A solid state NMR characterization of cross-linked polyaniline powder

A detailed structural characterization of annealed polyaniline (PANI) powder samples has been carried out by using solid state NMR (SSNMR) and FT-IR. Comparing annealed PANI with PANI that has been chemically reduced to the leucoemeraldine base (LB) form, the 13C and 15N SSNMR and FT-IR data clearly show the conversion of the quinoid rings to benzenoid rings upon heating at ∼200 °C in vacuum. Combining interrupted-proton decoupling with 15N SSNMR reveals the presence of tertiary amine nitrogens generated from the cross-linking of the annealed polymer. The tertiary amine sites occur upon new bond formation between quinoid rings of one chain with the imine site of an adjacent chain, yielding a N,N′-diphenylphenazine structure at the site of cross-linking. The NMR data also shows that upon heating cross-linking is the predominant reaction occurring in PANI powder and that it follows a single mechanism. Measurement of the NMR relaxation parameter also shows that the rate of chain dynamics of the polymer is modified upon cross-linking.