Insulating Polymer Matrix Research Articles

Electrical, magneto-transport and localization of charge carriers in nanocomposites based on carbon nanotubes

Here we investigate the electrical transport and magneto-transport properties of a network of singlewalled carbon nanotubes (SWNT) embedded into an insulating polymer matrix in order to build a nanocomposite. We show that the SWNT metallic character is lost in the nanocomposite. Intrinsic localization of the carriers at the bundle size is evidenced from conductivity results. It is confirmed by magnetoresistance measurements which exhibit quantum interference effects in the localized regime. By comparison with isolated SWNT, another difference is found in the role of the Coulomb interactions. Building an ensemble of SWNT allows a partial screening of these interactions as revealed from the ohmic and non ohmic conductivity temperature and field dependencies.

On the characterization of an electrically conductive polyaniline complex

This paper reports on the characterization techniques performed to evaluate the suitability of three different samples of a polyaniline (PANI) complex (PANIPOL™) for the production of composites exhibiting a phase separated morphology with continuous elongated structures of PANI embedded in the bulk of an insulating polymer matrix by following an in-situ deformation process. The characterization techniques included rheometry, differential scanning calorimetry and thermogravimetry. In addition, X-ray diffraction, gravimetry, infrared spectroscopy, conductivity measurements, and optical and scanning electron microscopy were used to fully characterize the samples. The thermal limitations and stability of the samples were determined. At the same time, their flow properties in the molten state under different levels of shear were also analysed. The experimental results assisted in the identification of the samples' components and revealed that the PANI particles were 7 μm in diameter or smaller.

To what extent is the structure of a random composite compatible with a percolation model?

Recent developments of local field microscopy allow a considerable improvement over more traditional techniques (like transmission electron microscopy) of the knowledge of the actual structure of composites made of conducting particles dispersed in an insulating polymer matrix. The Resiscope is an attachment to an atomic force microscopy (AFM) microscope that provides at the same time a classical image of the sample surface and a novel image representative of the local electrical conduction through the sample, between the tip and the back. Using the thickness of the sample as an independent variable, we have obtained original data on the 3D structure of the finite as well as on the infinite cluster and their relative variations with the particle concentration in a series of carbon black-filled elastomers. We have performed computer simulations on percolation cubic networks aimed at providing similar data when both the filling factor p and the sample “thickness” (number of planes) are allowed to vary. We have found that in spite of a general agreement between experimental and calculated data, there exist significant differences that seem to prove that the actual structure is more complex than the one predicted by a classical percolation model for random composites.

Conducting probe atomic force microscopy applied to organic conducting blends

Atomic force microscopy (AFM) is used in contact mode with a conducting tip to probe the conducting network of the conductive polymer polyaniline blended in an insulating polymer matrix. The high resistance contrast and sharp boundaries between conductive and insulating phases is observed down to scales in the 10 nm range. The very low scale electric dispersion corresponds to the morphologic phase segregation known from conventional AFM or transmission electron microscopy measurements, which is responsible for the ultralow electrical percolation threshold previously demonstrated in this system.

Direct current conduction in SiC powders

Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC-grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic and conduction mechanisms of the SiC powder itself are of interest. SiC powders have been studied by current–voltage measurements and the influences of grain size and doping have been investigated. The macroscopic current characteristics of green and black SiC powders can be described by the transport mechanisms at the grain contacts, which can be modeled by Schottky-like barriers. The SiC is heavily doped and tunneling by field emission is the dominating conduction mechanism over the major part of the nonlinear voltage range. It is suggested that preavalanche multiplication influences the current at the highest voltages, especially for p-type black SiC.

Alternate current characteristics of SiC powders

Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic of the SiC powder itself is of interest. The ac characteristics of SiC powders have been studied by dielectric response, capacitance–voltage, and ac-pulse measurements. The frequency, electric field, and pressure dependencies have been analyzed for green and black SiC, which have different doping. The ac characteristics of green and black SiC powders are governed by both the barrier regions at the SiC-grain contacts and the surrounding matrix. The nonlinear loss is determined by the conduction current at the contacts. Depending on the doping level of the SiC grains, the capacitance may be controlled by either the nonlinear capacitance of the barrier region or the linear capacitance of the surrounding matrix. Each contact zone may be modeled by a nonlinear resistance in parallel with both a nonlinear and a linear capacitance. The components are considered to be frequency independent. However, in order to explain the macroscopic frequency and field dependencies of the SiC powders, the use of a network of unique contact zones with dissimilar properties is suggested.

Piezoresistance of conductor filled insulator composites

Several series of electrically conducting composites composed of a conducting filler randomly dispersed into an insulating polymer matrix were prepared. The fillers were the tin–lead alloy powder, copper powder, aluminium powder and carbon black, and the matrices were polyethylene, polystyrene and epoxy resin. The piezoresistance effects of these composites have been investigated under uniaxial presses. It was observed that the piezoresistance depends on the applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus and potential barrier height. Piezoresistance increases with increase of applied stress, filler particle diameter and potential barrier height, but decreases with increases of filler volume fraction and matrix compressive modulus. A model based on the change in interparticle separation under applied stress, is developed. By analysing this model, the piezoresistance of composites is studied and the effects of influencing factors are theoretically predicted quantitatively, showing good agreement with the experimental data. © 2001 Society of Chemical Industry

Surface characterizations of conductive poly(methyl methacrylate)/polypyrrole composites

The composites of poly(methyl methacrylate) and polypyrrole (PMMA/PPy) were prepared by a chemical oxidation of pyrrole in a PMMA latex medium resulting in a network like structure of polypyrrole embedded in the insulating polymer matrix. Water was used as the dispersion medium. The content of polypyrrole was determined by elemental analysis as varying from 0.25 wt.% to 10 wt.%. The electrical conductivity of prepared composites depends on the concentration of polypyrrole and reached values of between 1 × 10− 9 S/cm to 0.1 S/cm The surface of powder form of PMMA/PPy composites was characterized by X-ray photoelectron spectroscopy (XPS) and by scanning electron microscopy (SEM). The antistatic properties of compression moulding form of composites were tested.

99/03147 Effect of thermal treatment on crystallization and PTC properties of conductive PVDFI/CB composite

The conduction mechanism in the organic positive temperature coefficient of resistance (PTC) materials composed of conductive carbon black (CB) particles and an insulating polymer matrix was investigated at the nanolevel using a scanning probe microscope. The atomic force microscope (AFM) observation showed the CB particles as circular projections of about 50–100 nm in diameter. On the other hand, nanoscopic current measurements conducted by scanning an Au-coated AFM tip on the sample surface revealed that some of these projections also appear as bright spots in the current-mode image, suggesting that these parts are electrically connected through the sample to a conductive substrate. Moreover, the area of the bright regions was observed to increase significantly with increasing applied voltages. These data suggest that the governing mechanism for these organic PTC compounds may be the electron tunneling or hopping processes through the dispersed CB particles rather than the electron transport through the contacted CB particles.

Effect of heating rate on positive-temperature-coefficient-of-resistivity behavior of conductive composite thin films

A phenomenon was discovered that leads to the selective detection of abrupt increases in the temperature of conductive composite thin films consisting of conductive ceramic fillers and an insulating polymer matrix. Examining the heating rate dependence of the positive-temperature–coefficient-of-resistivity (PTCR) effect provided information about this intelligent phenomenon. The anomalous PTCR effect was observed above 0.3 °C min−1 for all the prepared films. However, the magnitude of the anomaly decreased when the heating rate decreased below 0.1 °C min−1, and when the heating rate further decreased below 0.04 °C min−1, the anomalous resistivity–temperature relationship disappeared. The results suggest that these thin films can selectively detect abrupt increases in temperature, which could lead to an intelligent mechanism. Our results also suggest a PTCR mechanism, in which the expansion of crosslinked polymers in the thermodynamic nonequilibrium state essentially produces the anomaly.

Chemical preparation and characterization of conductive poly(methyl methacrylate)/polypyrrole composites

The processes of the preparation of highly conductive polymer composites of poly(methyl methacrylate) and polypyrrole (PMMA/PPy) have been studied. The composites were prepared by a chemical modification method resulting in a network-like structure of polypyrrole embedded in the insulating polymer matrix. Water was used as a solvent. The content of polypyrrole was determined by elemental analysis varying from 0.25 to 10 wt%. The electrical conductivity of compression-moulded samples depends on the concentration of polypyrrole and reached values of between 1×10 −9 S/cm to 0.1 S/cm. Highly conductive composites were also obtained from a mixture of coated and non-coated PMMA particles. The PMMA/PPy composites were characterized by elemental analysis, infrared spectroscopy and X-ray photoelectron spectroscopy (X.p.s.). The method of low-voltage scanning electron microscopy was used to study the potential contrast in dependence on the acceleration voltage between areas of different chemical composition. This method can be used to show the perfection of the PPy network structure in the composites. The antistatic properties of PMMA/PPy composites were demonstrated. The stabilizing effect of PPy on the thermal stability of poly(methyl methacrylate) was shown by thermogravimetric analysis.

Negative-resistance device using organic-dye-doped polymer film

In order to develop a new negative-resistance device, a single-layer device of organic-dye-doped insulating polymer was fabricated by dipping. The current–voltage characteristics at room temperature showed remarkable negative resistance. The ratio of peak current to valley current was 23 or higher and the difference between peak and valley voltages was 2 V. The negative resistance did not depend much on the combination of insulating polymer matrix and dye dopant or on temperature. This suggests that the negative resistance is caused by tunneling transport of holes or electrons. Two possible models are considered for the origin of the negative resistance. One is based on the polarization formed by holes or electrons trapped in the dye molecule. The other is based on the tunneling at the interface between electrode and dye molecules.

New acrylic resin composite with improved thermal diffusivity

Statement of problem. Studies have shown that physical characteristics of denture base materials may affect patient acceptance of denture prostheses by altering sensory experience of food during mastication. Thermal diffusivity is one material property that has been cited as being important in determining gustatory response, with denture base acrylic resins having low thermal diffusivity compared with denture base metal alloys. Purpose. This study prepared and characterized experimental acrylic resin composite material with increased thermal diffusivity. Material and methods. Sapphire (Al 2 O 3 ) whiskers were added to conventional denture base acrylic resin during processing to achieve loadings of 9.35% and 15% by volume. Cylindrical test specimens containing an embedded thermocouple were used to determine thermal diffusivity over a physiologic temperature range (0° to 70° C). Results. Thermal diffusivities of the sapphire-containing composites were found to be significantly higher than the unmodified acrylic resin. Thermal diffusivity was found to increase in proportion to the volume percentage of sapphire filler, which suggested that the high aspect ratio ceramic particles formed a pathway for heat conduction through the insulating polymer matrix. Conclusion. The thermal diffusivity of denture base acrylic resin was increased by the addition of thermally conducting sapphire whiskers. (J Prosthet Dent 1998;79:278-84.)

Synthesis, Electrical Properties and Stability of Polypyrrole-Containing Conducting Polymer Composites

Conducting polymer composites of polyethylene and polypyrrole (PE/PPy), polypropylene and polypyrrole (PP/PPy) and poly(methyl methacrylate) and polypyrrole (PPMA/PPy) were prepared by means of a chemical modification method resulting in a network-like structure of polypyrrole embedded in the insulating polymer matrix. The content of polypyrrole determined by elemental analysis varied from 0·25 to 17wt%. Electrical conductivity of compression-moulded samples depended on the concentration of polypyrrole and reached values from 1×10-11 to 1 S cm-1. The morphology of the composites and blends was studied by low-voltage scanning electron microscopy. The stability of PP/PPy composites was investigated by thermogravimetric analysis and by conductivity measurements during heating–cooling cycles. There was only a small drop in conductivity caused by the annealing of PP/PPy composites in air at temperatures up to 80°C. The results of thermogravimetric analysis showed a stabilizing effect of PPy on PMMA/PPy composites against thermal degradation. The antistatic properties of PMMA/PPy composites were demonstrated. © 1997 SCI.

Electrical properties and stability of polypyrrole containing conducting polymer composites

Conducting polymer composites of polyethylene and polypyrrole (PE/PPy), polypropylene and polypyrrole (PP/PPy), and poly (methyl methacrylate) and polypyrrole (PMMA/PPy) were prepared by means of a chemical modification method, resulting in a network-like structure of polypyrrole embedded in the insulating polymer matrix. The content of polypyrrole determined by elemental analysis varied from 0.25 to 17 wt.%. Electrical conductivity of compression-moulded samples depends on the concentration of polypyrrole and reached values from 1 × 10 −11 to 1 S/cm. The morphology of the composites was investigated by low-voltage scanning electron microscopy (LVSEM). Potential contrast measurements as a function of the acceleration voltage were used to prove the perfection of the PPy network structure. The electrical transport mechanism in PP/PPy composite was studied. The data of the temperature dependence of conductivity were fitted following the function for a charge-energy-limited tunnelling (CELT) model. There is only a small drop in conductivity caused by annealing of PP/ PPy composites in air at temperatures up to 80 °C. A stabilizing effect of PPy on thermal stability of polypropylene is shown by thermogravimetric analysis. The antistatic properties of PE/PPy and PMMA/PPy composites were demonstrated.

Nanoscopic analysis of the conduction mechanism in organic positive temperature coefficient composite materials

The conduction mechanism in the organic positive temperature coefficient of resistance (PTC) materials composed of conductive carbon black (CB) particles and an insulating polymer matrix was investigated at the nanolevel using a scanning probe microscope. The atomic force microscope (AFM) observation showed the CB particles as circular projections of about 50–100 nm in diameter. On the other hand, nanoscopic current measurements conducted by scanning an Au-coated AFM tip on the sample surface revealed that some of these projections also appear as bright spots in the current-mode image, suggesting that these parts are electrically connected through the sample to a conductive substrate. Moreover, the area of the bright regions was observed to increase significantly with increasing applied voltages. These data suggest that the governing mechanism for these organic PTC compounds may be the electron tunneling or hopping processes through the dispersed CB particles rather than the electron transport through the contacted CB particles.

Electrochemical study of poly(vinyl chloride)/polypyrrole blends

Blends were obtained by impregnation of an insulating polymer matrix, poly(vinyl chloride), with an oxidant, FeCl 3, followed by exposure to pyrrole vapor. The redox properties of films of the blends obtained by this method were studied by cyclic voltammetry and impedance spectroscopy. The results show that thin films (20 μm) are more electroactive than thick films (100 μm) although the proportion of polypyrrole is the same for both ( ca. 38%). This electroactive behavior is in agreement with morphology observations, which show different phase separation mechanisms as a function of the film thickness: a spinodal mechanism for the thin film and a binodal mechanism for the thick film.

Stability of processed poly(3-octylthiophene) and its blends

Electrically conducting poly(3-octylthiophene) (POT) and blends thereof have been processed and analyzed. Processing was done by extrusion using POT as the electrically active component the insulating polymer matrix being EVA, PE, PP, PS or PB. For these basic studies the in situ doping was done by I 2, FeCl 3 and sulfonic acid. Room temperature conductivity was used to probe the percolation of the conductivity as a function of matrix and dopant concentration. The measured conductivities varied between those of the insulating matrices and 1·10 −2 Ω −1 cm −1 . The long term stability of the conductivity was measured in various atmospheres at room temperature as well as elevated temperatures. The effect of stabilizers was also studied.

Conducting polymers of aniline I. Electrochemical synthesis of a conducting composite

We describe an electrochemical synthesis of a conducting composite of polyaniline. Poly(bisphenol A carbonate) was used as the insulating polymer matrix. Composite characterization was made using FT-IR, SEM and DSC data. The conductivities of the composites seemed to be in the order of pure polyaniline as prepared by the same method. Moreover, the above-mentioned methods reveal that the resultant composites have different properties compared to a simple mechanical mixture of the two polymers.

Charge transport in polyethylene–graphite composite materials

Deviation from the predictions of percolation theory for conduction in polythylene‐graphite composites (see figure) have been explained in tems of tunneling between the conducting graphite particles in the insulating polymer matrix, a mechanism which is believed to be prevalent at lower graphite concentration.magnified image