Effect of chain aggregation on the conductivity and ESR spectra of polyaniline

Abstract

Solutions of polyaniline in m cresol with and without camphorsulfonic acid (CSA), as well as films cast from these solutions were studied by ESR spectroscopy at 133-423 K and by optical spectroscopy in the range λ = 350-1100 nm. An analysis of the optical and ESR spectra shows that in the solutions and films without CSA polyaniline is fully doped but the conductiv ity of these films is low (~10 -8 S cm -1 ; cf. 100 S cm -1 for the films with CSA). Compared with the CSA containing samples, the samples without CSA are characterized by broader ESR lines and higher contribution of the Curie spins to the magnetic susceptibility. These facts indicate a weak aggregation of polyaniline chains without CSA, which leads to low conductivity. A formula was proposed, which describes the temperature dependence of the polyaniline ESR linewidth and allows the interchain distance and the mobility of electrons moving along polymer chains to be determined. The conductivity of polyaniline films is affected by moderate heating (363-388 K) of the films and solutions from which the films were cast. It was found that the interchain distances correlate with the conductivity of the films and with the broadening of their ESR lines caused by the effect of O 2 .Polyaniline films cast from polyaniline solutions in m cresol containing camphorsulfonic acid (CSA) addi tives have been a subject of intensive research because they exhibit recordly high conductivities (see, e.g., Refs 1-7 and references cited therein). It is accepted that CSA favors transformation of polyaniline into a protonated conducting form and that m cresol changes the confor mation of the polyaniline molecules from a coil to ex tended chain. Extended chains are characterized by closer packing and the film conductivity increases due to fast inter chain electron transfer.Protonation of the basic form of polyaniline leads to the appearance of paramagnetic polarons.Because of spatial periodicity of the "polaron" lattice the electron energy spectrum has a band structure. The upper band is partially filled, which is responsible for the high electronic conductivity and temperature indepen dent Pauli susceptibility. The conformation of polyaniline molecules can be affected by counterions X -and solvent molecules. Theoretical analysis 8 showed that high solu bility and extended conformation of the polyaniline molecules in solution in the m cresol-CSA system is due to the formation of a specific complex polyani line-m cresol-CSA owing to combination of the stack ing of phenyl groups, hydrogen bonding, and electrostatic interaction.Conducting polymers including polyaniline are called quasi 1D metals, because their properties strongly de pend on the interchain interaction. It was theoretically shown that the existence of true 1D metals is impossible, because any violation of periodicity leads to localization Base Salt m Cresol CSA

Triplet Paramagnetic Centers in Conducting Polymers – Study by ESR and SQUID

“…3, 4 and 5 the triangles oriented down, up, and sideways correspond to temperature decrease from 293 K to minimum, then to increase to 423 K, and to return to room temperature, respectively. ESR line of the solution shows no superposition of two lines and the line width is ~1 mT, which (according to (Kulikov et al, 2005)) indicates unfolded chain conformation. Characteristics of the solution remain unchanged for many months.…”

“…3. The emeraldine base was dissolved during a month, and polyaniline transformed into the doped (protonated) form PANi(m-cresol) 0.5 (Kulikov et al, 2005). In Fig.…”

“…This model can also explain the nonlinear χT-T dependences and requires high values of the exchange interaction, up to 1000 K. In our opinion, these values are unrealistically high. We have previously shown (Kulikov et al, 2005) that the maximum known value for the exchange interaction (~1 K) is observed for distance between polyaniline chains of ~0.6 nm. We believe that the model of exchange -coupled polaron pairs remains valid under suggestion that the singlet-triplet splitting is not caused by the exchange interaction between two isolated centers, but it is a property of a particular polymer fragment, for example, tetramer, and cannot be interpreted as a result of the interaction of isolated spins.…”

“…The lack of the half-field forbidden transition and ESR line splitting (see Fig. 1) suggests that the zero-field splitting is less than 1 mT, or 0.01 J mol -1 (Kulikov et al, 2005(Kulikov et al, , 2007a(Kulikov et al, , 2010b. (2) at L=2, E ZFS =0 and the uniform function F(E) with different values of E 1 and E 2 given in parentheses in kJ/mole (Kulikov et al, 2008).…”

“…According to the "triplet" model, conducting polymers consist of fragments only in singlet or triplet state (no doublet satates) with wide distribution of the singlet-triplet splitting, and magnetic properties of conducting polymers are described by an integral of fragment magnetization over this distribution. This Chapter is a mini-review of our papers (Kulikov et al, 2005(Kulikov et al, , 2007a(Kulikov et al, ,b, 2008(Kulikov et al, , 2010a(Kulikov et al, ,b, 2011. The most convincing confirmation of the "triplet" model gives an analysis of the dependence of magnetization of polymers at helium temperatures on magnetic field.…”