Does the susceptibility of polyaniline proof the metallic properties?

Does the susceptibility of polyaniline proof the metallic properties?

Polyaniline (PANI) is the oldest artificial polymer which is now known to be electrically conducting. It was first produced by Runge in 1834 [1] and investigated in more detail by Fritzsche in 1840 [2]. From that time on there was a lot of interest in structural and chemical properties of PANI named aniline black because it was widely used as a dye. More than 100 years later this polymer has attracted much attention due to its very high electrical conductivity [3] among organic compounds. Up to now interesting new results on PANI are continuously published.

The emeraldine form is the partly oxidised form of PANI (cf. Fig.1) where six benzene rings and two quinoid rings are present in an eight-ring repeating unit. The leucoemeraldine form is the completely reduced form of PANI. This form contains only benzene rings in the polymer chain (cf. Fig.1). Due to the lack of charge carriers this form of PANI is an insulator. There are different types of doping of conducting polymers. In the case of PANI usually the so called acid or non redox doping is widely used [10, 11]. In this case the oxidation state of the polymer is not changed despite the oxidation state of single atoms can change. Sometimes this doping mechanism is explained by an internal redox reaction [12] or a proton induced spin unpairing [13].

Figure 1
Three forms of investigated PANI. From top to bottom leuco emeraldine form, protoemeraldine form and emeraldine form. (The position of the quinoid groups is not established.)

The charge carriers produced upon doping of PANI are 1.) single charged polarons with spin ½, which are the origin of the magnetic susceptibility and 2.) double charged bipolarons which are spinless. The temperature dependence of the magnetic susceptibility of PANI was often described to be
a mixture of Curie and Pauli paramagnetism [14,15]. Carefully prepared samples of PANI doped with campher sulfonic acid (CSA) show a temperature independent magnetic susceptibility [16 - 20]. In the cited papers the temperature dependence and the low number of spins is explained by the metallic behavior of PANI. Considering this polymer from a chemical point of view, redox reactions have to be taken into account. It is to be stated that
the leucoemeraldine form of PANI can be easily oxidised by oxygen [21]. This way to produce charge carriers is to be taken into account for the explanation of the electronic state of PANI.

On the other hand the oxidised forms of PANI can be reduced. Especially redox reactions of oligomers of PANI are well investigated [22, 23]. The existence of redox equilibria in PANI is unequivocal [24 - 30]. Interpreting temperature dependent magnetic properties of PANI we have to consider the temperature dependence of the equilibrium constant of the redox reaction. The temperature dependence of the magnetic susceptibility of PANI is explained in this study on the basis of a redox equilibrium.

The content of quinoid rings in the three types of PANI referred in this paper can be drawn from Fig. 2. The absorption spectra taken from diluted solutions of the appropriate form of PANI indicate that the reduction of PANI with hydrazine results in the leucoemeraldin form. The remaining small absorption at 600 nm can not be further reduced by a prolongation of the reduction time or increasing the excess of hydrazine what is similar to [22, 31]. After access of the oxygen from the air, this absorption peak grows quickly due to oxidation of the polymer.

Figure 2
UV-Vis spectra of the emeraldine form, protoemeraldine form and leukoemeraldine form of PANI. The concentration of the solutions was appr. 4*10-4 mol/l in DMSO.

The EPR measurements were done at solid samples. The solvent was removed by holding the sample in high vacuum at elevated temperature and the access of oxygen was avoided by sealing the EPR tubes.

For the emeraldine form of PANI a spin concentration of 2.3*1019 spins /g was determined which corresponds to a susceptibility of 0.9 * 10-5 emu/(mole 2 rings). This value is in good accordance to [17] where an average value of 2 * 10-5 emu/(mole 2 rings) was found. The protoemeraldine form of PANI which has a lower content of quinoid rings has a higher spin concentration of 7.2*1019 spins/g which corresponds to a susceptibility of 2.8 * 10-5 emu/(mole 2 rings). This increase is well known from electrochemical doping experiments [26,27]. The highest concentration of spins is reached at relatively low oxidation stages of PANI.

The leucoemeraldine form of PANI should have no spins in the ideal case. In fact we measure a spin concentration of 5.7*1018 spins/g (0.22 * 10-5 emu /(mole 2 rings) which results from an oxidation to a very small extent during washing the polymer with water because the preparation of the polymer was not done in the glove box. Although no spins are expected for the pure leucoemeraldine form we used these spins to measure the temperature dependence of
the susceptibility by EPR. Fig. 3 shows the temperature dependence of the magnetic susceptibility and the curve expected for pure Curie magnetism. The calculation of this curve is based on a temperature independent number of spins. The value was that of the protoemeraldine form of PANI at room temperature.

Figure 3
Temperature dependence of the magnetic susceptibility of emeraldine – rhombi, protoemeraldine – squares and leucoemeraldine – lower triangles. The upper triangles show the theoretical curve for a fixed number of spins with Curie behavior.

From the literature [26 - 28] and from our own earlier investigations it is known that at least three different species exist in the doped PANI. A) Completely reduced polymer segments, which are diamagnetic. B) The singly oxidised form of a polymer segment, which has spin ½ and C) the doubly oxidised form, which is diamagnetic. The singly oxidised polymer segments are usually treated as polarons and the doubly oxidised form as bipolarons [32] or polaron pairs [33]. The difference between bipolarons and polaron pairs does not influence our consideration. From a chemical point of view the polaron is a mobile radical cation and the bipolaron is a spinless dication with a chinone diimine structure. For simplicity we will use the physical nomenclature for our further consideration. There is no doubt that the protonated bipolaron is able to form polarons. But the reaction mechanism is a matter of discussion. Usually the already mentioned internal redox reaction is assumed. Here we claim that the polarons can also be formed by a simple redox reaction between bipolaron and neutral polymer segments which is also assumed in [28]. This redox concept is quite common in organic chemistry and is also supported by mechanism of polymer growth were the double oxidised polymer reacts with monomer units [34, 35]. Taking such a reaction into consideration we have also to consider the influence of the temperature on this reaction. A simplified treatment of that problem is given next. First we have to take into account the equilibrium between bipolarons B, neutral segments N and polarons P.

 (1) 

For this reaction the equilibrium constant is:

(2) 

For that equilibrium constant the temperature dependence is given by

 (3)

Substituting   K  by equation (2)  we obtain: 

 (4)

In our case the concentration of bipolarons and neutral segmentsis nearly unchanged but the concentration of the polarons is changed
by a factor of 2. That means the natural logarithm of the polaron concentration versus the inverse temperature (see Fig. 4) should give a straight line

with a constant slope and an intercept containing  

but depending on the product of the concentration of the bipolarons and the neutral segments, which can be considered as  nearly constant for the emeraldine and protoemeraldine form of PANI. In the case of the leucoemeraldine form of PANI a little change of this product is considered to be the reason for a slightly increased slope.

Figure 4
Temperature dependence of the spin concentration of the emeraldine form - rhombs, protoemeraldine form – squares and leucoemeraldine form of PANI - triangles

A hint that the chemical explanation might be better than the metallic can be drawn from conductivity measurements. The conductivity of the emeraldine form of PANI was about 5 Scm^-1, that of the protoemeraldine form of PANI 2 Scm^-1. The conductivity of the leucoemeraldine form of PANI was less than  10^-4 Scm^-1. This low conductivity let us to draw the conclusion that the sample in an insulator or at least a semiconductor but not a metal. Therefore the metallic state is not necessarily the reason for the temperature dependent spin concentration in PANI since it can be also understood  by considering a chemical equilibrium between different oxidation states of the polymer units in dependence on temperature.

This result should remind us that a hypothesis can not be proven by an experiment and that we have to search always for alternative explanations.

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Prof. Lothar Dunsch