Dispersion Induced Insulator-to-Metal Transition in Polyaniline

PACS: 71.30.+h, 75.40.Cx, 61.10.-i

The metallic behaviour is still a matter of debate, as polyanilines, polyacetylenes, or polypyrroles show partially metallic features, but most are on the insulator side of the "insulator-to-metal (IM) transition". Quite a few are - sample dependent after solvent-borne "secondary doping" - in the critical regime, or cross the IM transition to the metallic side, however reproducibly only under pressure.^, Here we show, that melt dispersion processing - without solvents or "secondary dopants" - pushes polyaniline reproducibly to the metallic side of the IM transition, although the undispersed polyaniline is on the insulator side. This is the first time, that a conductive polymer is found there without applying pressure. The transition to the metallic state is associated with a decrease of the C₆-N-C₆ angle from 166° to 134°.

Processing of conductive polymers (Organic Metals) is a key problem for basic research and industrial applications. After the usual protonation of polyaniline (PAni) creating mobile charges, two approaches are used:

(i) A "secondary doping" process, e. g. with m-cresol, has been shown to increase conductivity and crystallinity of polyaniline, however without shifting it reproducibly to the metallic side of the IM transition except under pressure (ii) Melt dispersion of PAni in an insulating polymer matrix: for example, PAni protonated with p-toluenesulfonic acid (pTsA) has been dispersed in polymethyl-methacrylate (PMMA). Although the conducting polymer was the minor component in the blend, the low-temperature conductivity increased by many orders of magnitude, the temperature dependence of the thermopower became metallic, and the IR reflectance at low energies increased.

According to X-ray studies conducting (PAni) consists of nm-sized crystalline regions surrounded by amorphous material. This topology also becomes visible in morphological studies on liquid PAni dispersions, where so-called primary particles of about 10 nm size were found^,. The nanoscale structure of the material is reflected in the electric conductivity. According to Ref^., the charge transport in PAni takes place via two contributions: metallic conduction through a crystalline core of 8 nm, and thermally activated tunnelling (hopping) through an amorphous barrier of 1-2 nm diameter. Here, we focus on the question whether the above dispersion process can induce fundamental changes of (a) metallic properties and (b) structure (crystallinity). For this purpose we have investigated paramagnetic susceptibility, dc-conductivity, and crystal structure of PAni protonated with p-TsA as well as of various dispersions (see Table I). The raw PAni (#604, sample # 8) was synthesised using a commercial technique described in Ref.. It was used to prepare a series of PAni-PMMA blends with different concentrations. For the dispersion technique we refer to Ref . Only sample # 549 was synthesised following a usual synthetic procedure, but p-TsA instead of HCl. Sample # 604-pd45 is an extract residue powder of the PAni-PMMA blend # 7 (see below).

The first condition for metallic behaviour is a high charge carrier density, i. e. a high density of electronic states at the Fermi level, N(E_F). The latter one can be determined measuring the magnetic susceptibility:

(1)
where denotes the core diamagnetic susceptibility, is the Pauli susceptibility, and C is proportional to the number of singly occupied states, N_s.. We have used a SQUID magnetometer to measure in the temperature range from 2 to 300 K under an applied field of H=100 mT (see Fig. 1). The core value for the counterion was calculated to be -206 * 10^-6 emu/mole/2-ring, the core value of PMMA is -62.82 * 10^-6 emu/mole. and C are listed in Table 1. Even at the lowest concentration of PAni the values are much higher than those reported previously.

Besides a high charge carrier density also a good mobility is required to achieve a metallic dc-conductivity,. In our PAni-PMMA blends, thermally activated tunnelling through amorphous barriers is the dominant transport mechanism below 200 K. In such disordered systems, the reduced activation energy, W = dln(s )/dln(T), allows to detect a possible insulator-to-metal (IM) transition, a concept that is related to Anderson localization and Coulomb interaction. Fig. 2 displays W for two blends and a compacted pellet of the corresponding undispersed, pure PAni. The positive slope of W at low temperature indicates, that the blends are on the metallic side of the IM transition. This is not the case for the undispersed PAni, where the slope is negative: like other conductive polymers, it is on the insulator side. In contrast to our blends, other materials are at best in the critical regime, where W is independent of temperature. In general, pressures of several kbar have to be applied to reach the metallic state. Examples are highly doped and stretch oriented PAc and PPy-PF₆. CSA-protonated PAni behaves similar, even though its properties depend sensitively on the quality of the samples.
Below 4.2 K, where the dc-conductivity of the blends follows , we have also studied its variation under an external magnetic field, H. An applied field causes to decrease: , where for the magnetoconductivity holds. The interaction parameters of our blends (g Fs = 0.329 and 0.091, respectively) are small compared to those found in solvent processed "secondary doped" CSA-protonated PAni (g Fs = 0.5) or doped semiconductors near the IM transition. The exponent describing the field dependence is at low fields, while at high fields . This is characteristic of disordered metals, where tunnelling plays an important role, and is also observed in metallic Si:P near the IM transition.

In order to find out, whether the dispersion induced changes of conductivity are related to the crystal structure of PAni, we have recorded wide angle diffraction patterns of different PAni powders: (i) the raw material used for the above studies (#8 in Table 1 and Fig. 3), (ii) PAni extracted from the blend with the highest concentration (#7), and (iii) similar synthesized PAni powders with different conductivities¹¹. The results allow to evaluate the structure of the orthorhombic elemental cell and its parameters (see Fig. 3). These differ from those found in studies on PAni doped with various counterions^21,22,23. Already our unprocessed raw material, which is protonated during polymerisation, exhibits a different crystal structure than, e. g., PAni-CSA, which is made from the HCl salt via neutralisation to the emeraldine base^{, ,} .
We find a remarkable relationship between conductivity and the cell volume, V (Fig. 4): while V decreases by about 6% the conductivity increases by a factor of 8. With the exception of one powder (#549) this corresponds to an increase of conductivity with decreasing polymer chain angle delta, i.e., the angle of the C(6)-N-C(6) dimer (see Fig. 3). PAni #8, the raw material unsed for the preparation of the blends, has in its undispersed form a delta of 166°. The same material, after dispersion and extrction from blend #7, has a delta of 134°, indicating a much denser chain packing. Obviously, this results in a better overlap of the conduction bands, so that the blend is on the metallic side of the IM-transition (see Fig. 1). The transition seems to occur as soon as the polymer chain angle of the crystalline phase falls below a specific value. Although we did not yet evaluate it exactly, 134° for p-TsA protonated polyaniline. Surprisingly, this change is induced by melt dispersion. We interpret this as a shear induced recrystallisation. The shear rate applied in the process is over 1,000 s^-1 at a maximum process temperature of 160 °C.

The above correlation between the structure of the crystalline PAni regions and the macroscopic conductivity is surprising, since the latter one is determined by the highest resistances, i.e. by the amorphous barriers. From studies on solvent-borne processing it is assumed that the amorphous phase exists in various conformations, like"coil", "expanded coil" or "rod-like"²⁴ entities. The improvement of the chain alignment is therefore the aim of solvent-borne film casting and stretching. Our conductivity measurements show, that our dispersion process affects the degree of order in the amorphous shells and thus their resistance.

The above results are very relevant for basic science and technology of organic conductors. PAni processed by melt dispersion is the first conductive polymer which crosses the IM border in a reproducible way without applying pressure. It is remarkable that this was achieved with no solvents or secondary dopants in a production scale process. Colloidal melt dispersion of a "pre-metallic" component with a high density of states improves the structure of the crystalline regions and the degree of order in the amorphous shells.

Table 1

You can find further references and figures here.