Spectroscopic studies of chemically synthesised polyaniline and its ability to act as radical scavenger

Abstract

During the past almost three decades, conducting polymers have been the subject of intense scientific and industrial research and development worldwide. A general background of the fundamental principles and concepts of conducting polymers and their applications is presented in Chapter 1. In Chapter 2, the characterization of chemically synthesised emeraldine base polyaniline (EB-PANI), acetone-extracted to remove impurities, is examined. Spectroscopic studies PANI at different levels of oxidation are reported, and the structural changes in as-synthesised EB-PANI upon doping with HCl and iodine vapour are investigated. A maximum in the number of unpaired electron spins was found in 1M HCl doped PANI, corresponding to emeraldine salt PANI (ES-PANI), which indicates that bipolarons form at higher doping levels. Also, a maximum conductivity of 0.478 S cm-1 was observed for 1M HCl doped PANI, and no significant change in the conductivity was observed at higher doping levels. A new mechanism of iodine doping of EB-PANI was proposed, which occurs by the oxidation of the benzenoid diamine units, instead of the quinoid diimine units as previously proposed. In Chapter 3, the incorporation of planar Cu(II) complexes as dopants into ES-PANI was investigated by means of EPR and IR spectroscopy. A decrease in the intensity of the EPR signals from polarons in Cu(II) complex-doped PANI samples was observed, while the Cu(II) anion concentration increases and favors the formation of bipolarons in Cu(II) complex-doped PANI systems. The reactions of EB-PANI following different levels of reduction or HCl doping with CuCl2 aqueous solution are also investigated and new reaction mechanisms are proposed. The conductivities of the resulting samples have also been measured and discussed. The ability of aniline and PANI samples to act as antioxidants using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free-radical method are investigated by means of UV-VIS, FTIR and EPR spectroscopy in Chapter 4. A study of the kinetics of the reaction between DPPH and aniline showed that the reaction is first order with respect to DPPH and approximately second order with respect to aniline. A further investigation of the mechanism of the aniline/DPPH reaction and products of the reaction (aniline oligomers and DPPHH) by magnetic resonance techniques is discussed in Chapter 5. This study has resulted in the identification for the first time of uncapped dimers and trimers of aniline oligomers in solution by EPR spectroscopy. In Chapter 6, for the first time, the applicability of various 13C and 15N solid-state nuclear magnetic resonance (SSNMR) techniques in the investigation of the changes in the structure of PANI that occur upon reaction with DPPH radicals was investigated and discussed. The results indicate partial oxidation of PANI samples upon reaction with DPPH, which is consistent with previously obtained FTIR and EPR data in Chapter 4. In Chapter 7, some further research directions and projects involving chemically synthesised PANI are briefly described.