Investigation of the conduction properties of phosphoric and sulfuric aciddoped polybenzimidazole

When complexed with a strong inorganic acid such as sulfuric or phosphoric acid, films of polybenzimidazole (PBI) show two types of behaviour, dependi ng on the time of immersion in the corresponding acid bath. The first type, prepared at shorter doping times, has conductivity in the range 10(-5)-10( -4) S cm(-1), whilst the second, of conductivity > 10(-3) S cm(-1) is forme d after more prolonged immersion. The 'switch-over' from one state to the o ther is at 10-11 h in H3PO4, and 2-3 h in H2SO4. PBI has a remarkable capac ity to concentrate H3PO4 and, even in an acid bath of concentration 3 mol d m(-3), the acid concentration within a PBI membrane is ca. 14.5 mol dm(-3). IR spectroscopy performed on hydrated PBI membranes as a function of tempe rature, and on acid-complexed membranes as a function of the amount of sorb ed acid confirms proton transfer from H3PO4 to the imino groups of PBI and, at high doping levels, the presence of undissociated H3PO4.