Chemical and Physical Properties of Soluble Part of Emeraldine: Comparison with Insoluble Emeraldine
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CHEMICAL AND PHYSICAL PROPERTIES OF SOLUBLE PART OF EMERALDINE: COMPARISON WITH INSOLUBLE EMERALDINE. R. LAVERSANNE* and P. VACAt *Centre de Recherche Paul Pascal, CNRS, Chiteau Brivazac, F-33600 Pessac FRANCE kLaboratoire CPMOH, Universitd de Bordeaux I, F-33405 Talence FRANCE
ABSTRACT A preliminary physical study of the soluble part of emeraldine is presented. The electrical and magnetic properties of this low weight material are very similar of those of the pristine emeraldine salt. These results emphazise the possible role of short chain species in the conduction mechanism of polyaniline. In order to clarify this point, previous magnetic investigations on insoluble emeraldine salts are revisited. It appears that the fit of the magnetic susceptibility temperature dependence of polyaniline with a Curie law plus a Pauli term might not be justified at low temperature. A T-a law with ca= 0.75 gives a better fit suggesting a behavior similar to the one found in Random Exchange Heisenberg Antiferromagnetic Chain (REHAC).
INTRODUCTION Among conducting polymers, Polyaniline (PANI) has been of special interest because of its ability to encover insulator to conductor transition upon protonation [1, 21. The partly oxidized form of PANI called emeraldine [3], shows an electrical conductivity increasing from 10-10 Scm-1 in the base form up to 5 Scm- 1 for 50% protonation [1, 4]. Numerous studies have been performed to tentatively elucidate the conduction mechanism in this polymer [5]. Yet controversy is still alive upon the existence of metallic islands in emeraldine salts [6]. On the one hand the defenders of this picture have proposed that a Pauli susceptibility can be deduced from magnetic measurements [7]. Moreover studies on transport properties [8], where an exp(-(TTr0 )l/2) law is demonstrated for electrical conductivity, as well as microwave measurements [9] may be interpreted in agreement with this model. On the other hand the opponents to this model [10-12] give several arguments against a true metallic behavior. The electrical conductivity of emeraldine salt is too low compared to other polymers with genuine metallic conductivity, and can almost be reached with oligomers as short as eigth units [101. The temperature dependence of the electrical conductivity is understandable in terms of hopping of rather localized charges [111 and the disordered nature of this polymer should always be kept in mind when describing its properties [12]. Besides, in a recent study on a crystalline emeraldine salt [13], a Pauli susceptibility has been seen at high temperature from EPR measurements, while at lower temperature the susceptibility becomes temperature dependent. This has been interpreted as the onset of a homogeneous metallic behavior at high temperature, destroyed by disordered induced localization at low temperature. In the first part of this paper we describe an experimental study of the low weight species obtained as a soluble part of emeraldine base. Electrical conductivity and EPR are described at room temperature f
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