Electrical transport and EPR investigations: A comparative study for d.c. conduction mechanism in monovalent and multivalent ions doped polyaniline

Abstract

A detailed comparative study of electron paramagnetic resonance (EPR) in conjunction with d.c. electrical conductivity has been undertaken to know about the charge transport mechanism in polyaniline (PANI) doped with monovalent and multivalent protonic acids. This work is in continuation of our previous work for further understanding the conduction mechanism in conducting polymers. The results reveal that the polarons and bipolarons are the main charge carriers formed during doping process and these cause increase in electrical conductivity not only by increase in their concentration but also because of their enhanced mobility due to increased inter-chain transport in polyaniline at high doping levels. EPR line asymmetry having Dysonian line shape for highly doped samples shows a marked deviation of amplitudes A/B ratio from values close to one to much high values as usually observed in metals, thereby support the idea of high conductivity at higher doping levels. The nature of dopant ions and their doping levels control the charge carriers concentration as well as electrical conductivity of polyaniline. The electrical conductivity has also been studied as a function of temperature to know the thermally assisted transport process of these charge carriers at different doping levels which has been found to follow the Mott's variable range hopping (VRH) conduction model for all the three dopants used. The charge carriers show a change over from 3D VRH to quasi 1D VRH hopping process for multivalent ions at higher doping levels whereas 1D VRH has been followed by monovalent ion for full doping range. These studies collectively give evidence of inter-chain percolation at higher doping levels causing increase in effective mobility of the charge carriers which mainly seems to govern the electrical conduction behaviour in this system.

Mechanism of direct current electrical charge conduction in p-toluenesulfonate doped polypyrrole/carbon composites

“…The values of N(E F ) lies in the range of 10 20 À10 21 cm À3 eV À1 for all the PPy/C composite samples and are in good agreement with the values reported earlier for other p-conjugated systems. 17,19,20,24,[27][28][29][30][31][32] The present results are consistent with the Mott requirement that aR > 1 and W > k B T for conductivity by hopping of charge carriers to the distant sites. The estimated values of Mott's parameters and temperature dependent activation energy suggest that Mott's 3D-VRH conduction model is appropriate for explaining the mechanism of dc electrical charge transport behavior of PPy/C composite samples.…”

“…The observed trend in r dc vs. 1000/T appears similar to those reported earlier for other conjugated polymers. 17,19,20,24,[27][28][29][30][31][32] The dc conductivity r dc (Fig. 7) can be given 13,45 by the following relation:…”

“…It is evident from ESR investigations that ESR line shape for all the PPy/C composite samples is of Lorentzian type and almost symmetric indicating thereby that the oxygen attack on these samples is negligible. 44 A single Lorentzian ESR line shape having no Dysonian asymmetry 24 rules out the possibility of formation of metallic islands thus eliminating the probability of FIT conduction 16 in these PPy/C composite samples. 23,46,47 However, before the application of Mott's VRH conduction model 13 to explain the dc conductivity results, it was thought worthwhile to examine the temperature dependence of activation energy.…”

“…15 The other two models, widely used to explain the dc conductivity of conjugated polymeric systems, are fluctuation induced tunneling (FIT) 16 and Mott's variable range hopping (VRH) conduction. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In FIT conduction model, 16 the tiny conducting domains, formed during doping in a granular system, participates in the delocalization of charge carriers by tunneling between conducting metallic islands. However, the macroscopic conductivity in most of the conjugated polymeric systems is assumed to result from the hopping conduction process.…”

“…35 Several studies on dc conduction of different conducting polymers and their composite systems are available in literature. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] However, there is no report available on temperature dependence of dc electrical conductivity in pTS doped PPy/C composites.…”