Polythiophene degradation

The range of soluble polythiophenes has been growing rapidly to include side chains up to docosane 267), ether and amide links in the side chains 268), and water-soluble polymers with sulfonated side chains (Table 1) which are claimed to be self-doping in that the sulphonate may act as the counterion to the delocalized chain cation 269,270). In principle, these polymers can be p-doped and undoped by the transport of a proton or a small cation rather than a large anion, and so may respond more rapidly. By treatment of an aqueous solution with NOPF6, a doped solution can be made, which slowly degrades. 

In considering the potential applications of electroactive polymers, the question always arises as to their stability. The deterioration of a physical property such as conductivity can be easily measured, but the chemical processes underlying it are not as easy to be revealed. In order to understand them, XPS has been used to follow the structural changes which occur in the polymer chain and the counter-ions of the doped polymer. The following sections present some XPS findings on the degradation of electroactive polymers, such as polyacetylene, polypyrrole, polythiophene and polyaniline, in the undoped and doped states. 

Oxide, flouride, and polymeric films, as well as certain others, are used as protective coatings for HTSC materials (for example, see [505]). The electrodeposition of conducting polymers such as polypyrrole [433,491, 493, 506], polythiophene and its derivatives [493, 507], and polyaniline [478] is the most effective process. Anodic electropolymerization in acetonitrile solutions proceeds without any degradation of the HTSC substrate and ensures continuous and uniform coatings. Apparently, this method is promising not only for the fabrication of compositions with special properties based on HTSC [50, 28,295] as mentioned above, but also for the creation of junctions with special characteristics [507]. 

From X-ray diffraction, it is known that semicrystalline polythiophene powder consists of completely co-planar molecules [64], in contrast to the oligomers with chain length of and above three. The crystallinity of powders of chemically coupled polythiophene prepared by monomer oxidation with iodine, increases from 35% as synthesized up to 56% after annealing at 753 K for 30 minutes [65]. At the same time the residual iodine content decreased from 3.17% as synthesized to 0.13% after the heat treatment. Whereas annealing at 753 K leads to a first degradation of the polymer, heat treatment at 673 K results in polythiophene with chains of approximately 1200 thiophene units. Electrochemically polymerized polythiophene gives a completely different X-ray diffraction pattern [66],... 

Figure 16.5. Oxidative degradation of polythiophene (a) random chain scission accompanied with loss of conjugation (b) random chain scission with addition on C=C bond leading to loss of conjugation (c) addition on C=C bond without chain scission leading to loss of conjugation and (d) substitution on C=C bond without chain scission leading to retention of conjugation. Adapted from Synth. Mel. 66, 33 (1994), permission of Elsevier Science S.A., Lausanne.

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... 

Figure 16.27. Arrhenius plot of first-order rate constants of isothermal degradation of polythiophene. Adapted from Bull. Mater. Sci. 18(3), 255 (1995), with pemiission of the Indian Academy of Sciences.

Table 16.10. Changes occurring in and band gap as estimated from IJV-VIS spectra of electrocheniically prepared polythiophene containing BF4 counter-ion during thermal degradation at 80°C in air. Adapted from Bull. Elecirochein. 9(2/3), 109 (1993), with permission of C.E.C.R.I. (India).

Substituted-polythiophenes showed a remarkable effect on degradation and stability in electrochemical systems where poly(3-methylthiophene) was observed to be the most stable and poly(3,4-bis(ethylmercapto) thiophene) to be the least stable in the experiments conducted by Tsai et al. [272] on poly(3-ethyl-mercapto-thiophene), poly(3-hexylthiopliene), poly(3-methyl-thiophene) and poly(3,4-bis(ethylmercapto)thiophene). 

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