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Medium voltage underground cables were aged in air at temperatures between 70°C to 140°C. The changes of the structural properties of the crosslinked polyethylene (XLPE) insulating layer were characterized by physico-chemical techniques: Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). The performed analyses revealed chemical ageing (oxidation, chemical consumption of additives, post-crosslinking...) and physical ageing (migration of chemical species). The accelerated thermal tests on stripped XLPE samples showed, as expected, a mild oxidative degradation. In contrast, ageing tests conducted in the presence of the semi-conductive layers revealed a striking difference, with a migration of extra carbonyl species into the XLPE insulation.

The degradation of perfluorinated sulfonic acid ionomer (PFSA) binder in catalyst layer of Proton Exchange Membrane Fuel Cell (PEMFC) was investigated on Membrane-Electrode-Assemblies (MEA) operated up to 10400 h in base load conditions for telecom relay. Soxhlet extractions in water media were performed on the catalyst layer deposited on the Gas Diffusion Layer (GDL). The Soxhlet solutions for anode and cathode sides were analyzed by 19F NMR spectroscopy showing degradation products originating from the ionomer in the catalyst layer. At the cathode side, 1,2,2-tetrafluoro-2-(1,2,2,2-tetrafluoroethoxy)ethanesulfonic acid were clearly identified. This suggests that PFSA binder degradation originated from an H• and/or •OH radical attack of the side chain. The NMR quantification of the degradation products allows a fruitful comparison with the literature. The degradation mechanism of the PFSA binder is different from that of the membrane.

International audience; The encapsulation of organic photovoltaic (OPV) devices can help mitigate the degradation induced by environmental factors like water and oxygen and thus potential to increase OPV lifetime. Because flexibility is an important parameter for targeted OPV applications, this paper proposes a fundamental study on the impact of the roll-to-roll flexible encapsulation process. Both performance and mechanical reliability of encapsulated devices have been scouted. Furthermore, it has been demonstrated that a relatively simple peeling technique allows understanding the role of the interfaces inside a multilayered OPV device supported by a flexible poly(ethylene terephthalate) substrate. For this purpose, the peeling strengths between each layer were measured using a series of partial devices. This provided a quantitative analysis of the mechanical strength or quality of each interface. Two interfaces revealed pronounced weaknesses: active layer with hole transporting layer and transparent conducting electrode with electron transporting layer. Among various surface treatments applied to improve these interfaces, an optimized UV-ozone (UVO3) treatment proved to modify substantially the surface properties of used zinc oxide (ZnO) and thus improved its adhesion to the neighboring layers. The physicochemical and structural changes of ZnO have been confirmed by IR spectroscopy and contact angle measurements. It has also been shown that better interfaces within the device improve the overall performance of the devices and their resilience to roll-to-roll encapsulation.

International audience

The long-term thermal performance of vacuum insulation panels (VIP) is brought by the capacity of their barrier envelope to maintain the core material under vacuum. This study is focused on the detailed modelling of gas transfer through the defects of aluminium-coated polymer films used for VIPs’ envelopes. The 3D simulations were performed with monolayer and multilayer metal-coated polymer films. They have been carried out in dynamic conditions with the SYRTHES® software developed by EDF R&D. The results show that the water vapour and air permeations through a monolayer film slightly depend on the polymer substrate thickness, diffusivity and solubility, but primarily, on the defects geometry and arrangement. Regarding multilayer films, the permeation can be deduced from the ideal laminate theory. We are now able to provide and operate a numerical model, which can calculate the permeance of monolayer or multilayer metallized polymer films as a function of the coating quality and the geometry of the layers. Even if calculated permeances are ten times higher than measurements, this study improves our understanding of gas transports through VIPs’ barrier envelope and allows to manage more efficiently the relations between the films microstructures and their overall permeability. This paper is split into 6 parts: physical phenomena, methodology and modelling tools, simulation results, experiments and model validation and then, discussion and conclusion.

Electrical stability and homogeneity of silver nanowire (AgNW) networks are critical assets for increasing their robustness and reliability when integrated as transparent electrodes in devices. Our ability to distinguish defects, inhomogeneities, or inactive areas at the scale of the entire network is therefore a critical issue. We propose one-probe electrical mapping (1P-mapping) as a specific simple tool to study the electrical distribution in these discrete structures. 1P-mapping has allowed us to show that the tortuosity of the voltage equipotential lines of AgNW networks under bias decreases with increasing network density, leading to a better electrical homogeneity. The impact of the network fabrication technique on the electrical homogeneity of the resulting electrode has also been investigated. Then, by combining 1P-mapping with electrical resistance measurements and IR thermography, we propose a comprehensive analysis of the evolution of the electrical distribution in AgNW networks when subjected to increasing voltage stresses. We show that AgNW networks experience three distinctive stages: optimization, degradation, and breakdown. We also demonstrate that the failure dynamics of AgNW networks at high voltages occurs through a highly correlated and spatially localized mechanism. In particular the in situ formation of cracks could be clearly visualized. It consists of two steps: creation of a crack followed by propagation nearly parallel to the equipotential lines. Finally, we show that current can dynamically redistribute during failure, by following partially damaged secondary pathways through the crack.

The hydrolytic stability of poly(ethylene terephthalate) (PET) has already been largely reported. The chemical reactions induced by damp-heat exposure are well–known, and various kinetic expressions for the degradation have been presented. Using the data from previous studies, a new model for degradation is proposed. This model combines the effect of temperature and humidity in a single equation. Three parameters are utilized: the classical pair of activation energy ( Ea ) and pre-exponential factor ( f 0 ), and the reaction order ( n ) to the relative humidity ( RH ). The model may be used to fit the degradation data from various sources describing the hydrolysis over a large range of conditions (40–100 %RH, 60–160 °C). In addition a prediction of the crystallinity changes brought about by hydrolytic chain scission was performed. Prediction of useful lifetime in moist heat is also possible (hydrolysis of 0.2% ester moieties in the polymer).

International audience

There is a need for increasing the durability of proton exchange membrane fuel cell systems. Membrane failure is usually generated by a chemical attack led by peroxide radical species. The chemical attack promotes pinhole formation that ultimately induces the shutdown of the fuel cell. An electronic short circuit between the anode and the cathode through the membrane has also been identified as a failure mode. However, the current resulting from $\hbox{H}_{2} $ crossover is estimated to be an order of magnitude larger than the electronic current. As a consequence, the electronic short circuit is often disregarded. In the present work, we revealed numerous local hotspots in pristine membrane electrode assemblies (MEAs) that are sensitive to short circuits. Catalyst layer heterogeneities were found responsible for these hotspots. In use, these flaws do not directly impact the global performance but may induce premature degradation. With the use of an electrical passive technique, one can identify the electronic short-circuit resistance of membranes simply by charging and discharging the double-layer capacitor of the MEA. The integration of this technique into fuel cell systems was possible, and measurements were performed at different ageing times. They revealed a gradual increase in the number of cells with short circuit annunciating the failure of the entire stack.

In order to improve their commercial liability, many scientific and technological efforts are being performed on fuel cell systems. The bipolar plates (BPP) represent a major part of the stack and play crucial roles in fuel cell. As a result an optimal material for BPP application should present an unusual balance of properties, essentially high electrical conductivity and good flexural strength. To date, even if different materials have been tested, polymer composites remain the most plausible solution. We propose a generic approach for the optimization of composite formulations for BPP applications. The most relevant parameters to be controlled were first determined. A performance index was thereby defined and optimized with the help of both experimental data gathered from literature and experimental results from the current study. The optimized composites were in the range of the DOE specification.

Vacuum Insulation Panels (VIPs) are super-insulating products that can be used for different applications. When they are used for building thermal insulation, they have to maintain a very high thermal performance in long-term. The initial thermal conductivity of VIPs is around 4 mW.m − 1 .K − 1 . This very low value will increase in time, depending on the nanoporous property of the core material and on the barrier envelope efficiency which prevents the increase of moisture and internal pressure. To estimate the long-term thermal performance of VIPs, modelling is required. Simulations have shown the very great importance of the core material characteristics, and the external temperature and humidity conditions. That is why it is necessary to study the VIPs behaviour with realistic solicitations in service life for each VIP configuration. This paper proposes a methodology for determining the long-term thermal performance of VIPs when they are installed in envelope components for various building thermal insulation applications. All the methodology including the climate conditions, the building modelling and the insulation systems, is described. Severity criteria and performance indicators are proposed in order to estimate the thermal efficiency of VIPs in various applications and climates.

The effect of nitrogen insertion in a benzothiadiazole (BT) moiety was investigated via the synthesis of a novel copolymer based on dithienosilole (DTS) and pyridalthiadiazole (PT), the heterocyclic counterpart. Stille cross coupling of the electron rich and electron poor units yields poly[(4, 4’-didodecyldithieno[3, 2-b:2’, 3’-d]silole-2, 6-diyl)- alt -(2, 1, 3-pyridalthiadiazole-4, 7-diyl), a nonsoluble polymer. Dense π–π stacking interactions that inhibited polymer solubility were cured by the introduction of two solubilizing hexylthiophene spacers, and a new soluble low-bandgap copolymer, namely poly[(4, 4’-didodecyldithieno[3, 2-b:2’, 3’-d]silole-2, 6-diyl)- alt -{4, 7-bis[2-(3-hexyl)thienyl]-2, 1, 3-pyridalthiadiazole-5, 5’-diyl}] (PDTSDTPT), was alternatively obtained. For reasons of comparison, the corresponding oligomer, i.e., ODTSDTPT, was also synthesized. Their chemical, thermal, optical, electrical, and electrochemical properties were investigated. UV–visible spectroscopy revealed that PDTSDTPT alternating copolymer is more absorbing than its BT analog. It has a lower optical bandgap, higher absorptivity, and red-shifted absorption spectrum into the near-infrared region up to 900 nm. Photovoltaic applicability was investigated for PDTSDTPT and its derived oligomer in bulk heterojunction solar cells employing PC 60BM, [6,6]-phenyl-C60 butyric acid methyl ester, as the n-type semiconductor. Significant improvement of open-circuit voltage was evidenced with PT-based materials. The short-circuit current ( J SC) and fill factor ( FF ), however, remain a material of continuous improvement by morphology control.

In this paper, dynamic percolation of carbon nanotubes in silicone oil was probed by means of electrical measurements. Carbon nanotubes were first dispersed in silicone oils with ultrasounds. The obtained suspension was then slowly diluted. After each dilution, the changes in electrical properties was monitored over time. This procedure enabled to collect a large number volume fraction and define both the percolation threshold and the critical exponents with unmatched accuracy. The percolation threshold was found to decrease with time as a consequence of the already described aggregation of conductive nanoparticles. More surprisingly, the critical exponent also largely varied over time. This changes were correlated to the quality of the dispersion of carbon nanotubes within the host matrix. Finally, statistic percolation model was adapted into one that take into account for conductivity changes as a function of time and filler content.

Through a fruitful collaboration between three academic laboratories and a proton-exchange membrane fuel cell (PEMFC) manufacturer, important insights into the origin of the cell voltage losses monitored during on-site 110-cell-PEMFC-stack long-term operation (12,860 h i.e. 1.5 years) were provided. The decline of electrical performance is heterogeneous at the stack and the membrane-electrode assembly (MEA) scales, as shown by the monitoring of individual cell voltages during the ageing process and segmented cell characterizations at the end-of-life. The long-term electrical performance losses were bridged to the degradation of the MEA constitutive materials. They mostly originate from the emergence of large size holes in the proton exchange membrane (PEM), which lead to detrimental hydrogen crossover. The hydrogen crossover in turn causes the formation of radical species that actively participate in the irreversible corrosion of the high surface area carbon support in the cathode region neighbouring the hole. The degradation of the carbon support further prevents the efficient access of the reactive gazes and the proper removal of the reaction products. Depending on the size of the hole in the membrane, the localized loss of electrical performance may extend to the whole MEA, and depreciate its global performance.

cited By 8; International audience; Two donor-acceptor (D-A) copolymers containing dithienosilole (DTS) donor unit and unsubstituted benzothiadiazole (BT) or fluorinated benzothiadiazole (ffBT) acceptor unit, PDTSBT and PDTSffBT, were synthesized by Stille cross coupling polymerization and tested for application in polymer solar cells (PSCs). The new alternating copolymer (PDTSffBT) possesses both a low optical bandgap (Eg = 1.54 eV) and a deep highest occupied molecular orbital energy level (HOMO) (-5.46 eV). It was found that the introduction of the two electron-withdrawing fluorine atoms on the benzothiadiazole unit results in a decrease of the HOMO energy level with slight effect on the lowest unoccupied molecular orbital (LUMO) as observed through cyclic voltammetry (CV) analysis. Inclusion of fluorine atoms also leads to an increase in the interchain interaction in the PDTSffBT copolymer with respect to its analogue PDTSBT copolymer according to the X-ray diffraction measurements (XRD). When PDTSffBT:PC71BM blends are tested in bulk heterojunction (BHJ)-polymer solar cells, the cells display a short-circuit current (J SC) of 8.82 mA/cm2, a high open circuit voltage (V OC) of 0.74 V, and a Fill Factor (FF) of 45%, giving an overall power conversion efficiency (PCE) of 2.93%, compared to 1.64% for the BT-containing cells prepared in parallel under identical conditions. The significant performance enhancement results from the higher VOC and J SC in the ffBT-containing cells. These results unambiguously indicate that the fluorination is an efficient method to modulate the energetic levels and improve the photovoltaic performances of the widely used benzothiadiazole-based low bandgap copolymers. © 2014 Elsevier Ltd. All rights reserved.

International audience; Through a tight collaboration between chemical engineers, polymer scientists, and electrochemists, we address the degradation mechanisms of membrane electrode assemblies (MEAs) during proton exchange membrane fuel cell (PEMFC) operation in real life (industrial stacks). A special attention is paid to the heterogeneous nature of the aging and performances degradation in view of the hardware geometry of the stack and MEA. Macroscopically, the MEA is not fuelled evenly by the bipolar plates and severe degradations occur during start-up and shut-down events in the region that remains/becomes transiently starved in hydrogen. Such transients are dramatic to the cathode catalyst layer, especially for the carbon substrate supporting the Pt-based nanoparticles. Another level of heterogeneity is observed between the channel and land areas of the cathode catalyst layer. The degradation of Pt3Co/C nanocrystallites employed at the cathode cannot be avoided in stationary operation either. In addition to the electrochemical Ostwald ripening and to crystallite migration, these nanomaterials undergo severe corrosion of their high surface area carbon support. The mother Pt3Co/C nanocrystallites are continuously depleted in Co, generating Co2+ cations that pollute the ionomer and depreciate the performance of the cathode. Such cationic pollution has also a negative effect on the physicochemical properties of the proton-exchange membrane (proton conductivity and resistance to fracture), eventually leading to hole formation. These defects were localized with the help of an infrared camera. The mechanical fracture-resistance of various perfluorosulfonated membranes further demonstrated that polytetrafluoroethylene-reinforced membranes better resist hole formation, due to their high resistance to crack initiation and propagation. WIREs Energy Environ 2014, 3:540–560. doi: 10.1002/wene.113For further resources related to this article, please visit the WIREs website.Conflict of interest: The authors have declared no conflicts of interest for this article.

Infrared and Raman microscopies are complementary tools for analyzing the chemical structures of polymer. In this paper, the study is specially devoted to the encapsulant materials of a flexible PV panel. With different characterization methods (optical microscopy, infrared spectroscopy, infrared and Raman microscopy, DSC, TGA), the structure of encapsulation system was characterized: the number and nature of the layers were determined. The materials used and the structure of the complex are especially designed for photovoltaic application. The encapsulation system was further characterized after an accelerated ageing at 80 °C/85% RH for 2000 h. The chemical modifications were studied by infrared and Raman microscopies.

International audience; The water sorption properties (i.e. diffusivity and solubility) of three unfilled vulcanized EPDMs were investigated in wet atmosphere (typically between 0 and 95% RH) and in pure distilled water (denoted as 100% RH) at 70 °C with an IGAsorp dynamic sorption analyzer. In the 0–0.5 activity range, all EPDMs obey to the usual Fick and Henry's laws. In contrast, at activities higher than 0.5, the water sorption behavior of all EPDMs deviates from these two laws because of the formation of water clusters. Park's equation was used for simulating accurately the sorption isotherms and thus, accessing Henry's constant and the average size of water clusters. In wet atmospheres, clusters would be composed of about 8 ± 4 molecules corresponding approximately to the size of free volume holes. In contrast, in pure distilled water, clusters consist in a much larger number of water molecules (between 55 and 71) presumably because of an exacerbated swelling of EPDM samples. Thus, immersion in water solution is more drastic than exposure to fully saturated wet atmosphere. The impact of the EPDM formulation (nature and content of crosslinking agent) on these experimental results is clearly of the second order of magnitude.

One of the most harmful degradation process in PEM fuel cell is the development of pinholes in the membrane. There is therefore a need for an effective experimental characterization to allow ab initio membrane comparison. In this paper, the mechanical fracture resistance of various PFSA membranes was studied using the essential work of fracture (EWF) and tensile tests. PTFE reinforced membrane better resists pinholes formation due to its high resistance to crack initiation and propagation. Additionally, energy partitioning showed that the necking and tearing stage of the layered structure membrane accounts for the main part of the total fracture energy due to enhanced plastic deformation of PTFE. Moreover, cracks were found to initiate and propagate easily in the direction parallel to the polymer chains which suggest that the fracture control could be optimized by pointing the direction of the gas channel perpendicularly to the orientation of the polymer chains, i.e. to rolling process during manufacturing. Finally, EWF technique was found to be more relevant for assessing the differences in the mechanical behaviour of the membranes compared to standard tensile tests.

Polymer films with enhanced dielectric and breakdown properties are essential for the production of high energy density polymer film capacitors. By capitalizing on the synergistic effects of forced assembly nanolayer coextrusion and biaxial orientation, polymer multilayer films using poly(ethylene terephthalate) (PET) and a poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDF-TFE)] copolymer were produced. These films exhibited breakdown fields, under a divergent field using needle/plane electrodes, as high as 1000 kV mm−1. The energy densities of these same materials, under a uniform electric field measured using plane/plane electrodes, were as high as 16 J cm−3. The confined morphologies of both PET and P(VDF-TFE) were correlated to the observed breakdown properties and damage zones. On-edge P(VDF-TFE) crystals induced from solid-state biaxial stretching enhanced the effective P(VDF-TFE) layer dielectric constant and therefore increased the dielectric contrast between the PET and P(VDF-TFE) layers. This resulted in additional charge buildup at the layer interface producing larger tree diameters and branches and ultimately increasing the breakdown and energy storage properties. In addition to energy storage and breakdown properties, the hysteresis behavior of these materials was also evaluated. By varying the morphology of the P(VDF-TFE) layer, the low-field dielectric loss (or ion migration behavior) could be manipulated, which in turn also changed the observed hysteresis behavior. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 882–896

The dynamic and equilibrium water vapor sorption properties of amorphous polyethylene terephthalate were determined via gravimetric analysis over a wide range of temperatures (23-70 °C) and humidities (0-90% RH). At low temperature and relative humidity, the dynamics of the sorption process was Fickian. Increasing the temperature or relative humidity induced a distinct up-swing effect, which was associated with a plasticization/clustering phenomenon. For high temperatures and relative humidity, a densification of the polymer was evidenced. In addition to the classical Fickian diffusion, a new parameter was introduced to express the structural modifications of PET. Finally, two partial pressures were defined as thresholds that control the transition between these three phases. A simplified state diagram was finally proposed. In addition, the thermal dependence of these sorption modes was also determined and reported. The enthalpy of Henry's water sorption and the activation energy of diffusion were independent of vapor pressure and followed an Arrhenius law.

Polymer–metal multilayer films provide attractive barrier properties to air and water, provided by several films bonded together. Used in high temperature and humidity, the delamination is the most common degradation mechanism. This mode of failure is critical because it leads to premature loss of barrier properties. Therefore, delaminations limit the use of multilayer in these conditions. Delamination should result from interfacial shear stress, and plausibly from difference of shrinkage of the films, due to relaxation of internal stress. A method was developed for measuring the deformations of each individual film. The measurements indicate that the shrinkage is anisotropic and controls the decohesion in the multilayer. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016.

For the first time, an extrusion process is used to produce a perfectly nanostructured organic photoactive layer. Extrusion is an interesting, inexpensive and environment-friendly (solventless) option allowing an industrial production scale for polymer solar cell technology. In this paper, this plausible alternative to the usual large scale wet-processing techniques (inkjet printing, roll-to-roll coating…) was considered in order to elaborate bulk lamellar ordered heterojunctions employing polythiophene and fullerene based materials, as the electron donor and acceptor respectively, which were specifically elaborated for this work. An alternating donor/acceptor (D/A) lamellar structure close to the ideal interpenetrating system was targeted and successfully realized. Continuous D and A domains of about 24 nm were obtained, i.e. size close to the exciton diffusion length (20 nm) for π-conjugated organic materials; a photovoltaic response was evidenced under AM1.5 light simulator conditions. This photovoltaic effect, observed in spite of the presence of insulating polymers (∼61 wt%) added in both semi-conductor materials in order to adjust their viscosities, proves the possible achievement of such a perfect working nano-architectured structure via the extrusion process. The next step, consisting in the use of pure D and A extrudable polymers, should thus lead to high and durable photovoltaic performances. Therefore, this paper opens the way to a new promising OPV processing method: extrusion.

International audience

A novel low bandgap copolymer based on dithienosilole and pyridalthiadiazole (PDTSDTPT) is synthesized via Stille cross coupling. Its chemical, thermal, optical, electrical, and elctrochemical properties were investigated. We evidenced by UV-vis Spectroscopy that PDTSDTPT with modulated HOMO and LUMO energy levels exhibit higher absorptions over a larger fraction of the solar spectrum than the BT analogue with an absorbance that reaches around 900 nm in the infrared region. The photovoltaic applicability of this specific polymer was confirmed by preliminary tests in devices based on bulk heterojunction (BHJ) employing PC 60 BM, [6,6]-phenyl-C 60 butyric acid methyl ester, as the n-type semiconductor under irradiance of 100 mW cm−2. The results indicate that despite the low mobility of PC60BM and without control of the morphology, PDTSDTPT show promising improvable performances. Open circuit voltage (Voc) close to 0.7 V, short circuit current (J SC ) close to 4.6 mA cm−2 and power conversion efficiency (PCE) of 1.2% were obtained. However, in current work, we expect that further improvement of device performance will be achieved via the substitution of PC 60 BM by semi-conducting carbon nanotubes (s-SWNT) in the photoactive layer that should increase the charge mobility and enhance the device performance.

We here report the synthesis of a new donor copolymer family based on benzobistriazole and dithienosilole. In the present work, the separation and purification of bromo-benzobistriazole isomers was realized for the first time, allowing the synthesis of a series of monomers with different positions of the solubilising alkyl chains. The thermal, photophysical and electrochemical properties of resulting synthesized copolymers are depicted. It should be noted that polymers with optical bandgaps ranging from 1.11 to 1.65 eV were obtained allowing to optimize the covering of the solar emission spectrum.

This paper describes the effect of electric field on the structuration of carbon nanotubes in silicone oil. The particles have been dispersed in the liquid by ultrasounds and diluted in order to vary the filler content. Electrical measurements were performed under different electric fields and over time on each filler content, in order to probe the particles structuration. From a critical value, the electric field was found to both increase the conductivity of the composite and reduce percolation threshold. This effect is further enhanced with time. Spectacular percolation thresholds, as low as 22 ppm have been evidenced. This result was attributed to a strengthening effect between filler contacts. More surprisingly, the conductivity remained high after the electric field was stopped, showing the irreversible nature of this effect. In addition, an analytical model has been developed to describe the conductivity of the composite as a function of three parameters: nanotubes content, time and electric field.

Dispersion and percolation of MultiWall Carbon NanoTubes (MWCNT) was studied in silicon oils by means of electrical measurements. After MWCNTs were homogenously dispersed with ultrasounds in silicon oil, the composite was slowly diluted to obtain a large number of filler content. Conductivity measurements were done for each filler content allowed us to determine the percolation threshold and the corresponding critical exponent with an unmatched accuracy. A decrease of percolation threshold over time has been observed over time and was correlated to MWCNT aggregation. More surprising, critical exponent also varies over time, which suggests a strong correlation with the quality of the filler dispersion in the matrix. Owing to the large number of data collected, a model derived from static percolation was established. The latter describes conductivity as a function of time and filler content.

The dielectric lifetime and corresponding damage morphology of polycarbonate/poly(vinylidene fluoride-co-hexa- fluoropropylene) (PC/P(VDF-HFP)) layered systems are studied under constant direct current (DC) field. Melt blends of the two polymers are also considered for comparison. The dielectric lifetimes of the latter are systematically much shorter than the layered systems. The interfaces between the polymers act as flaws that induce up to two orders of magnitude difference between the layered and blend systems. The capacitance val- ues versus time during breakdown progression exhibit an inversed S-shape pattern. The three regimes in the S-shape pattern are consecutively attributed to randomly isolated break- downs, interconnecting breakdowns, and wearing-out of the capacitor film. The film breakdown images during dielectric lifetime test confirmed the transition from randomly isolated breakdowns to interconnecting breakdowns. This transition was further evidenced by a bimodal distribution in the Weibull analysis. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 993-1003, 2012

Polyelectrolyte membrane used in proton exchange membrane fuel cell (PEMFC) should act as a barrier to hydrogen and electrons. In the present work, fast and robust specific tools were developed both to characterize these properties and localize defects, such as pinholes or short-circuits, within the membrane electrode assembly (MEA). The identification of defective cells without disassembling the Axane's stack was performed in a single step through a passive electrical technique so-called “relaxometry” and by internal gas leak measurements. The flaws were further tracked inside MEA using Infrared Thermography in order to obtain their precise location. The latter, performed after stack disassembly, permitted to reveal the defects responsible for the Fuel Cell shutdown. A third step dealt with SEM examinations of the flaws in order to identify the origin and nature of the membrane failure.

Micro/nanolayer coextrusion was used to fabricate polycarbonate (PC)/poly(vinylidene fluoride) (PVDF) layered films with significantly reduced dielectric losses while maintaining high energy density. The high-field polarization hysteresis was characterized for layered films as a function of PVDF layer thickness (6000 to 10 nm) and composition (10 to 70 vol % PVDF), and was found to decrease with decreasing layer thickness and PVDF content. To gain a mechanistic understanding of the layer thickness (or nanoconfinement) effect, wide-angle X-ray diffraction, polarized Fourier transform infrared spectroscopy, and broadband dielectric spectroscopy were employed. The results revealed that charge migration, instead of dipole flipping, was responsible for the hysteresis in multilayered films. The absence of PVDF dipole-flipping was attributed to the nonuniform electric field distribution in the layered structure, where the field in PVDF layers were calculated to be significantly lower than that in PC layers due t...

The ion exchange capacity (IEC) of conventional ion-exchange perfluorinated membranes based on the so-called Long-Side-Chain (LSC) polymers and their Short-Side-Chain (SSC) Aquivion® derivatives was determined using a series of experimental techniques. Newly developed and already used experimental analyses were compared in terms of sensitivity to the determination of the ion exchange capacity. The use of complementary techniques allows better determination of IEC with an uncertainty of about 3%. This IEC determination method was then applied to membranes with large chemical differences such as copolymers and reinforced membranes. In addition, based on these analyses, relationship between DMA, WAXS, TGA and conductivity parameters and either IEC or the molar number of tetrafluoroethylene (TFE) per comonomer unit are underlined, leading to a more comprehensive approach of architecture in perfluorosulfonic acid (PFSA) polymer membranes.

Layer-multiplying coextrusion was used in conjunc- tion with isothermal recrystallization to study the confined crys- tallization of polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-tetrafluoroethylene (PVDF-TFE) using polycarbonate (PC) and polysulfone (PSF) as confining materials. Three layered systems were produced (PC/PVDF, PSF/PVDF, and PC/PVDF-TFE) with layer thicknesses ranging from 525 to 28 nm. The crystal morphology was affected by both layer thickness and recrystalli- zation temperature. Specifically, increased recrystallization tem- perature and decreased layer thickness facilitated the formation of high aspect ratio in-plane crystals in both PVDF based poly- mers. On the other side of the spectrum, thicker layers and lower recrystallization temperatures produced on-edge PVDF crystals and isotropic PVDF-TFE crystals. The morphology was corre- lated with oxygen permeability, which decreased by almost two orders of magnitude compared with the bulk. A variety of crystal structures were obtained and explained with nucleation and dif- fusion theory. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1750-1761, 2011

The changes in properties within membrane electrode assemblies (MEAs) aged in a stack functioning at constant-power operation (0.12 W cm −2 ) for several durations (0, 347, 892, and 1397 h) were characterized. An important effort was placed into better understanding interfaces. Two tests were thus developed to investigate the changes in each active layer/membrane interface. Both techniques demonstrated that the mechanical bounding of both cathode and anode to the polymer membrane improve with the functioning time in fuel cell. This phenomenon was further attributed to Pt dissolution and diffusion/precipitation within the polymer membrane and to a diffusion/crystallization of the binding agent in the vicinity of the electrode/membrane interfaces.

Different accelerated tests in 12 fuel cells stack were performed in laboratory, namely on/off, back-up, and base-load regimes. In parallel, membrane electrode assemblies (MEA) were integrated in two ''on- site'' systems for GSM relay application. One of them was dedicated to base-load power applications while the second fuel cells coupled with photovoltaic panels operated in semibase load mode. To investigate the influence of the power profiles on MEA degradation, over 80 CCB MEAs (5 layers) were studied at different scales using ex situ characterizations such as tensile tests, TGA-MS, DMTA, and SEM. A series of complementary microstructural age- ing markers were thereby identified. The isolated influence of dry-wet cycling on MEA properties was also established after passive hydro-thermal (HT) ageing performed contin- uously for 10 months in the laboratory. The changes of each marker as a function of HT ageing time permitted to define a temporal benchmark. Based on these indicators, the main changes occurred in the MEA properties appear after a 5 months dry-humid cycling (up to about 1800 cycles). The trends observed were useful to compare and estimate the degree of degradation of each ageing tests. Thus, the accelerated tests performed in laboratory for at least 500 h in stack did not reveal systematic MEA modifications. On the contrary, the 1500 h ''on-site'' system operation results in some MEA degradations which origins are discussed. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3501-3510, 2011

The influence of controlled cationic contaminant on the thermal behaviour of proton conducting perfluorosulfonic acid membrane (PFSA) was studied. A series of Ni2+-exchanged membrane was prepared by varying the ratio between positive charges resulting from the contaminant to the negative charges originating from the sulfonate groups. A large change in properties was evidenced at a ratio close to 0.2. Above this threshold, the PFSA exhibited significantly higher main relaxation and degradation temperatures. The set of experimental data could be described with the help of a phenomenological microstructural model essentially based on ionic interactions. The impurity model could then be employed to understand changes in protonic conductivity from the literature.

This article focuses on description and evaluation of multilayer films, which are used for vacuum insulation panels (VIP). The materials investigated include a neat polyethylene terephthalate (PET) and PET films coated by chemical vapor deposition with different aluminum thicknesses. The aim of these investigations was to evaluate the influence of structural features on the physical properties of Al/PET systems. The morphological analyzes associated to water vapor permeability measurements showed that the metallized surface exhibited defects through which a small amount of water vapor can permeate. It is shown that the permeance varies linearly with the surface fraction occupied by pinholes which in turn changes linearly with the aluminum thickness.

International audience; In a first part, this paper focuses on the interest of using a multilayer for vacuum insulation panels envelope. It is shown that piling up layers of metallized poly(ethylene terephthalate) does not seem to improve the initial barrier performance. In parallel, a permeation model to predict barrier properties of multilayer films was developed and validated with experimental results. In a second part, durability of a multilayer composite was investigated using accelerated hydrothermal ageing tests. It is shown that physical properties and in particular barrier properties decrease with ageing time. Polyurethane layer used as adhesive system in multilayer films is responsible for complete degradation of aluminum layer after 6 ageing months at 70 degrees C - 90% relative humidity. Copyright (C) 2009 JohnWiley & Sons, Ltd.

Some properties of perfluorosulphonated ionomer membranes contaminated by a series of 10 counter ions were investigated by infrared spectroscopy (FTIR), thermogravimetric analysis coupled to mass spectroscopy (TG-MS), and dynamic mechanical spectrometry (DMA). Distinctive parameters were extracted and regarded as a function of the cations' properties. An optimum interaction between sulfonate group and cation was found for cations with Lewis Acid Strength (LAS) in the 0.2―0.3 range. This critical value is found to be the Lewis Basic Strength (LBS- SO - 3 ) of the sulfonate anion in Nafion membrane. Thermal stability analyses also point out the influence of this cation parameter on the polymer degradation process. Cations with LAS values lower than LBS- SO3 improve the thermal stability of the side chains while cations with LAS values higher than LBS- SO - 3 enhance the thermal degradation. Moreover, the temperature of the modulus drop increases with the LAS of the counter ion. For cations with values lower 0.5, the transition is attributed to the glass relaxation of the polymer while for cations showing LAS values higher than 0.5, the loss of stiffness originates from the polymer thermal degradation process. The overview of the experimental data allows the definition of calibration curves as a function of the cations' LAS.

The fracture toughness is key parameters to select polymeric films. The essential work of fracture (EWF) is a phenomenological but efficient way to characterize this resistance to fracture. One can gain valuable information on the resistance to perforation and propagation of flaws. A new technique was developed to better understanding the EWF experiments. A tensile test combined to photoelasticimetry allows following in situ the geometry and amount of plastic deformation on double edge notched specimen. The EWF parameters are determined when the plastic deformation appears constant, so when the fracture energy Wf only contributes to rupture filament. This new methodology requires just a single sample, whereas at least five specimens are required for general method. It will help characterize expensive polymeric films or reveal the heterogeneous behavior, for instance after polymer ageing. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 132, 42854.

International audience; This study focuses on the characterization of two complete stacks of a proton exchange membrane fuel cell that have operated in stationary mode for 12,860 h. We compare cell voltage, and in situ and ex situ leak tests with local cell performance from a segmented cell, showing large differences in degradation between the stacks. Cells near the warmest region show more membrane degradation in the hydrogen inlets. We build a membrane thickness profile over a large area from thousands of optical microscopy images, covering seven channels and lands around the hydrogen inlet region, and identify a dramatic difference between channels 1 and 2, where dry hydrogen enters, and channel 3, where hydrogen has travelled 26 cm. Using a polytetrafluoroethylene reinforced membrane, we discriminate thickness variation for each layer and we show that degradation commences from the anode side.

This paper describes the effect of electric field on the structuration of carbon nanotubes in silicone oil. The particles have been dispersed in the liquid by ultrasounds and diluted in order to vary the filler content. Electrical measurements were performed under different electric fields and over time on each filler content, in order to probe the particles structuration. From a critical value, the electric field was found to both increase the conductivity of the composite and reduce percolation threshold. This effect is further enhanced with time. Spectacular percolation thresholds, as low as 0.0024 vol %, have been evidenced. This result was attributed to a strengthening effect between filler contacts. More surprisingly, the conductivity remained high after the electric field was stopped, showing the irreversible nature of this effect. In addition, an analytical model has been developed to describe the conductivity of the composite as a function of three parameters: nanotubes content, time and electric field.

This paper describes both an experimental study and a numerical investigation of the transverse mechanical behaviour of unidirectional polypropylene/glass fibre composites. A deterministic numerical simulation of the mechanical damage that accounts for the organization within real composites is proposed. This numerical method is based on classical finite element analysis, with tailored phase arrangement that unequivocally accounts for the real samples. This technique permitted some light to be shed on the origin of the transverse brittleness of these composites. It is shown that the number of damaged zones as a function of the applied strain follows a Weibull distribution. The influence of the volume fraction of filler was also estimated, both through experimental and numerical ways. It can be concluded that the reduction of the average distance between fibres increases the local stress enhancement and favours the degradation mechanisms of composites, especially at the glass fibres/polymer interface. The paper also proposes a novel analytical relationship linking the number of damaged zones to the macroscopic transverse mechanical behaviour of commingled composites.

International audience; The low thermal conductivity of Vacuum Insulation Panels (VIPs) degrades with time due to gas perme-ation through VIPs barriers. There is additional ageing of their envelope and core material. Accelerated experiments are carried out in order to better understand this ageing process but they are not sufficient to predict the long term performance of panels. Models have to be used to connect the short term evaluation and the long term behaviours in order to improve the prediction of the thermal conductivity evolution over 50 years. This paper describes the development of a VIP model in the Dymola ® software. This model takes into account the envelope and core material hydro-thermal characteristics and behaviours, and integrates the actual solicitations of the panels during accelerated ageing tests. The thermo-activation of the envelope permeance is integrated. Many properties of the core material are modelized: type of core material, sorption isotherm, hygro-thermal ageing, pore size distribution, etc. Simulations in constant conditions in temperature and humidity have been carried out. The results show that the real behaviour of the VIPs conductivity can't be simply evaluated through a linear extension of its short-term evolution. Given the important impact of the core material detailed characteristics, it is absolutely necessary to get an accurate determination of the core material sorption curve and of the ageing of this curve.

The aim of this study was to develop Carbon–polymer composites with extreme electrical conductivity (100 S/cm) combined with good flexural strength. Despite the many optimization methods described in the literature, no comprehensive optimization procedure was to be found because the formulation did not control by itself the final properties. This study showed the major influence of the processing conditions with these peculiar materials. A detailed study of the influence of the processing conditions on the microstructural and macroscopic properties was performed. We, thereby, proposed a comprehensive way to optimize the properties of the final product. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42274.

International audience; 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.

Aging of epoxy based composites used as insulators in the industry are investigated. The analysis of a composite aged for 20 years in actual service conditions points out chemical degradations, a structural recovery, a tendency for particle debonding from the polymer matrix and with respect to the unaged sample, and no significant changes in the permittivity values measured at 50°C. To understand this latter result, a purely physical aging was carried out on fresh composites with similar formulation as the insulator aged 20 years in service. A detailed analysis of the changes in properties during physical aging is proposed. It is concluded that the constant value at 50°C found for the 20 years aged composite results from two opposite phe- nomena: an increase in the relative dipole amount due to structural recovery and/or the presence of polar species at the polymer/filler interface that both increase and the loss of polar species in the bulk that decreases the . © 2006 Wiley

International audience; This paper reports in situ measurements of electrical resistivity during mechanical solicitation of insulator-conductor composite. Electrical and mechanical behaviors will be discussed on 3 examples namely (i) unidirectional carbon fiber reinforced polymers (CFRP), (ii) stick-like whiskers reinforced soft thermoplastics and (iii) carbon nanotubes reinforced soft thermoplastics. Resistivity measurements are described as a useful tool to monitor the damage of these composites. In the case of composite materials consisting in reinforced insulated matrix by electrically conducting fibers or fillers, resistivity (or conductivity) measurements show classically several features such as strong anisotropy in long fiber composites or the existence of continuous direct current (DC) conducting pathway for short fiber or particulate composites. Moreover, and this is the topic discussed here, in terms of mechanical stress/strain properties, the fibers or fillers conducting network may exhibit resistivity changes due to fiber ruptures, delamination, filler-filler debonding within filler network, etc.