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International audience; This work deals with the study of thermoset-thermoplastic blend forming an epoxy-amine/poly-etherimide phase separated to assess the dielectric and thermal performances. These materials would be good candidates to replace the passivation layer in semiconductors, particularly ones used as switches in power electronic applications. Polyimide and Parylene are usually quoted and studied. Mixtures based on polymers would be a novel candidate that can manage to be an insulator for the system. The modified and non-modified systems where characterized by transmission electron microscopy, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, dielectric analysis and analytical simulation was carried out on dielectric measurements. These complementary techniques were used first to investigate the presence of the phase separation phenomenon and second to quantify the separated nodules size. The effect of this phase separation was examined and showed enhancement in the dielectric values compared to the pure epoxy system. It was finally simulated to show a close assumption of what is found experimentally.

In power electronics, electrical insulation encapsulation at high temperatures and/or high voltages is a challenge for the systems such as IGBT. Different materials are used and tested in industry and literature. Silicone elastomers for example associated with a primary passivation layer of polyimide are considered to be good candidates but are limited to 180 – 200 °C. Epoxy thermosets are known to be good insulator to protect electrical devices from moisture, dust and short circuits electrical breakdown. Thermoset/thermoplastic blends can be another good candidate for electrical insulation properties. The thermoplastic, its percentages as well as the curing conditions control the final morphology (phase separation with disperse thermoplastic phase, bicontinuous phases or inverted structure …) and thus the electrical properties of the sample. In our study, polyetherimide (PEI) and Polystyrene (PS) thermoplastics were introduced with different percentages into the epoxy system. Electronic microscopy images showed similar phase separation phenomenon with different size of nodules of the thermoplastics. The effect of the morphology on mechanical, electrical and thermal properties has been investigated. The addition of 6 wt% of PS and 10%wt of PEI gives nodules of thermoplastic with a micrometer size inside the epoxy amine network where it did not affect either the storage modulus or the thermal resistance under air or nitrogen atmosphere. Whereas, the dielectric spectroscopy points out that, the addition of polystyrene into the epoxy system increased the relative permittivity of the blend. Contrariwise, the addition of polyetherimide into the epoxy system has decreases the relative permittivity by 15 % in all the temperature and frequency ranges compared with the pure epoxy amine network.

In power electronics, electrical insulation encapsulation at high temperatures and/or high voltages is a challenge for the systems such as Insulated-gate bipolar transistor (IGBT). Different materials are used and tested in industry and literature. Silicone elastomers for example associated with a primary passivation layer of polyimide are considered to be good candidates but are limited to 180–200 °C. Epoxy thermosets are known to be good insulator to protect electrical devices from moisture, dust and short circuits electrical breakdown. Thermoset/thermoplastic blends can be another good candidate for electrical insulation properties. The thermoplastic percentages as well as the curing conditions control the final morphology (phase separation with disperse thermoplastic phase, bicontinuous phases or inverted structure …) and thus the electrical properties of the sample. In our study, polyetherimide thermoplastic was introduced with different percentages into the epoxy system. Electronic microscopy images showed the different morphologies depending on the thermoplastic amount. The effect of the morphology on mechanical, electrical and thermal properties has been investigated. For example, the addition of 10%wt of PEI gives nodules of thermoplastic with a micrometer size inside the epoxy amine network where it did not affect either the storage modulus or the thermal resistance under air or nitrogen atmosphere. Whereas, the dielectric spectroscopy points out that the addition of polyetherimide into the epoxy system has decreases the permittivity by 15 % in all the temperature and frequency ranges compared with the pure epoxy amine network.

International audience; Two set of single core cables with PVC insulation with different decreases of electrical resistivity have been analyzed to determine the aging mechanism responsible for this decrease of electrical properties. Four different aging mechanisms can be considered: oxidation, effect of water ingress, loss or migration of plasticizers and dehydrochlorination. IR spectroscopy has not revealed any oxidation nor the presence or the effect of water. IR microspectroscopy has confirmed the uniform composition of the polymer throughout the sample. SEM and EDX analyses have highlighted clusters of lead chloride. The presence of this compound results from the reaction of the thermal stabilizer due to the mechanism of dehydrochlorination. However, these techniques did not distinguish between the two samples of different resistivity. UV spectroscopy revealed the presence of double bonds in the two samples and show that the concentration is higher in the cable with the worse electrical properties. The formation of double bonds is a consequence of the dehydrochlorination and cannot result from one of the three other aging mechanisms considered. Although the measurement of chloride ion concentrations by coulometry should have confirmed this aging mechanism, the surprisingly low measured levels of chloride ions are discussed with regards to the adequacy of the method to measure lead chloride in cluster forms.

Among materials used for electrical insulation in power electronics systems like IGBT, silicone elastomers used are considered as good candidates but are limited to 180–200°C. In this work, new thermoset/thermoplastic composites are investigated. The addition of some thermoplastics to epoxy amine systems can lead to system exhibiting phase separation. The thermal degradation under some conditions reaches 300°C, which is interesting for power electronics applications. The mixture of thermoplastic inside epoxy prepolymer governs the final microstructure. For example, the addition of 10 % wt of PEI gives inclusions of thermoplastic with a micrometer size. Even if the T alpha of epoxy matrix was changed upon the addition of PEI, the dielectric spectroscopy points out that the addition of some thermoplastic decreases the permittivity compared with the pure epoxy amine network.

Semi-interpenetrating polymer networks (semi-IPNs) based on crosslinked polyurethane (PU) and linear polyvinylpyrrolidone (PVP) were synthezised, and their thermal and dynamic mechanical properties and dielectric relaxation behavior were studied to provide insight into their structure, especially according to their composition. The differential scanning calorimetry results showed the glass transitions of the pure components: one glass-transition temperature (T g ) for PU and two transitions for PVP. Such glass transitions were also present in the semi-IPNs, whatever their composition. The viscoelastic properties of the semi-IPNs reflected their thermal behavior; it was shown that the semi-IPNs presented three distinct dynamic mechanical relaxations related to these three T s values. Although the temperature position of the PU maximum tan δ of the α-relaxation was invariable, on the contrary the situation for the two maxima observed for PVP was more complex. Only the maximum of the highest temperature relaxation was shifted to lower temperatures with decreasing PVP content in the semi-IPNs. In this study, we investigated the molecular mobility of the IPNs by means of dielectric relaxation spectroscopy; six relaxation processes were observed and indexed according the increase in the temperature range: the secondary β-relaxations related to PU and PVP chains, an α-relaxation due to the glass-rubber transition of the PU component, two α-relaxations associated to the glass-rubber transitions of the PVP material, and an ionic conductivity relaxation due to the space charge polarization of PU. The temperature position of the α-relaxation of PU was invariable in semi-IPNs, as observed dynamic mechanical analysis measurements. However, the upper α-relaxation process of PVP shifted to higher temperatures with increasing PVP content in the semi-IPNs. We concluded that the investigated semi-IPNs were two-phase systems with incomplete phase separation and that the content of PVP in the IPNs governed the structure and corresponding properties of such systems through physical interactions.

Hybrid organic/inorganic films have been prepared using different complexes soluble in a (BTDA/4,4′-ODA) poly(amic acid) solution. A silver based complex and two palladium based complexes were used and the metallic nanoparticles were formed by a single step process during the cure cycle applied to the film. Depending on the complex or the curing conditions used, either the nanoparticles formed were uniformly dispersed in the nanocomposite films, or the particles were also located at the film surface. The presence of the nanoparticles, whatever their composition, led to a decrease of the thermal degradation temperature in air atmosphere. The presence of 15 wt% of crystalline silver particles did not induce significant variation of the glass transition temperature and of the gas transport properties demonstrating low nanoparticle/matrix interactions. On the contrary, a pronounced effect in reducing the gas permeability for a wide variety of gases was observed for the nanocomposites containing the palladium based nanoparticles and this even for low palladium amount (5 wt%). Specific interactions between hydrogen and the Pd based nanoparticles were evidenced and the gas transport properties were discussed as a function of following parameters: the gas nature, the nanoparticle structure and composition, the developed morphology and the particle/matrix interactions.

The dielectric properties of the heat-resistant heterocyclic polymers are governed by their chemical structure and the absence of unwanted dipoles. A polyphenylquinoxaline is taken as an example of the effect of the dipole symmetry. In the particular case of polyimides, it is obvious that the dehydrocyclization of the intermediate polyamic acid has to be completed to produce the lowest achievable permittivity. This is illustrated by the polycondensation in solution of the polyimides based on 4,4'-(9H-fluoren-9-ylidene)-bisbenzeneamine. In addition, the spatial conformation of the macromolecular chain can also play an important role. The chemistry of a polyimide prepared from 4,4'-(1,3-benzenedicarbonyl) bis(1,2-benzenedicarboxylic acid dianhydride) allows to discuss this topic. However, a dielectric constant of 2.7 seems to be the ultimate value achievable for dense films prepared with the most performing polymers. The introduction of homogeneously distributed micro-voids allows the preparation of dielectric films with a permittivity lower than 2.0. The modification of a polymer with tertiobutyl carbonate (t.BOC) groups is a promising approach because it offers a large processability window.

International audience; This paper presents an investigation on the current and breakdown voltage measurements in thermoset-thermoplastic blends based on an epoxy-amine/poly-etherimide phase separated material. Thermosetthermoplastic separated blends could be a novel material for insulation applications and can be compared to epoxy/inorganic composites systems. Pure epoxy network as well as the blends with 10 wt% of PEI were studied in terms of conduction and transient currents. An ohmic behavior below the threshold field (E th) and space-chargelimited conduction (SCLC) above E th are pointed out. Contact emission phenomenon was investigated by means of Schottky model and seems to be valid for both materials. The addition of 5 and 10 wt% of PEI into the epoxy system showed an increase in the values of breakdown voltage with the increase of the PEI amount.