Effect of thermal processing conditions on the intrinsic oxidation states and mechanical properties of polyaniline films

Emeraldine (EM) film prepared from concentrated N-methylpyrrolidinone (NMP) solution gel at 150 °C in air (EM-150) has a significantly higher mechanical strength than that of the EM film prepared from dilute NMP solution at room temperature (EM-25). The nigraniline (NA) films prepared from oxidation of either EM-25 or EM-150, or the leucoemeraldine (LM) film prepared from reduction of EM-150, suffer a small loss in mechanical strength. The LM film prepared from reduction of EM-25, however, lacks any appreciable mechanical strength, although its mechanical strength is substantially enhanced when heated to 200 °C. The mechanical strengths of all the other films increase only slightly when exposed to temperatures above room temperature and up to 200 °C. X-ray photoelectron spectroscopy (XPS) results suggest that the intrinsic oxidation state ([=N−] / [−NH−] ratios) for both types of NA films decreases steadily when heated above room temperature. For EM-25, the intrinsic oxidation state increases slightly when heated to about 100 °C before showing a decrease at higher temperatures. The intrinsic oxidation state of EM-150 does not vary significantly with temperature up to 200 °C. However, the intrinsic oxidation states of both types of LM films increase steadily when heated above room temperature. After exposure at 300 °C, all films have about the same [=N−]/[−NH−] ratio and have lost all mechanical strength. The effect of intrinsic oxidation state of polyaniline (PAN) on its mechanical properties is discussed.