SELF-ASSEMBLY OF THE ELECTROACTIVE COMPLEXES OF POLYANILINE AND SURFACTANT

An electroactive material with remarkable solubility, processibility as well as mechanical properties has been developed by complexation (thermal doping) of polyaniline (PANi) emeraldine base with dodecylbenzenesulfonic acid (DBSA) in the solid state. Isothermal treatment of such a mixture was found to promote the complex formation. Optimum conditions of complexation were established with respect to the formation of layered structure, electrical conductivity and solubility. The optimal temperature for the doping process was found to be in a range of 100-120 degrees C while the best ratio of DBSA to PANi was between 3:1 and 4:1 by weight, a nearly stoichiometric equivalence of aniline repeat units and DBSA molecules. The time of isothermal treatment should be controlled within 30 min. Thermal doping induced orientation to polymer chains in a layered structure, whereby the hydrophobic tails of the surfactants function as spacers between parallel stacks of the main chains. This anisotropy was achieved by the self-assembly during the thermal doping rather than ordinary drawing or stretching of the polymers. A unique liquid crystalline mesophase with a smectic-like optical texture was observed for the soluble portions of some specimens. The excess DBSA in the samples is considered to function as a solvent and to give rise to the liquid crystalline fluidity of the phase. The scanning tunneling microscopy (STM) image 5000 x 5000 Angstrom on a submicrometer scale obtained from a PANi/DBSA thin film exhibits a surface morphology with a granular size of 200-300 Angstrom. The image of 150 x 150 Angstrom on a molecular scale obtained from multilayer PANi/DBSA deposited on a highly oriented pyrolytic graphite (HOPG) surface provides a direct observation of a self-assembled structure and close layer packing of the polymer backbone with dimensions in accord with the results found by X-ray diffraction. Our results indicate that the thermal doping process of polyaniline by DBSA offers new possibilities to obtain optimal structures through a self-assembly.