Electroactive polymers doping

All the spectroscopic observations described in the case of PPP are similar to those of polyparaphenylenevi-nylene [40]. A band appears at 757 cm" (Figs. 21.10c and 21.10d) after cesium ion implantation with low energy ( = 30 keV) and the same dose level as with PPP (D = 4 X 10 ions/cm ). This behavior is typical of electroactive polymers doped by ion implantation. 

The effect of doping on electrical properties such as (1) direct current conductivity ouc (dc conductivity), (2) thermoelectric power characterized by the Seebeck coefficient 5, and (3) alternating current conductivity cTac (ac conductivity) has until recently essentially been studied in the case of electroactive polymers doped by chemical or electrochemical processes. 

The earliest XPS work on modem electroactive polymers appears to be that of Hsu et al. [63] on the chemical states of the dopant in iodine-doped (CH)X films. In this work, combined XPS and Raman scattering studies revealed the presence of I3 and IJ species. The latter species resulted from the equilibrium process of the type I2 + IJ = I5. Similar findings were also made in at least two separate studies [64, 16]. The presence of polyiodide anion species was also observed... 

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. 

One challenge within the area of conducting electroactive polymers such as polypyrrole (PPy) is increasing the stability of the materials. By doping PPy with [Co(C2B9Hn)2] , the material becomes more resistant to overoxidation by 300-500 mV. ... 

The trimethylsilyloxy (TMSO) group is stable under the coupling conditions in acetonitrile (Table 4, number 11). After oxidative dimerization the TMS ether can be mildly hydrolyzed (H and H2O) to the phenol or converted to a dibenzofuran. 1,2-Dialkoxybenzenes have been trimerized to triphenylenes (Table 4, numbers 9, 12, and 13). The reaction product is the triphenylene radical cation, which is reduced to the final product either by zinc powder or in a flow cell consisting of a porous anode and cathode [60]. Dibenzo-crown ethers are converted by anodic oxidation to electroactive polymers. Films of these polytriphenylenes exhibit unusual doping properties 62-64]. 

Gas-solid interface reaction can provide a simple method of producing ordered polymers. In our laboratory, we have used reactions with AsF, to produce polymers. The advantage of this method is that it produces electroactive polymers (enhanced electrical conductivity) in a doped state. The polymer formed, however, generally contains disorder as revealed by their x-ray... 

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

The earliest application of the XPS technique to the study of CT interactions in modem electroactive polymers appears to be that of Hsu et al. [144] on the iodine-doped (CH) films. The presence of multiple chemical states for the iodine dopant, such as the and Is species, has been revealed by the combined XPS core-level and Raman scattering studies. The Is species was postulated to have residtcd from the equilibrium process of the type U + Is Is. with the 13ds/2 BEs for the I2 and Ij species lying at about 620.6 and 619 eV respectively. Similar results were... 

E.V. Ovsyannikova, O.N. Efimov, E.P. Krinichnaya, and N.M. Alpatova, Cathodic doping of thin-layered composites. Formed by electroactive polymers and rubbed single-walled carbon nanotubes, Rus. J. Electrochem., 43, 1064-1068 (2007). 

Bay, L., S. Skaarup, K. West, T. Mazur, O. Joergensen, and H.D. Rasmussen. 2001. Properties of polypyrrole doped with alkylbenzene sulfonates. Presented at the Proceedings of the SPIEs 8th International Symposium on Smart Structural Materials, Electroactive Polymer Actuators and Devices (EAPAD), Newport Beach, CA. 

Berry, B.C., A.U. Shaikh, and T. Viswanathan. 2003. Lignosulfonic acid-doped polyaniline (ligno-pani) for the corrosion prevention of cold-rolled steel. In Electroactive polymers for corrosion control, eds. P. Zarras, J.D. Stenger-Smith, and Y. Wei, 182. Washington DC American Chemical Society. 

Paik, W, I.-H. Yeo, H. Suh, Y. Kim, and E. Song (2000). Ion transport in conducting polymers doped with electroactive anions examined by EQCM. Electrochim. Acta... 

The redox properties of the conducting polymer film are the primary interest of the present chapter, because most of the important applications are associated with switching the electroactive polymer films from the neutral (reduced) state of the doped (oxidized) state. The voltage range in which representative polymers show electroactivity is shown in Figure 2.2, compared with inorganic materials. Li metal is chosen as the reference because of the interest in using the intercalation materials in lithium battery systems. 

The oxidation or doping process of electroactive polymers involves the simultaneous insertion of charge and a counterion to balance the charge. For redox polymers such as polyvinylferrocene the process may be written... 

On the other hand, the electrodes attaching electroactive polymer films and polymer CMEs having ionic, electronically conductive, or redox property have been widely studied. Electronically conducting polymers, which are usually more conductive than redox polymers, can be doped to increase their intrinsic conductivity. Such films are now popular because they are technically easier to modify by applying polymer onto an electrode compared to covalent monolayer formation. Polymer films generally adhere satisfactorily to electrodes simply by forces of chemisorption or by being insoluble in the contacting solvent. 

The capability of polypyrrole to be reversibly doped and dedoped by electrochemical methods makes this electroactive material adequate for the construction of rechargeable batteries [181-185]. The electroactive polymer can be either the anode or the cathode of the cell, although construction of anodes is most common, due to difficulties in inserting negative charges into polyheterocycles. Technical factors such as cyclability, energy density, and stability have to be optimized in the future, before commercial application of polypyrrole-based batteries. 

On going to polypyrrole we find that for heavily doped polypyrrole, the resistance is proportional to the logarithm of temperature in the temperature range 1 K to 1 mK. This behavior is appropriate for an amorphous metal [57]. This is supported by the temperature independence of paramagnetic susceptibility that is sometimes seen for heavily doped polypynole. Such a result is also inconsistent with spinless excitations such as bipolarons. Thus one can see various types of temperature behavior for charge transport in electroactive polymers in the dry state. 

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