In situ Raman spectroscopy study of the oxidative polymerization of aniline in media of different acidity

In situ Raman spectroscopy was used for the first time to study the reaction intermediates/products of the oxidative polymerization of aniline to polyaniline (PANI) with peroxydisulfate in aqueous solutions of different initial acidity. Raman spectra were recorded for the drops of reaction mixture taken at various reaction times. Five reaction systems with different pH of the monomer solution (pHmon), leading to 1-D nanostructured PANIs (pHmon =5.8, 4.6 and 4.5) and granular PANIs (pHmon= 3.4 and 1.0) were explored. Spectral characteristics of different reaction phases - athermal, exothermic and post-propagation were identified and correlated with the proposed mechanisms of aniline polymerization and nanostructures formation. For the nanostructured PANIs, the athermal phase was characterized by pronounced strengthening of the delta(N- H)/v(C--N)Q band at 1510-1520 cm-1, while the characteristic of the second exothermic phase was the strengthening of the v(C-C)B/ v(C-C)Phz band at c.a. 1636 cm-1 which becomes stronger than the v(C--C)Q/v(C-C)SQ band at c.a. 1590 cm-1. For granular PANIs, the band at 1590 cm-1 was stronger than that at 1630 cm-1 for all reaction phases. For all reaction systems, characteristic feature of the post-propagation phase was strengthening of the v(C-N+center dot) band at 1330-1340 cm-1 due to delocalized polarons. For granular PANIs, the band associated with phenazine-like units at c.a. 1415 cm-1 disappeared after the exothermic phase, while for nanostructured PANIs it was seen until the end of the reaction. The obtained results are expected to be useful in further investigations of the intermediates/ products and mechanisms of polymerizations of other aromatic amines.