Oxygen transport through electronically conductive polyanilines

Abstract

Emeraldine base of polyaniline was synthesized by a chemical oxidation polymerization technique. The resulting emeraldine base film was treated with 4M HCl, 1M NH4OH, and subsequently with varying dopant HCl concentrations. The oxygen and nitrogen permeabilities through the doped polyanilines decreased with their doping level, while their oxygen selectivity over nitrogen increased up to 12.2 when doped with 0.0150 M HCl solution. When doped with 0.0175 M HCl, the membrane selectivity is expected to be higher but immeasurable because of extremely low permeability of nitrogen. The origin of such high selectivity is explored in terms of the facilitated transport and free volume change upon doping. Because the polarons generated upon doping react with oxygen molecules specifically and reversibly, these polarons can act as oxygen carriers and thereby facilitate oxygen transport. It is, therefore, expected that the oxygen permeability increases with the polaron (carrier) concentration. However, permeability for oxygen in these materials decreased with the increase in polaron concentration. Instead, the permeability correlated well with the d-spacing or free volume, measured via x-ray diffraction. It is found that although facilitated oxygen transport may occur, its contribution to oxygen permeability is insignificant. The free volume change upon doping seems to play a major role in determining gas permeation.