Synthesis of alkyl and N-alkyl-substituted polyanilines: A study on their spectral properties and thermal stability

Polyaniline (PANI), N-methyl- and N-ethyl-PANI, 2- and 3-ethyl-PANI as well as 2,6-dimethyl- and 3,5-dimethyl-PANI have been synthesized using ammonium persulfate as catalyst. These polymers have been studied in the undoped and doped state by FT-IR and electronic spectroscopy in the range from the UV–VIS to the NIR portion of the spectrum. As doping agent camphorsulfonic acid was used. The chemical structures of the N-methyl- and N-ethyl-PANI have been found to contain quinoneimine units along the polymer chain. In the doped state the N-alkyl-PANIs show the polaronic band transition in the NIR part of the spectrum, at longer wavelength than doped unsubstituted PANI. A similar but less pronounced phenomenon is observed in the case of ethyl-substituted phenyl rings of PANI. Dimethyl-substitution at the phenyl ring of PANI hinders the formation of high molecular weight polymer. Only oligomers are formed. The thermal stability of unsubstituted PANI in the undoped state is very high and N-alkyl substitution and phenyl ring substitution lowers the thermal stability of PANI. Doped samples show a significantly worse thermal resistance in comparison to the corresponding undoped samples, this is due essentially to the volatility and the decomposition of the protonic acid used as dopant. Good general agreement has been found between the predicted thermal stability on the basis of increment groups calculations and the experimental results. The thermal decomposition of unsubstituted and undoped PANI has been followed by FT-IR spectroscopy.