L. A. Apresyan

Anomalies and error limits in electrical-conductivity measurements in plasticized transparent poly(vinyl chloride) films

The anomalous behavior of the electrical conductivity of transparent poly(vinyl chloride) films is experimentally studied at field voltages approaching the breakdown threshold, above the threshold, and far below the threshold. The effects of strong nonlinearity, relaxation drifts, and spontaneous reversible transitions between states with high and low electrical conductivities are revealed for the first time in “thick” (20–100 μm) plasticized transparent poly(vinyl chloride) films. When a certain threshold is exceeded, relaxation oscillations in the test sample are generated and their frequency is proportional to the applied voltage. During a “mild” breakdown, the polymer film passes into the high-conductivity state (the current increases by four or more orders of magnitude), which is identical to the states observed during low-voltage transitions. A simple qualitative model explaining the anomalous character of the electrical conductivity of plasticized poly(vinyl chloride) films is advanced.

On depolarization factors of anisotropic ellipsoids in an anisotropic medium

Depolarization factors for an anisotropic ellipsoid in an anisotropic medium are defined. It is shown that conventional depolarization factors, which are valid for isotropic media, can be extended to anisotropic media by introducing the concept of “reduced ellipsoid,” in which the anisotropy of the ellipsoid shape, as well as anisotropy of the medium, is taken into account.

Nonlinear response and two stable electroconducting states in transparent plasticized PVC films

The electric conductivity of transparent plasticized poly(vinyl chloride) (PVC) films with thicknesses about 30–50 μm has been studied in electric fields with strengths significantly below the breakdown level. It is established that the PVC films exhibit spontaneous reversible transitions between two stable states—with high and relatively low conductivities, in which the bulk resistivity amounts to ∼103 and 106 Ω m, respectively. Relaxation current-voltage characteristics have been measured in a continuous regime, which allowed the Debye relaxation processes to be taken into consideration and effects related to the nonlinearity and transitions between indicated states to be separated. A regime with deterministic switching between the two conducting states has been observed. A simple qualitative model that describes the anomalous character of conductivity in polymer films is proposed.

Synthesis of Composites with Alternating Layers of Poly(vinyl chloride) and Single-Wall Carbon Nanotubes Homogeneously Dispersed in Carboxymethyl Cellulose

For preparation of water-resistant and thermally stable nonlinear optical elements containing single-wall carbon nanotubes, an original method for the formation of layered structures based on alternating layers of poly(vinyl chloride) and a water-soluble polymer (carboxymethyl cellulose) with dispersed singlewall carbon nanotubes is proposed. An analysis of the optical properties of the resulting composites by means of optical-absorption spectroscopy and Raman scattering makes it possible to confirm that the nanocomposites contain individual (not united in bundles) single-wall carbon nanotubes.

A study of the temperature dependence of electrical insulating properties of films of PVC subjected to thermolysis modelling thermal aging in solution

Experiments on measuring the conductive properties in a series of “thin” (10–12 µm) and “thick” (170–250 µm) film samples of unplasticized and plasticized PVC previously subjected to thermolysis (partial dehydrochlorination in solution) simulating the thermal aging of polymer PVC insulation in electrical cables and coatings have been carried out. In this case, PVC macromolecules contain chains of polyene conjugated bonds (PCBs), so that the samples subjected to the treatment are copolymers of vinyl and vinyl chloride. In all cases, the temperature dependence of electrical conductivity was studied in the range close to the performance (from 15 to 85°C) using a standard (GOST) measurement technique. At the same time as the temperature increases for the “thin” samples, abrupt changes in the conductivity by more than ten orders of magnitude were observed with the transition from low conductivity state (LCS) into a high conductivity state (HCS). This transition has a pronounced two-stage character, which is a quasi-stable intermediate semiconducting state (SCS, Rv = 104–105 Ω) was clearly recorded between the limit states of LCS (resistance of the samples Rv>1012 Ω) and HCS (Rv = 0.5 Ω). For a series of “thick” samples of PVC films, the temperature dependence has the shape with completely different, smooth character. It has been noted that the greatest influence on the degradation of electrical properties of the PVC film samples (simulating traditional types of flexible PVC cable insulation) exerts not only the decrease (volatilization) in concentration of traditional plasticizers (such as DOP), but the appearance of PCB formed because of their thermal aging, which eventually lead to the total degradation of their electrical insulating properties by 1.5–2 orders of the magnitude as compared to the initial value.

Temperature dependence of electrical conductivity for poly(vinyl chloride)-polyacetylene copolymer films

The temperature dependence of the electrical conductivity for films of poly(vinyl chloride) whose macromolecules are crosslinked with chains of polyene conjugated bonds—the copolymer of vinylene and vinylchloride obtained as a result of the thermolysis of poly(vinyl chloride) in solution—is investigated. The experiments are performed for a series of samples 10 ± 1 μm in thickness, including pure poly(vinyl chloride) and copolymers with various concentrations of conjugated bonds, in the temperature range 15–85°C. At quite a high concentration of conjugated bonds, stepwise changes in conductivity of more than 10 orders of magnitude are observed. With an increase in temperature, the samples containing conjugated bonds transition from the state of low conductivity to the state of high conductivity. For the first time, it is found that the transition has a pronounced two-step character; i.e., between the extreme states of low conductivity (the sample resistance Rv > 1012 Ω) and high conductivity (Rv = 0.5 Ω), an intermediate quasistable semiconducting state with Rv = 104–105 Ω is registered. With an increase in the concentration of conjugated bonds, a tendency for the temperature transition—a jump in the electrical conductivity—to increase is observed. The contents of the conjugated bonds in the copolymer samples are evaluated according to the absorption spectra in the visible range.

On the effective medium model for particles with a complex structure

We consider a generalization of the effective medium approximation to the case of matrices containing macroscopically inhomogeneous particles with an arbitrary structure (cermet topology). The form of the result is determined to a considerable extent by the heuristic choice of effective cells used to estimate the field and induction values averaged over the volume. The simplest choice of a particle in an unperturbed field as a cell leads to the Maxwell–Garnett approximation, while the self-consistent effective medium approximation corresponds to the replacement of the unperturbed field by the mean field. As an example, we describe particles with a shell, as well as statistically anisotropic media with a single preferred direction.

Switching of the electrical conductivity of plasticized PVC films under uniaxial pressure

The jumplike switching of the electrical conductivity in wide-band-gap polymer (antistatic plasticized polyvinylchloride) films under uniaxial pressure is studied. In various plasticized PVC materials, the uniaxial pressure inducing a conductivity jump by four orders of magnitude or higher changes from several to several hundreds of bars, and this effect is retained at a film thickness of several hundred microns, which is two orders of magnitude larger than the critical film thicknesses known for other wide-band-gap polymers. In addition to the earlier interpretation of the conductivity anomalies in plasticized PVC, we proposed a phenomenological electron-molecular dynamic nanotrap model, in which local charge transfer is provided by mobile molecule segments in a plasticized polymer.

Catalytic synthesis of tubular carbon nanofibers using propagating combustion wave in acetylene

A new method for the pyrolytic synthesis of carbon nanostructures is proposed and experimentally implemented. The synthesis takes place during the propagation of an acetylene combustion wave catalyzed by ferrocene vapor in the absence of oxygen. Under these conditions, long (>10 µm) tubular carbon nanofibers with an external diameter of 10–60 nm and a wall thickness of up to 5 nm were obtained. The proposed method is characterized by a high rate of synthesis and low energy consumption, which implies good prospects for the development of an effective catalytic technology for the synthesis of carbon nanotubes.

Experimental study of the influence of hydrogenation on synthesis of carbon nanostructures in a reactor with activated hydrogen

The role of backward pyrolysis of carbon during hydrogenation of unstructured carbon black and nanotube growth in a reactor with hydrogen activated by diffusion through a heated metallic wall is studied. If the wall is heated nonuniformly, the amorphous unstructured carbon deposit “climbs” in the course of hydrogenation by active hydrogen and deposits again when falling on the heated metal surface, self-organizing into more ordered nanofibers and nanotubes as a result of forward pyrolysis. It is shown that this effect can be used for raising the concentration of carbon nanotubes growing on catalyst particles in the deposited layer.