Raman spectroscopic study on the structural changes of polyaniline during heating and cooling processes

Polyaniline (PANI) in three ground states—emeraldine base (EB), leucoemeraldine base (LEB), and pernigraniline base (PNB)—were studied by Raman spectroscopy in the temperature scale of −195 to about 375°C. The Raman spectral results demonstrated that a crosslinking reaction occurred on EB chains at high temperatures, whereas the phenyl torsion angles of EB chains decreased during the cooling process. LEB was unstable and was gradually converted to EB during the heating process, although its chain conformation remained stable by cooling. The spectral change tendency of PNB was similar to that of EB during the heating process. Although the torsion angle of PNB also decreased with decreasing temperature, like that of EB, this behavior was hard to observe experimentally because of the relatively “free ” rotation of its phenyl rings.