T. V. Vlasova

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.

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.

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.

Self-organizing cyclolinear methylcyclohexasiloxane polymers carrying reactive vinyl groups

Atactic cyclolinear organosilicon polymers containing vinyl substituents in RSiO1.5, R2SiO or both moieties have been synthesized through the heterofunctional polycondensation of trans,cis-2,8-dihydroxymethyl(vinyl)cyclohexasiloxanes with 2,8-dichloromethyl(vinyl)cyclohexasiloxanes. The structure of the polymers has been studied by 1H and 29Si NMR and IR spectroscopy, molecular mass measurements, and elemental analysis. The phase behavior of these copolymers in the bulk has been examined by DSC, X-ray diffraction, and polarization optical microscopy. It has been shown that the copolymer can exist in the mesomorphic state in the temperature range from −100 to +200°C. The X-ray data indicate changes in the interlayer spacing and the type of packing of cyclolinear poly(methylvinylsiloxanes) with an increase in the content of vinyl substituents in the repeating units of the polymer. The ability of cyclolinear poly(methylvinylsiloxanes) to spread over the water/air interface and to form mono-and multilayers has been investigated. As the content of vinyl substituents in the polymer unit is increased to two or four, the ability of polymers to form multilayers is preserved. The incorporation of vinyl substituents into RSiO1.5 or R2SiO moieties of polymer units is accompanied by the formation of monolayers.

On Improving the Running Ability of Cables with Polymer Insulation

The main factors that prevent the improvement of the running ability of electrical cables with polymer insulation have been considered. Today, compositions based on polyvinylchloride (PVC) are mainly used as polymer insulation in most electrical cables in Russia. PVC, as are most polymers, is rather sensitive to thermal aging, which is considered as one of the main reasons for the occurrence of various polymer-insulation defects that lead with time to short circuit and fire. It has been shown based on the experimental data that the sewn-in chains of polyene conjugated bonds are formed in PVS macromolecules during the thermal aging. The chains proper are the main reasons for deterioration of the dielectric properties of PVS insulation. The incorporation of conventional plasticizers like phthalates, phosphates, etc., into a polymer matrix plays a much smaller role. Thus, to develop a new generation of polymer insulation, first of all it is required to produce new types of stabilizers that can both efficiently eliminate the formed polyene conjugated carbon bonds during thermal aging and additionally improve all the physical, mechanical, performance, and technological properties of polymer insulation.

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.

Synthesis of carbon nanofibers and nanotubes in an activated-hydrogen reactor

The parameters of multilayer carbon nanotubes and nanofibers synthesized by pyrolysis of acetylene in a reactor filled with hydrogen activated by diffusion through a hot metallic wall, as well as synthesis products, are studied. The results of synthesis with catalysts applied on the substrate and in the gaseous phase are reported.