The stability of polyaniline in strongly alkaline or acidic aqueous media

Polyaniline (PANI) base has been suspended in 9 M potassium hydroxide at 20 degrees C or 90 degrees C for various time intervals extending to 4 months. The fraction of acetone-soluble material increased from 1.2 wt.% to 4.5 wt.% after exposure to an alkaline medium for 60 days at 20 degrees C. Gel-permeation chromatography indicates that the aggregation of PANI is reduced, while the chain degradation itself is negligible. FTIR spectroscopy confirms this trend and the absence of hydrolytic changes in the PANI structure. Polyanifine retains the ability to be reprotonated with a I M sulfuric acid to a conducting form. No marked changes in the molecular structure have been found, even after suspension of PANI in 9 M KOH at 90 degrees C for 60 days. Similar immersion of PANI salt in 5 M sulfuric acid at 20 degrees C was responsible for changes in the protonation, and the mass increased by 11 wt.%. This was explained by the exchange of the original sulfate or chloride counter-ions for hydrogen sulfate anions or by the protonation of secondary amine sites in PANI in addition to imine ones. The changes in the molecular structure are discussed on the basis of FTIR spectra. The conductivity decreased from 1.2 S cm(-1) to similar to 10(-3) S cm(-1) but no time-dependence of conductivity was observed. There was no fraction of PANI soluble in acetone. PANI in the protonated state is thus stable also in the strongly acidic medium. The study is supplemented by the assessment of the thermal stability of PANI base, which is of importance for the processing of PANI. Loss of moisture has been observed after exposure to 250 degrees C for 10 h in both nitrogen atmosphere and in air. Good stability was found at 350 degrees C only in the nitrogen atmosphere, while a marked mass loss in weight was registered in air. (C) 2008 Elsevier Ltd. All rights reserved.