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Simultaneous Electrochemical-ESR Studies Of Conducting Polyaniline And Its Derivatives

Posted on:2005-10-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q ZhouFull Text:PDF
GTID:1101360182467723Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Conducting polymers have been attracting much attention from researchers owing to their unique properties and technological potentials. Among them polyaniline (PANI) has been studied most extensively because of its high conductivity, good redox reversibility and stability in air. For about two decades, the study of the electropolymerization of aniline, the conduction mechanism of polyaniline and the application to batteries, bioelectrochemistry, etc. has been active. Poly-o-phenylenediamine (PoPD), a derivative of polyaniline, has also been a hot topic of research in recent years.However, the conduction mechanism of conducting polymers is still not fully understood. Polaron conducting and bipolaron conducting are two main theories. These two models have been in conflict for decades, but both of them are in lack of decisive new experimental supports. In the opinion of the author of this thesis, therefore, it is necessary for promoting the understanding of the conduction mechanism to have new experimental techniques and gain new data, which are different from previous ones in terms of quantity. Electron spin resonance (ESR) is an important technique in the study of conducting polymers and had played a unique role in the development of conduction theory for conducting polymers. In history, the different interpretations of ESR findings lead to the conflict of polaron versus bipolaron conducting theories. In the literature, there were not many ESR studies conducted simultaneously with electrochemical measurements for conducting polymers. To the best knowledge of the author, there had been no report on the simultaneous ESR and conductance measurement in electrochemically controlled environment for conducting polymers. Besides, none had reported in-situ variable temperature ESR measurement for conducting polymers. As well known, the structure and properties of conducting polymers are sensitive to environment, it is therefore necessary to carry out ESR and conductance measurements under careful electrochemical control to acquire reliable data. In this work, the techniques of simultaneous electrochemical-ESR-conductivity measurements and in situvariable temperature ESR technique were established and were used to explore the conduction mechanism of conducting polymers. By using the specially designed ESR cells, polyaniline and poly-o-phenylenediamine were studied thoroughly. Besides, some interesting ESR behaviors encountered in the course of the thesis, including the asymmetrical line shape of ESR spectra and the influence of the paramagnetic redox couples on polyaniline, were also studies. The main results are as follows:1. Simultaneous electrochemical-ESR-conductivity measurements of polyanilineWith a new cell design, simultaneous electrochemical ESR and conductance measurements were carried out on polyaniline in different media to gain detailed information about the changes of ESR characteristics (intensity, line width, etc.) and conductance with electrode potential. In aqueous HC1, the combined measurements revealed the whole process of the appearing and increasing of polarons, the increasing of polaron mobility and the transfer of polaron to polaron lattice. In the high conductance potential region, the motion broadening of the ESR line width was found to well correspond to the change of conductance, indicating the mobility and concentration of charge carriers in the polaron lattice are the determining factors for the conductance. The ESR signals gained at different potentials showed different behaviors of ESR saturation, serving as a new evidence for the transfer from Curie spin to Pauli spin. In sulfuric acid solutions, similar results were obtained. All these results are in favor of the polaron lattice model for the conductance of conducting polymers.2. In situ variable temperature ESR studies for polyanilineBy using a specially designed coaxial ESR cell and appropriate mixed solvents, in situ variable temperature ESR measurements were realized to verify the spin nature of polyaniline in HC1 and H2SO4 media at controlled potentials. According to the temperature dependence of ESR intensity, it can be clearly seen that the spin nature changes from typical Curie at low potential (low conductivity) to typical Pauli at high potential (maximum conductivity). In addition, the onset potential of spin conversionwas found to depend on the medium. These findings are new direct and strong evidence in support of the polaron lattice model for conducting polymers.3. Simultaneous electrochemical-ESR studies of poly-o-phenylenediamineThe results of in situ ESR measurements of poly-o-phenylenediamine at room temperature appeared similar to those of polyaniline. The ESR data seemed to imply that with increasing potential polarons were appeared, increased and became more mobile and eventually transferred to polaron lattice. Accordingly, a high conductance was expected for higher potential region. However, direct conductance measurement showed an increase of about 7% in conductance at lower potentials compared to higher potentials, in contrast to the ESR prediction. This controversy was explained with the difference between microscopic and macroscopic conductivities of the material. The directly measured was the macroscopic conductance while the ESR predicted was the microscopic one. It may be imaged that if the contacts between polymer particles become less intimate in the potential region of high microscopic conductivity the macroscopic conductivity may be lower than that in the low microscopic conductivity region. This deduction was supported by a literature reported STM finding that the morphology of PoPD became rougher on potential moving to positive.4. Coexistence of two kinds of polarons in polyaniline in aqueous sulfuric acid solutions For polyaniline in aqueous H2SO4, spectral simulation and decomposition of high quality ESR spectra and the study of ESR saturation indicated the coexistence of two kinds of polarons. According to the correlation between the potential dependences of ESR line width and conductance, these two polarons were assigned to those in the ordered and disordered domains, respectively, in the polymer. With increasing doping level, both kinds of polarons experienced changes from Curie to Pauli spins, and both were charge carriers. However, no such coexistence of different polarons was found for polyaniline in HC1 solutions, implying strong influence of the medium on the structure and properties of polyaniline.5. Simultaneous electrochemical-ESR-conductivity measurements of polyaniline in the presence of paramagnetic redox coupleIt was found in our experiment that addition of Fe(III)Fc/Fe(II)Fc or Fe3+/Fe2+to the solution caused remarkable increase in ESR line width in potential region where the iron center was in Fe(III) valence state. However, the conductivity was essentially the same as that in the absence of the redox couples. The unusual line broadening was attributed to the secular broadening due to the spin-spin interaction between Fe(III) and polarons. Because the mobility of polarons was not affected, the conductivity remained unchanged.
Keywords/Search Tags:conducting polymers, electron spin resonance, polyaniline, poly-o-phenylenediamine, conduction mechanism
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