Synthesis of polyaniline films by plasma polymerization

Abstract

This work presents a study on the formation of polyaniline films by plasma polymerization using RF glow discharges with resistive coupling between stainless-steel electrodes. The polymer was obtained at a frequency of 13.5 MHz and pressure in the range (2–8)×10⁻² Torr. Polyaniline and I₂-doped polyaniline films were synthesized as thin films that can be adhered to glass and metal surfaces. The IR analysis shows the characteristic polyaniline peaks with evidence that some benzene rings are broken by the discharge energy. The thermogravimetric analysis shows an increase in polymer decomposition as a function of the time of synthesis. This effect, the physical characteristics and solubility of the film suggest crosslinking in the polymers due to the continuous impact of the electronic rain. The way in which the relative humidity affects the electric conductivity of plasma polyaniline films was established. At room temperature and relative humidity of 40% the conductivity was between 10⁻¹⁰ and 10⁻¹² S/cm. When the films were synthesized with I₂, the electric conductivity ranged from 10⁻⁴ to 10⁻¹¹ S/cm with relative humidity from 40 to 90%.

Introduction

Aniline is considered as a starting monomer to produce conducting polymers, as homopolymer or by doping with anions. Polyaniline (PAn) is usually obtained in solution by direct oxidation of aniline or by electrochemical oxidation [1]. This process is used to obtain linear PAn and sometimes is combined with simultaneous doping to increase the conductivity. Linear PAn is an electric insulating polymer but can be converted to a semiconductor by doping the compound with halogens and other anions. Crosslinked PAn shows similar behavior with lower values of conductivity.

Films of PAn are difficult to synthesize by conventional chemical methods because they form powder when recovered from the starting acid solution. Some PAn films over insulating substrates have been prepared by chemical oxidation using very dilute aqueous solutions of aniline [2]. Thin films of around 1 μm in thickness of chemically synthesized PAn have also been prepared by thermal and/or vacuum evaporation of the polymer powder [3]. Paloheimo et al. [4]recently used the layer-by-layer self-assembly method for fabricating thin films of PAn.

Another method of PAn film synthesis less commonly used is by plasma polymerization. In this method, the reaction begins in the gas phase and, as the polymer grows, goes to the nearest wall, where the film is formed. The chemical reactions are promoted by excitation and/or ionization produced by collisions of the monomer molecules with the electrons immersed in a plasma generated by electric discharges. The best suited plasma for polymerization is the glow discharge. In this process the polymerization occurs without the presence of other reagents, such as the oxidant agent and the solvent in the liquid phase, giving a product without major contamination. The reactions can also be combined with other compounds looking for specific characteristics in the final polymer. There is not much information about PAn synthesized with this technique. Bhat and Joshi [5]synthesized PAn by plasma polymerization using the RF inductive coupling mechanism. The plasma polymerization method has also been used to synthesize other conducting polymers 6, 7, 8.

Fig. 1 shows the four idealized oxidation states of PAn: leucoemeraldine, emeraldine, pernigraniline and emeraldine salt. Different structures result in different electrical behaviors of the material. Emeraldine salt is a partially oxidized compound, protonated, with electrical conducting characteristics. Leucoemeraldine is a fully reduced compound with electrical insulating characteristics. There are no double bonds between the aromatic rings and the N–H groups. Emeraldine base is an insulating compound, partially oxidized with few N–H groups in the main chain. Emeraldine changes from insulator to conductor when it is protonated with proton donor acids, such as HCl. This change is one of the most interesting properties of PAns. The structure of emeraldine PAn can be changed to emeraldine salt by removing an electron from the N–H group. Pernigraniline is a fully oxidized compound without conducting characteristics. There are no N–H groups in the structure. The level of protonation in the structure causes dramatic changes in the conductivity.

Another factor that affects conductivity in doped PAns is moisture. It is known that moisture increases the conductivity in doped PAn by changing the electrostatic interaction between anions and positive charges. The increment in conductivity reported for PAn doped with HCl or camphor sulfonic acid under moisture is around one order of magnitude [9].

In this work we present the process to obtain PAn films and in situ I₂-doped PAn films by plasma polymerization using RF glow discharges with the resistive coupling mechanism. We study the influence of the dopant, the synthesis parameters and the relative humidity on the electric PAn properties. Both films, undoped and doped PAns, are studied by thermogravimetry, IR spectroscopy and scanning electronic microscopy to determine the structure and the oxidation states of plasma PAns.