,4-/>]pyridine (94), whose dedoped conductivity was 10 S/cm and whose conductivities after doping with NO BF4 . I2. and FeCIi were 2 X 10 3 X 10 , and6 x 10 S/cm, respectively [22]. No band gap data were presented for either polymer. 

There is no doubt that doped polypyrrole is very much more stable than is polyacetylene, but reports are variable of exactly how stable it is. Street 393) reported that the polymer loses less than 20% of its initial conductivity after one year in air at ambient temperature and Diaz and Kanazawa 589) claimed that the polymer is stable at 100-200 °C, depending on the counter-ion the latter authors also reported that polypyrrole is undoped reversibly by ammonia treatment. Munstedt 590) found that the conductivity of doped polypyrrole was unchanged after 200 days at 80 °C in... 

An outline of a quantitative description of the dispersion phenomenon is given below. This theory provides a description of the backbone structure of a polymer insofar as it evaluates a distribution function for conjugation lengths on the chains. In certain cases this was found to correlate with the conductivity of the polymer after doping and thus provided important information concerning the conduction mechanism. Fig. 4.8-5b... 

Despite their inherent electronic advantages, CT complexes and radical cation salts tend to be brittle and unprocessable. This problem might be overcome by the incorporation of oligomeric tetrathiafulvalenes in polymers, whereby the TTFs can be part of a main-chain or side-chain polymer. The key concern thereby is to achieve the suitable packing of the donor moieties, which is, of course, less perfect than in the crystalline state. Remarkably, the rigid-rod poly-TTF 164 could be made recently by a precursor route in which 164 is made by dimethyl disulfide extrusion of the precursor polymer (scheme 39). The electrical conductivity after iodine doping amounts to 0.6 S/cm [221]. Other examples of TTF-containing polymers, either in the backbone [222] or in the side-chain [223], are summarized in chart 25. 

In general, polymers composed of alternating silylene units and n electron systems are insulators. However, when the polymers are treated with an oxidizing agent, they became conducting. Preliminary doping studies showed, when cast films of selected polymers were exposed to iodine vapor under reduced pressure, that the conductivity of the films increased and reached almost constant values after 10 h. The conductivity at this point was found to be 3.6 x 10 S cm (13) and 8.9 X 10 S cm (17). 

Depending on their structure, the polymers containing heterocycles have various applications. For example, poly(furfuryl alcohol) is used in composite materials with fillers such as sand and concrete, in copolymers with formaldehyde, etc. Some of the polymers from this group have special properties such as good electrical conductivity (after appropriate doping). Among these polymers are poly(thiophene-2,5-diyl) and particularly polypyrrole, CAS 109-97-7, (usually in carbon black doped with an organic acid anion). The structure of this polymer is shown below ... 

Mechanical properties of poly(thiophene-2,5-diyl) are not suitable for many practical uses. Polymers with higher flexibility and still good thermal resistance or with other special conductivity properties (after doping) can be obtained from the polymerization of substituted polythiophenes. Some examples are shown below ... 

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