We have studied the behavior of a chemically synthesized polypyrrole as electrode material. The different electrochemical methods that we have used for this characterization, namely, cyclic voltammetry, ac impedance measurements, and cyclic chronopotentiometry, have evidenced the role of the low frequency capacitance Cnormals characteristic of the doped state of polypyrrole. This capacitance seems independent of the synthesis conditions of the polymer, and we show that it partially explains the large value of the total charge false(Cnormalm>100 normalAh/normalkgfalse) stored in this polypyrrole. Analyzing the effect of residual water in the polymer, we show that a water content of about 3% (w/w) increases the total charge Cnormalm and lowers the redox potential E0 . A quasi linear relationship has been found between E0 and Cnormalm , the massic capacity: Cnormalm increases when E0 decreases. This relation can be also attributed to the charging of the capacitance Cnormals:Qnormalcs=Cnormals false(Enormald−E0false) , where Enormald is the degradation potential. We discuss the origin of this capacitance Cnormals .

Anomalously large current plateaus observed in the cyclic voltammograms of some conducting polymers such as polypyrrole have been recently interpreted as the charging of a large capacitance taking place in the polymer in the oxxdized state. The nature and the orxgin of such capacitance is actually discussed either in terms of a static charge associated with a double layer formation in the porous electrode, or in terms of an overdoping of the polymer in a non-Nernstian redox process. In order to clarify this situation we made a systematic electrochemical study by cyclic voltametry, ac impedance measurements, and charging-discharging cycles on some chemically synthesized polypyrrole samples. Analyzing the response to the ac signal as a function of frequency, it appears that the capacitance effect can be described in terms of an ionic relaxation mechanism. A large part of the ions involved in the electrochemical doping exhibits a relatively short relaxation time, follow the low frequency ac signal, and are responsible for the capacitance effect. Another part of the doping ions appears to be deeply trapped in the polymer chain: they do not follow the ac signal, and do not participate in the capacitance effect. In this way we have identified two types of currents in the cyclic voltammetry: a "capacitive current" without hysteresis effect, and a "noncapacitive current" arising from the deeply trapped ions giving a large hysteresis responsible for the broadening of the reduction peak. From those results we have derived a model for the electrochemical doping assuming the existence of two types of ionic trapping sites in the polymer chains, and the capacitive effect is analyzed as a contribution to the doping process itself.

This article is the first one of a series devoted to the study of hydration processes in epoxy resins using infrared (FTIR) spectrometry, with a view to eventually determining the role of ambient H 2 O molecules in the degradation of these resins, particularly when they are exposed to irradiation. The starting experiment consists of recording spectra of thin films of these resins which allows us to propose a preliminary assignment of the bands of interest. The second experiment consists of heating these resins. As the main effect of a temperature elevation is to break hydrogen bonds, the analysis of the corresponding spectra enables us to more precisely assign bands which will be of importance in the hydration process. The third experiment consists of hydrating these resins in a humid atmosphere. The pressure of the water vapor of this atmosphere is directly measured in the IR spectra where the corresponding, extremely narrow bands are easily isolated. The analysis of the spectra obtained, after subtraction of these bands due to water vapor, allows us to determine the interaction configurations of H 2 O molecules which are embedded in the resins, that is, the various ways they establish or accept hydrogen bonds to or from hydrophilic groups.

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