engleski

EPR study of triplet states in conductive polyaniline

The magnetic properties of the conductive polymer polyaniline (PANI) and its deriva-tives have been studied extensively during the past decades. These properties are rele-vant for a better understanding of the nature of the charge carriers in the polymer struc-ture. For example, most of the studied PANI derivatives showed a nearly linear temperature dependence of the magnetic susceptibility multiplied by temperature which was attributed to disorder-induced localised polaron pairs (radical cation with S=1/2). A further study of the magnetic susceptibility suggests the coexistence of polarons and spinless bipolarons and the possible formation of bipolarons upon changing the temperature or doping level. A slight deviation from the linear dependence of the magnetic susceptibility multiplied by temperature vs temperature usually was detected by SQUID at low temperature intervals, at T < 10K, for various PANI samples. In this low temperature region, the presence of magnetic field dependence of the magnetic moment was also noted. Recently, both of these magnetic properties were described by employing a “triplet’’ model which used the distribution of the singlet-triplet splitting (E) with the density distribution function having a narrow peak near E=0. The present study was undertaken to look for the presence of such local triplet states in two types of PANI samples in order to more closely examine the nature of their distribution in these samples. It was expected that a characteristic EPR spectrum could be found and attributed to the excited triplet state with corresponding energy at low temperatures. The two types of PANI emeraldine salt samples investigat-ed were the commercial “PANI K” (average Mw > 15, 000, from Aldrich) and “PANI 1” produced enzymatically (with horseradish peroxidase) in aqueous solution by employing AOT vesicles as templates. Evidence for the presence of thermally activated states was obtained by studying the temperature dependence of EPR spectral intensities and line widths in the low temperature region. The obtained activation energies for the thermally activated process are discussed in terms of expected energies for single-triplet transitions and possible exchange interactions. The obtained exchange interactions are correlated with different distributions of the polaron pairs in the PANI samples.

EPR; polyaniline; condactive polymer

nije evidentirano