SYNTHESIS AND CHARACTERIZATION OF POLYANILINES AND POLYANILINE-ACCEPTOR COMPLEXES

Abstract

Two oxidation states of polyaniline were studied. They correspond to the intermediate oxidized state or emeraldine (EM) and the completely reduced state. or leucoemeraldine (LM). Initially, the chemical polymerization of aniline under different synthesis conditions reported in the literature was carried out. The characteristics and physicochemical properties of the resulting electroactive polymers were subjected to a comparative study by elemental analysis, thermogravimetric analysis, infra-red measurements and electrical conductivity measurements. The synthesis conditions, such as the oxidizing agents, the preparation media, the type and concentration of protonic acids and the washing and drying conditions were found to cause certain degree of variations in the structure and properties of the resulting polymers. When EM hydrochloride was dried at 105°C, its electrical conductivity dropped by two orders of magnitude, probably due to the loss of absorbed moisture. In thermogravimetric analysis, EM base and EM salt of several acids lost their absorbed moisture when heated from 30°C to 100°C. Acids in the EM salts were probably lost above about 200°C. Next, EM base was reacted separately with six proton-free organic electron acceptors and the electrical and physicochemical properties of the resulting complexes were studied. The complexing ability of the acceptor, based on the maximum attainable electrical conductivity of the resulting complex, could be arranged in the following ascending order: tetrachloro-p-benzoquinone (p-chloranil), tetrafluoro-p-benzoquinone (p-fluoranil), tetrabromo-obenzoquinone (o-bromanil), 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), tetrachloro-o-benzoquinone (o-chloranil) and tetracyanoethylene (TCNE). Electrical conductivity in the order of 10-1 S/cm was achieved in the EM/TCNE complex. Interactions of EM base with TCNE and DDQ probably resulted in the formation of cyano radical anions and some of these anions could be covalently bonded to the polymer backbone. X-ray photoelectron spectroscopy (XPS) results suggested the presence of halide ions and halobenzoguinone anions in the EM/halobenzoguinone complexes. The interactions of EM with the halobenzoquinones was found to be analogous to that of protonation, with the charge transfer interactions occuring preferentially at the imine repeating units of the EM base. Two possible structures for the EM/halobenzoquinone complexes were deduced from the experimental evidence. The stability, acid protonation and oxidative doping of LM were then studied. XPS and other analytical results suggested that the structures of the LM hydrochloride and LM chlorine complexes appeared to resemble that of the partially protonated EM hydrochloride. In the LM/ochloranil and LM/o-bromanil complexes, the acceptors first reacted with the amine units of LM to form some oxidized imine units and positively charged nitrogen species. The imine units formed in the first step could react subsequently with the acceptor molecules at high acceptor concentrations. Finally, some halogen substituted anilines were also polymerized according to the synthesis method for EM salts. The effect of the type and position of halogen substituents on the electrical conductivity and structure of the resulting polymer was studied. However, the electrical conductivity of these halogen substituted polyanilines was found to be only of the order of 10-7 S/cm or less.