On the Nature of Heterogeneity in Vacuum Deposited Polyaniline Films
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construct working circuits. In this respect vacuum thermal evaporation is of particular interest. Earlier studies [13,14] revealed that thin films can be produced also by this method, and many questions arose concerning the nature of the layers obtained. Further investigations have substantiated the complexity of this problem [ 15,16,17,18,19,20,21,22]. For example, when either emeraldine salt or base were used as a starting material XPS studies reveal almost fully reduced state of PAn in the deposited films. It is of value that the thickness of these films > 0.1 rLm. According to our data [22] formation of leucoemeraldine-like form during evaporation is due to the hydrolysis of quinone-imine fragments by strongly bonded water resulted in formation of appropriate amine- and oxygen-containing functional groups. Molecular weight and other properties of the thermally deposited PAn depend strictly on the temperature range of evaporation. The most PAn-like material (MW - 1500 D) was obtained in the temperature range 275-325°C. Immediately after evaporation the deposited films (in contrast to leucoemeraldine) may be easily dissolved in many common organic solvents like ethanol, benzene, acetone etc. Optical absorption spectra also indicate some changes in the electronic structure, as they are not identical to those of leucoemeraldine [16]. In addition, the films are characterized by the absence of ESR signal and poor conductivity (-10-7 S/cm) [17]. According to our data, regeneration of PAn-like properties may be achieved by the cyclic acid-base treatment of the as-deposited films on air [16]. After this treatment electronic spectra, ESR data and cyclic voltammetric curves of the vacuum evaporated PAn are the same as these ones for common PAns. Furthermore, as a 221 Mat. Res. Soc. Symp. Proc. Vol. 600 @ 2000 Materials Research Society
result of such a treatment the vacuum evaporated PAn also loses the solubility in common organic solvents. The latter fact was attributed to the formation of three-dimensional intermolecular aggregates of donor-acceptor type by their nature. Nevertheless, the electrical conductivity of these films (-10-4 S/cm [17]) was much less than that for common PAns (102 S/cm and higher). It should be particularly emphasized that the conductivity of very thin (< 100 A) acid doped vacuum deposited PAn films evidently may be very high concidering plasmons exist in the far-IR [21 ]. So, it is reasonable to assume that the local structure in microscopic scale for vacuum deposited films, in general, is identical with this one for common PAn, whereas character of self-organization on higher structural levels is not. EXPERIMENTAL Emeraldine salt powder was prepared by chemical oxidation by ammonium persulfate [15]. Then the salt was treated by concentrated ammonia solution to obtain emeraldine base. PAn
films were deposited on polished glass substrates in the course of vacuum thermal evaporation of the emeraldine base at 275-325 'C. The fractions having lower evaporation temperature were preliminary sublim
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