Moisture Effects in the Conducting Polymer, Polyaniline
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MOISTURE EFFECTS IN THE CONDUCTING POLYMER, POLYANILINE
J.P. TRAVERS, C. MENARDO and M. NECHTSCHEIN DRF/SPh/DSPE (UPR CNRS 216), Centre d'Etudes Nucldaires de Grenoble, 85 X, 38041 Grenoble-Cddex (FRANCE) S.K. MANOHAR and A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323 (USA) ABSTRACT Water content as well as conductivity have been measured as a function of water vapour pressure on polyaniline and poly-N-methylaniline samples. It is shown that i) absorption is governed by the acid protons of the polymers and ii) enhancement of conductivity needs the existence of protonated imines. It is concluded that moisture favours conductivity through an increase of the interchain transfers. INTRODUCTION Among conducting polymers, polyanilines (PANI) have attracted a considerable interest during the last five years. This class of electroactive polymers offer the unique situation in which the conductivity depends upon both the oxidation state, as in standard conducting polymers, and the protonation level [1-3]. In particular, number of works have been devoted to the study of the Insulator-toConductor transition which takes place on the emeraldine form upon protonation, while the oxidation level is held constant. Several results have been obtained which support the assumption of a phase segregation mechanism : protonation proceeds via the formation of fully protonated conducting islands embedded in an unprotonated insulating matrix [4-5]. Spin dynamics studies have suggested that conducting islands could consist of only one polymer chain and that the DC conductivity could be controlled by the interchain hoppings [6-7]. In addition, to oxidation and protonation, hydration has been shown also to control the transport properties (8]. For instance the conductivity can be increased by an order of magnitude when water is absorbed in the polymer. Several assumptions have been proposed to explain the role of moisture, invoking either a granular scale mechanism [9]- the role of water would consist of decreasing the insulating barriers between conducting grains- or a microscopic mechanism [10-11]- involving a role of the acid protons at the polymer chain level-. In this paper, we present a comparative study of the water absorption process, and the induced effects on the conductivity, in the parent polyaniline and the substituted poly-N-methylaniline (PNMA). The aim of the experiments was to follow the behaviour of conductivity variation versus moisture content with respect to a modification of the acid-base properties of the polymer. We, first, report experimental determination of water absorption isotherms and we establish a relation between the water absorption and the protonation level, which holds for both coumpounds. Then, we present conductivity measurements upon hydration. Finally, the different models are discussed to the light of the present results.
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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EXPERIMENTAL PANI samples were chemically prepared using
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