Oriented Polyacetylene Films Research Articles

Impact of synthesis conditions on the electric conductivity of oriented and doped polyacetylene

Abstract We carried out the measurement on a set of oriented polyacetylene films synthesized horizontally and vertically. For every type of a film and for a different thickness, we determined the d.c conductivity as a function of temperature in the range 120–430 °K. In the case of the two types of films analysed, the values of the activation energies E a obtained experimentally are in the same order of magnitude regardless to film thickness ( E H =0.47 eV and E v =0.49 eV).The respective values of the collision factor A were obtained by extrapolation. It is worth noticing that extrapolation of A at 300 K instead of T =∞, confirm the overall behavior of the curves. The conductivity measurement for T =∞ corresponding to oriented undoped polyacetylene samples, show different behavior which is related to the deposition procedure of oriented polyacetylene vertically or horizontally, hence exhibiting morphology difference. The experimental conditions leading to optimal conductivity are determined. The study has shown a number of criteria that allow to define the optimal conditions of polyacetylene synthesis of a highly fibrillary morphology.

THE EFFECTS OF PRESSURE AND MAGNETIC FIELD ON THE CONDUCTIVITY OF FeCl4 DOPED POLYACETYLENE: THE INFLUENCE OF SCATTERING BY LOW-ENERGY EXCITATIONS

The effects of hydrostatic pressure and magnetic field on the conductivity of oriented polyacetylene films doped with FeCl 4- up to metallic state have been studied at T = 0.35-300 K. It was found that application of pressure up to 10 kBar increases the conductivity at 300 K by a factor of 1.3 and suppresses the resistivity minimum in ρ (T) at 270 - 280 K. At T < 2 K the temperature dependence of resistivity ρ (T) ~ ln T (at ambient pressure and at 10 kBar) which remains unaltered by a magnetic field up to 14 Tesla. Transverse magnetoresistance (MR) was found to be negative, linear and temperature independent at T < 2 K (at ambient and high pressure). We attribute the observed ρ (T) and MR behavior to weak localization accompanied by effect of electron dephasing time saturation due to scattering by two-level systems of a special type. More strong behavior of ρ (T) and MR at T > 2 K related to further suppression of weak localization due to scattering by low-energy excitations of a "glassy" type.

Electrical Transport Properties of Perchlorate-Doped Oriented Polyacetylene

Abstract The electrical conductivities of perchlorate-doped highly oriented polyacetylene films were measured from 290 to 1.5 K. The conductivity and its anisotropy were significantly enhanced by stretching. The value of the conductivity was 18000 S cm−1 at 190 K, and remained at 14000 S cm−1, even at 1.5 K. The extraordinary high conductivity at very low temperature showed that the film is really metallic. The temperature dependence of the conductivity could be expressed as the sum of two terms : One is almost a temperature-independent term, which may be due to a very weakly localized free carrier; the other is due to a three-dimensional variable-range hopping mechanism of a more localized carrier. The conduction mechanism is discussed in terms of the polson model of doped polyacetylene.

Pressure dependence of the conductivity and magnetoconductance in oriented iodine-doped polyacetylene.

The temperature dependences of conductivity and magnetoconductance (MC) have been measured, both parallel and perpendicular to the chain axis, in oriented polyacetylene films doped with iodine. The evolution of anisotropy in conductivity as a function of doping time has been studied. Hydrostatic pressure increases the conductivity for pressures up to 4 kbar; at higher pressure the conductivity gradually decreases. The reduced activation-energy function [W=-T[d(ln\ensuremath{\rho})/dT] is temperature independent in the intermediate temperature region (T\ensuremath{\approxeq}180--60 K) suggesting that the material is just on the metallic side of the critical regime of the disorder-induced metal-insulator transition. However, at 8 kbar, W(T) exhibits a positive temperature coefficient from 180 to 1.2 K, implying that the system is more metallic due to enhanced interchain transport. The ${\mathit{T}}^{1/2}$ dependence of conductivity and the enhanced negative MC, at very low temperatures, indicates that the role of electron-electron interactions is significant. MC measurements have been carried out in all the five different combinations of current with respect to the chain axis and field directions. The sign of MC is positive (negative) when the field is perpendicular (parallel) to the chain axis. The anisotropy in MC is attributed to the anisotropic diffusion coefficient. This is shown by the enhanced positive contribution to the MC at 8 kbar, when the field (current) is parallel (parallel) to the chain axis.At 1.2 K, when both the magnetic field and current are perpendicular to the chain axis, the field-induced crossover from positive to negative MC shows the subtle interplay of weak localization and electron-electron interaction contributions to MC. The inelastic scattering lengths, at 1.2 K, in parallel and perpendicular directions to the chain axis are found to be 1163 and 210 \AA{}, respectively. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}3/4}$ dependence of the inelastic scattering length at low temperatures is consistent with the ${\mathit{T}}^{3/4}$ dependence of conductivity. This indicates that the inelastic electron-electron scattering known to be of importance in disordered metals is the dominant scattering mechanism in iodine-doped oriented polyacetylene.

Structural characterization of oriented polyacetylene films grown by the liquid crystal method

Structural characterization of oriented polyacetylene films grown by the liquid crystal method

Thermal conductivity of oriented polyacetylene films

Preliminary results on the effect of stretching on the thermal conductivity of polyacetylene films are presented. Also, data on the in-plane anisotropy and on the effect of of FeCl 3 content will be discussed.

Mechanical and electrical properties of highly oriented polyacetylene films

An improved technique was developed for tensile drawing of polyacetylene films prepared by the non-solvent polymerization method of Akagi et al.; draw ratios as high as 15 were obtained. Tensile drawing and the associated orientation resulted in major improvement of both mechanical properties and the electrical conductivity (after doping). Maximum values for the modulus and tensile strength were 50 GPa and 0.9 GPa, respectively. The electrical conductivities of highly drawn thin films, doped with Iodine, along the orientation direction reached values at room temperature of up to 3×10 4 S/cm, with an anisotropy, σ ∥/ σ ⊥ > 250. The results demonstrate that the electrical conductvity increases linearly with the modulus and tensile strength, with polymer chain orientation as the implicit variable.

Morphology and structure of highly oriented polyacetylene

Highly oriented polyacetylene films have been synthesized under high magnetic field (20 KG; 47 KG); using as solvent of the catalyst the nematic phase of a liquid crystal. We report here on morphology of undoped and iodine doped samples studied by transmission electron microscopy (T.E.M.) and electron diffraction (E.D.). Results about orientation, crystalline order, crystallite size are reduced.

Physical properties of highly oriented polyacetylene films

This paper is devoted to the characterization of highly oriented (CH) x films synthesized onto liquid crystal oriented by a strong magnetic field. The morphology of the films is studied by SEM and TEM. The fibril orientation and crystallinity are investigated by X-ray and electron diffraction. The anisotropic characteristics are measured by optical and transport measurements on undoped and iodine-doped samples. A longitudinal d.c. conductivity as high as 20 000 Ω −1 cm −1 was obtained.

Synthesis and characterization of oriented polyacetylene films

Synthesis and characterization of oriented polyacetylene films

Morphology and characterization of polyacetylene films synthesized in a liquid crystal solvent

The synthesis of oriented polyacetylene films was investigated by using a nematic liquid crystal as a polymerization solvent. As a result, when polymerization was carried out on a quiesent surface of the nematic catalyst solution, partially oriented polyacetylene films were obtained. On the other hand, in the polymerization under a flow of the nematic catalyst solution, highly oriented films were obtained. The order parameter of the oriented sample, estimated from polarized absorption spectra, was about 0.57. The maximum value of parallel electrical conductivity of the films doped with iodine was 4600 S/cm, while that of the films doped with arsenic pentafluoride was 11000 S/cm.

Highly Conducting Polyacetylene Films Prepared in a Liquid Crystal Solvent

Polyacetylene films prepared by the liquid crystal polymerization method with gravity flow technique are investigated through measurements of scanning electron micrographs, polarized infrared absorption spectra, and electrical conductivities. It is explored that the films have significant orientation of conjugated polyene chains along fibril alignment, and that upon doping by I2 or AsF5 they exhibit the highest electrical conductivity among those reported so far. By taking account of both morphological structure and activation energy for electrical conduction, the high conductivity is rationalized with preferential increase of parallel fraction of fibril achieved in the highly oriented polyacetylene films.

Synthesis of highly-oriented polyacetylene film in a liquid crystal solvent

The synthesis of oriented polyacetylene films is investigated by using a nematic liquid crystal as polymerization solvent. Highly-oriented thin films are obtained under a flow of catalyst solution in which a nematic liquid crystal is used instead of common solvents such as hexane and toluene. Nematic liquid crystals that could be used as solvents for the Ziegler-Natta catalysts are limited due to their reactive substituents with the active species and/or the components of the catalyst. Among the available nematic liquid crystals, phenylcyclohexane type is stable in the presence of Ziegler-Natta catalysts. Since fibril alignment in the oriented films is found to be along the flow direction of the catalyst solution, it appears that the direction of fibril growth coincides with that of the liquid crystal orientation, which is caused by an alignment effect of the liquid crystal molecules under a flow of the solution. This new and simple method allows quite large highly-oriented films to be synthesized, which makes it possible to investigate various kinds of anisotropic properties of polyacetylene.