Emulsion polymerization process for organically soluble and electrically conducting polyaniline

An emulsion process has been developed for the direct synthesis of the emeraldine salt of polyaniline (PANI) that is soluble in organic solvents. The process entails formation of emulsion particles with a mean hydrodynamic diameter of 150 nm and consisting of a water-soluble organic solvent (e.g., 2-butoxyethanol), a water-insoluble organic acid (e.g., dinonylnaphthalenesulfonic acid), aniline, and water. Aniline is protonated by the organic acid to form a salt which partitions into the organic phase. As oxidant (ammonium peroxydisulfate) is added to the reaction mixture, PANI intermediates are formed in the organic phase. As the reaction proceeds, the emulsion flocculates, forming a two-phase system. The reaction features an induction period followed by an exothermic polymerization, at which time soluble PANI forms in the organic phase. The reaction progress is conveniently monitored by temperature, pH, and open circuit potential. When dinonylnaphthalenesulfonic acid (DNNSA) is employed as the organic acid, the resulting product is highly soluble in organic solvents such as xylene and toluene (not a dispersion), has high molecular weight (M_w > 22 000), and forms moderately conductive (10^-5 S/cm) films. We have also found that the conductivity of PANI-DNNSA films may be enhanced (up to 5 orders of magnitude) by treating the films with surfactants such as benzyltriethylammonium chloride (BTEAC) or low-molecular-weight alcohols and ketones such as methanol and acetone. Electron microscopy shows that the conductivity enhancement phenomenon observed upon treatment with surfactants is due to self-assembly of PANI-DNNSA molecules into an interconnected network morphology. In the case of alcohol or ketone treatment the film conductivity is enhanced due to extraction of excess dopant, densification of the polymer, and a concomitant increase in crystallinity.