Lionel Flandin

Effect of strain on the properties of an ethylene–octene elastomer with conductive carbon fillers

Composites that incorporate a conductive filler into an ethylene- octene (EO) elastomer matrix were evaluated for DC electrical and mechanical properties. Comparing three types of fillers (carbon fiber, low structure carbon black, and high structure carbon black), it was found that the composite with high structure carbon black exhibited a combination of properties not generally achievable with this type of filler in an elastomeric matrix. A decrease in resistivity at low strains is unusual and has only been reported previously in a few instances. Reversibility in the resistivity upon cyclic deformation is a particularly unusual feature of EO with high structure carbon black. The mechanical and electrical performance of the high structure carbon black composites at high strains was also impressive. Mechanical reinforcement in accordance with the Guth model attested to good particle-matrix adhesion. The EO matrix also produced composites that retained the inherent high elongation of the unfilled elastomer even with the maximum amount of filler (30% by volume). The EO matrix with other conducting fillers did not exhibit the exceptional properties of EO with high structure carbon black. Composites with carbon fiber and low structure carbon black did not maintain good mechanical properties, generally exhibited an increase in resistivity with strain, and exhibited irreversible changes in both mechan- ical and electrical properties after extension to even low strains. An explanation of the unusual properties of EO with high structure carbon black required unique features of both filler and the matrix. The proposed model incorporates the multifunctional phys- ical crosslinks of the EO matrix and dynamic filler-matrix bonds.

Interrelationships between electrical and mechanical properties of a carbon black-filled ethylene–octene elastomer

Abstract The mechanical and dc electrical properties of a carbon black-filled ethylene–octene elastomer (EO) are reported. The stress–strain curves of the composites, scaled with that of the unfilled polymer up to approximately 500% strain, suggest good filler–matrix adhesion. The large reinforcement effect of the filler followed the Guth model for non-spherical particles. Electrical behavior under large strain was divided in four qualitatively or quantitatively different regimes that differentiated carbon black composites with the EO matrix from carbon black composites with chemically vulcanized matrices. Among the most notable features of the EO composites was the completely reversible variation of the resistivity, up to 20% strain, suggesting that these materials might be useful as strain gauges. In addition, depending on the carbon black content, the composites retained low resistivity to high strains. From the results of various thermo-mechanical treatments, a microstructural model for the response to stretching was proposed. This model incorporated dynamic junctions specific to the EO matrix to describe the mechanical properties, the decrease in resistivity at low strains (20%), and the weak increase in resistivity at intermediate strains (up to about 300%).

Enhanced breakdown strength of multilayered films fabricated by forced assembly microlayer coextrusion

There is a need in electronic systems and pulsed power applications for capacitors with high energy density. From a material standpoint, capacitive energy density improves with increasing dielectric constant and/or breakdown strength. Current state-of-the-art polymeric capacitors are, however, limited in that their dielectric constant is low (2–4). Our approach to improve polymer film capacitors is to combine, through microlayer coextrusion, two polymers with complementary properties: one with a high breakdown strength (polycarbonate) and one with a high dielectric constant (polyvinylidene fluoride-hexafluoropropylene). As opposed to the monolith controls, multilayered films with various numbers of layers and compositions subjected to a pulsed voltage exhibit treeing patterns that hinder the breakdown process. Consequently, substantially enhanced breakdown strengths are measured in the mutilayered films. It is further shown, by varying the overall film thickness, that the charge at the tip of the needle electrode is a key parameter that controls treeing. Based on the acquired data, a breakdown mechanism is formulated to explain the increased dielectric strengths. Using the understanding gained from these systems, selection and optimization of future layered structures can be carried out to obtain additional property enhancements.

Dielectric response of structured multilayered polymer films fabricated by forced assembly

The effect of introducing a multilayer microstructure on the dielectric properties of polymer materials is evaluated in 32- and 256-layer films with alternating polycarbonate (PC) and polyvinylidene-hexafluoropropylene (coPVDF) layers. The permittivity, dielectric loss, dielectric strength, and energy density were measured as a function of the relative PC/coPVDF volume concentrations. The permittivity follows an effective medium model while the dielectric strength was typically higher than that predicted by a volume fraction based weighted average of the components. Energy densities as high as ∼14J∕cm3, about 60% greater than that of the component polymers, are measured for 50% PC/50% coPVDF films.

Dynamic mechanical properties of precipitated silica filled rubber: Influence of morphology and coupling agent

Composites that incorporate precipitated silica into a vulcanized rubber were investigated for dynamic mechanical properties. Comparing different types of filler, it was found that the mean distance between particles did not alter Payne effect. On the contrary, the amount and morphology of the fillers played a major role on the macroscopic properties. The nature and amount of coupling or covering agents was also found to be an important parameter. A direct relationship between length and efficiency of interface agents was evidenced: longer silanes were more effective than shorter once independently from a covalent bounding to rubber. The set of studied parameters affecting Payne effect can be reduced to only two independents variables: the total amount of silica-rubber interface (a function of the amount of filler and its BET surface) and the quantity and nature of interface agent. From these data an attempt to relate the rubber to filler cohesion to Payne effect is proposed as well as a molecular mechanism derived from Maier and Goritz model. A mathematical treatment of the proposed mechanisms is currently being investigated that might help giving further insights on novel ways to further reduce Payne effect.

Influences of degree of curing and presence of inorganic fillers on the ultimate electrical properties of epoxy-based composites: experiment and simulation

This paper describes both an experimental study and a numerical investigation of the breakdown field in particle reinforced thermosets. The experimental study revealed that the increase of the conversion degree (α) of the epoxy matrix improved the breakdown voltages both in divergent and quasi-homogeneous fields. It was also observed that the presence of the second phase (mineral filler) dispersed within the polymeric host led to apparently contradictory results depending on the nature of the electric field. The neat matrix exhibited a higher field to breakdown than that of the composite in a quasi-homogeneous field configuration whereas the opposite behaviour was evidenced in a point–plane configuration. It thus appeared that the filler can either increase or decrease the breakdown voltage, depending on the nature of the electrode configuration. To further understand this behaviour, a deterministic numerical simulation of the dielectric breakdown that accounts for the organization within real composites was developed. This numerical method, based on the resistor–short breakdown model was improved to directly account for the real phase arrangement within the samples through scanning electron micrographs. The simulation furnished the explanation for the apparent disagreement, showing that the inorganic particles protect the composite in divergent fields through a mechanism similar in nature to the so-called barrier effect, whereas the same fillers were responsible for a local intensification of the electric field and thereby a reduction of the dielectric strength in a quasi-homogeneous field.

Dielectric breakdown of epoxy-based composites: relative influence of physical and chemical aging

The effect of aging on the dielectric strength of epoxy-inorganic particle composites used for insulators in the high voltage industry is reported. A differential scanning calorimetry analysis of an insulator aged twenty years in actual service conditions indicated both a chemical degradation and a structural recovery of the polymer network. This composite exhibited however a breakdown field comparable to that of a fresh sample with the same formulation. An accelerated physical aging was thus performed which lead to a large increase in the high voltage performance of the newly processes composite over time. This improvement was attributed to a densification of the thermoset resin, which impeded tree growth. It was also observed that the choice of the electrode geometry greatly alters the measurements under high electric field. In a quasi-homogeneous field configuration, the breakdown was mainly governed by the major flaws at the sample scale, namely the reinforcing particles. On the contrary, under a divergent field (with a point-plane electrode arrangement), the field was essentially localized at the point electrode tip, and the major flaws might not be reachable by the damage tree. It hence appeared that the measurements performed in a quasi-homogeneous field are not very sensitive to the variations within the polymeric matrix as are the measurements under a divergent field.

Effect of filler auto-assembly on percolation transition in carbon nanotube/polymer composites

A series of composites with various content of multi-walled carbon nanotubes dispersed in polymer was tested for electrical properties. During isothermal annealing in the melt, dynamic percolation transition induced a tremendous increase in conductivity. The unstable structure also experienced a more than one order of magnitude reduction in percolation threshold. The insulator to conductor transition concurrently became softer, as revealed by a monotonous increase in the critical exponent, gradually departing from the universal value. These large and concomitant changes in percolation transition with annealing time were ascribed to the self-organization of the filler that favors the completion of the conductive network.

Characterization of the Degradation in Membrane Electrode Assemblies Through Passive Electrical Measurements

A passive technique to characterize the degradation of membrane electrode assemblies is presented; it is based on simple measurement of electrical characteristics as a function of time. This method relies on the experimental evidence that the assembly behaves like a supercapacitor with a tremendous capacity in series with a small resistance. In a first approximation, a resistor-capacitor (RC) circuit can thus be utilized to model the charging and discharging behavior. The experimental data demonstrated that a more sophisticated equivalent circuit was necessary to understand the experimental results. The most favorable set of parameters was determined, thanks to a Monte Carlo type numerical analysis. Overall, a larger sensitivity to damage detection than the well-accepted electrochemical impedance spectroscopy is demonstrated that suggests a promising future to in situ detection of failure and understanding of degradation mechanisms.

Understanding dynamic percolation mechanisms in carbonaceous polymer nanocomposites through impedance spectroscopy: Experiments and modeling

Dynamic percolation refers to the impressive increase in conductivity in polymers filled with a constant filler content as a function of annealing time. We present a detailed study of the driving forces for this phenomenon. The organization of carbon nanotubes in a polymer melt is probed with a.c. conductivity. In contrast with the static percolation studied as a function of the filler content, two peaks are observed in the relative permittivity. We show that this new feature results from two distinct filler auto assembly mechanisms. The first one is ascribed to the relaxation of macromolecules and could be eliminated using the proper thermal treatment. The second mechanism is observed later in time. It is likely to correspond to a diffusion process of the carbonaceous filler similar to a phase separation. A phenomenological model is finally proposed to describe the changes in dispersion and distribution states with annealing time.