Compensated polythiophene

Cao et al. [133] studied the air stability of re-doped polythiophene which was prepared electrochemically and then compensated by aqueous ammonia as detailed in Table 16.8. Ammonia-compensated polythiophene was found to be quite stable when stored in an ambient atmosphere for 3 months, as neither any weight gain nor any change in infra-red spectmm was observed. Both the chemically re-doped and electrochemically prepared polythiophene showed much better stability as compared to polyacetylene and the air stability of the polymer was found to be dependent on the doping counter-ion as well as the solvent used in electrochemical polymerization. Electrochemically prepared polythiophene from a mixture of CH3CN and CH3NO2 (1 1 by volume) maintained their electrical conductivity, whereas the polymer re-doped chemically by FeCls was observed to be most stable in ambient air. 

Figure 16.26, First-order weight loss of polythiophene on isothermal heat treatment 1, NH4OH compensated polythiophene with BF4 at 377°C 2, as-prepared polythiophene with BF4 at 372°C 3, NH4OH compensated polythiophene with CI04 at 400 C 4, NH4OH compensated polythiophene with BF4 at 462°C 5, as-prepared polythiophene with CIO4 at 475 C 6, as-prepared polythiophene with BF4 at 464°C 7, NH4OH compensated polythiophene with CIO4 at 475°C 8, as-prepared polythiophene with 0104 at 475"C and 9, NH4OH compensated poljdhiophcnc with BF4 at 483°C. Adapted from Bull. Mater. Sci. 18(3), 255 (1995), with permission of the Indian Academy of Sciences.

Figure 16.29. Effect of ageing on LTV-VIS spectra of ammonia compensated polythiophene with BFa at I75°C 1, compensated polythiophene 2, after 4 days 3, after 6 days 4, after 15 days 5, after 23 days 6, after 30 days and 7, after 37 days. Adapted from Bull. Electrochem. 9(2/3), 109 (1993), with pennission of C.E.C.R.I. (India).

Figure 16.35. AsFj re-doping of ammonia compensated polythiophene with BF4. ...

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

We have made a detailed study of the degradation of polythiophene containing various counter-ions and their ammonia-compensated counterparts in air and in nitrogen both isothermally and with a programmed temperature increase [113]. The observed weight loss for 5-10 micron thick films of electrochemically... 

In our compensation studies on electrochemically prepared polythiophene with BF4 counter-ion [45], when exposed to 50 tor of ammonia, a massive decrease in electrical conductivity from 10-50 down to 10 X I0 ohm cm was observed without any shift... 

Shortly after discovery of methods for preparation of organic soluble derivatives of poly(phenylene vinylene) and polythiophene, Wudl and coworkers reported preparation of each of these polymers with sulfonated side groups. The specific objective of this work was based on the idea that the anionic sulfonate groups could make the polymers self-doping , wherein the covalently linked anionic units could serve to provide charge compensation when the polymer backbone is in the oxidized (semiconducting) state. These first studies focused on the synthetic monomer and polymer chemistry. However, the authors reported in their papers that their newly synthesized conjugated... 

Polythiophene and its derivatives show remarkable air stability in both oxidized and reduced form [111]. However, there are variable reports on their stability in different types of enviromnents. The ionization potential of poly thiophene is estimated to be above 5 eV, which is high enough to protect the polymer from forming a charge-transfer complex with otygen to cause oxidative instability. Both electrochemically as-prepared polythiophene and polythiophene redoped after ammonia compensation showed much better air stability compared to polyacetylene [112,113]. 

With polythiophene electrogenerated and reduced electrochemically in a solution of tetrabutylammonium perchlorate in acetonitrile, we confirmed [112] that the initial p-type doping of the film (cr 10 S/cm and S 600 pV/K) could be compensated by implantation of alkali ions (with Cs ions, = 30 keV, and D = 10 ions/cm we get cr = 10" S/cm and 5 -5pV/K). The junctions made finally by Cs implantation through a mask with a diameter of 0.5 mm produce a pn junction type of characteristic, with a perfect blocking current... 

It has also been found that the optical absorption spectrum of various polythiophene derivatives is pressure-dependent [19,28]. For instance, the nonplanar (high temperature) form of poly(3-dodecylthiophene) is red-shifted with increasing pressure (Fig. 24.6). These results seem to indicate that the conformational change of the polymer and the increase of the band gap induced by heating can be compensated for by the applied pressure. 

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