Corine Bas

Tensile mechanical properties of PEEK films over a wide range of strain rates. II

Tensile mechanical properties of poly(aryl ether ether ketone) (PEEK) films showing different thermal histories have been investigated at room temperature to point out the main key microstructural features governing properties over a wide strain rate range, i.e., from 10−5 to 300 s−1. The strain rate sensitivity of the mechanical properties of amorphous PEEK films significantly depends on the analyzed strain rate range: i.e., 1) from 10−5 to 10 s−1, the strain rate dependence of both apparent Youngs modulus and yield stress is weak; and 2) from 10−1 to 200 s−1, both parameters significantly increase. Thus, based on the definition of the relationships between temperature, strain rate, and frequency respectively used for tensile tests and dynamic mechanical spectrometry, it was shown that the mechanical behavior of PEEK films at room temperature could be governed by similar molecular mechanisms as those giving rise to the β1 and β2 transitions. The Eyring analysis shows that motions of five or six monomers are implied at the beginning of the plastic deformation of amorphous and semi-crystalline PEEK films, while at higher strain rates, shorter chain segments are concerned. Thus, the crystalline phase only induces an increase in the stress level because of the reinforcement effect but does not modify the molecular mechanisms governing the plastic deformation of PEEK films at room temperature.

CRYSTALLIZATION KINETICS OF POLY(ARYL ETHER ETHER KETONE): TIME-TEMPERATURE-TRANSFORMATION AND CONTINUOUS-COOLING-TRANSFORMATION DIAGRAMS

Abstract The isothermal and nonisothermal crystallization of poly(aryl ether ether ketone) (PEEK) films have been investigated using differential scanning calorimetry, density and wide-angle X-ray scattering. An original method to determine the crystallization domains taking into account the crystallized fraction is proposed. Based on metallurgy concepts, Time-Temperature-Transformation (TTT) and Continuous-Cooling-Transformation (CCT) diagrams for isothermal and nonisothermal conditions, respectively, are established for PEEK. The “C” shape of the TTT diagram gives evidence for the competition between nucleation and growth phenomena. At high temperature, crystallite growth predominates while at low temperature, nucleation phenomenon prevails. Changes in lamellae thicknesses, according to the crystallization temperature, have been related to the predominance of nucleation or growth phenomenon. The CCT diagrams show a similar shape and give evidence for a decrease in crystallization ability of PEEK with increasing the cooling rate. Isothermal and nonisothermal crystallizations are discussed through the Avrami analysis.

Copolyimides with trifluoromethyl or methoxy substituents. NMR characterization

Abstract In the first stage, a series of aromatic diamine compounds such as 2-methoxy-5,4′-diaminodiphenyl ether (ODAOMe) and 2-trifluomethyl-4,4′-diaminodiphenyl ether (ODACF 3 ) were synthesized. These aromatic diamines and 4,4′-diaminodiphenyl ether (ODA) were then used to prepare copolyimides with 4,4′-oxydiphthalic anhydride (ODPA) and bicyclo[2.2.2]-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA). Both chemical composition and the arrangements of repetitive units were characterized by 1 H and 19 F NMR. It was shown that solubility and thermal stability are related to the BCDA fraction in the copolymers and to the chemical structure of the diamine.

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.

Reinforcement effect and molecular motions in semicrystalline PEEK films: Mechanical and physical modelings. I

The influence of the crystalline phase on the viscoelastic behavior of poly (aryl ether ether ketone) (PEEK) films is assessed by dynamic mechanical spectrometry. Prediction of the viscoelastic behavior near T g of semicrystalline films is performed through mechanical and physical modelings. Changes in the a relaxation induced by the crystalline phase are related to both the mechanical coupling between phases and the decrease in the molecular mobility of chains, which is improved for samples showing a broad crystallite size distribution. Crystalline phase also induces some modifications in the characteristics of the β spectrum. The reinforcement effect brought by the crystalline phase in such a temperature range is predicted through a mechanical model. Then, changes in tan δ level in the β 1 region induced by the crystalline phase result from the mechanical coupling between phases. The magnitude of such changes only depends on the crystallinity ratio and it is not controlled by the crystallite size distribution. The crystalline phase also induces changes in the pattern of the β 2 transition, which could be attributed to modifications in the conformations of the chains near the crystalline entities and/or the magnitude of interactions between chains. Such modifications seem to be sensitive to the thermal history of PEEK samples.

In Situ Quantification of Electronic Short Circuits in PEM Fuel Cell Stacks

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 H2 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.

Contribution of the dynamic mechanical spectroscopy for studying the microstructure of polyimides

Different polyimide films based on various aromatic diamines and dianhydrides have been studied by dynamic mechanical thermal analysis. Polyimides exhibit three mechanical relaxations related to specific molecular motions. We have analyzed the sub-glass gamma relaxation which appears at 1Hz in -140°C to -50°C temperature range. This relaxation originates from water molecules in polyimides. Its temperature location strongly depends on the chemical structure of polyimides. Then, the temperature of this relaxation process was correlated to microstructural parameters. It was found that the gamma relaxation shifts towards higher temperatures with: (i) decreasing the free volume; (ii) decreasing the intersegmental distance determined X-ray diffraction; (iii) increasing the wavelengths of 50% transmission determined by UV-visible spectroscopy.

Water Vapor Sorption Properties of Polyethylene Terephthalate over a Wide Range of Humidity and Temperature

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 Henrys water sorption and the activation energy of diffusion were independent of vapor pressure and followed an Arrhenius law.

A study of positronium formation in anodic alumina

Al2O3 samples grown with natural well-ordered channels (having sizes of 30–90 nm in diameter) perpendicular to the surface, one side closed, were measured by means of Doppler broadening (DB) by variable energy positron (VEP) beam and positron annihilation lifetime (PAL) spectroscopy. The samples annealed at 300 and 560 °C showed amorphous structure and no positronium (Ps) formation. The sample annealed at 850 °C showed γ-phase polycrystalline structure and Ps formation. We were not able to prove from the PAL spectroscopy results themselves whether Ps is confined to the channels. We discuss the results of decomposition of the DB annihilation peak into four Gaussians. We conclude that the greater Ps fraction is confined to the channels. From the Compton-to-peak ratio analysis, the fraction of all the injected positrons which form o-Ps and in this form escape from the sample (for a VEP experiment, we have e+ injected from the open side of the channels) has been estimated as being ~20%.

Positron interaction in polymers

A series of dense copolyimide membranes was characterized using positron annihilation spectroscopy. The positron annihilation lifetime spectroscopy performed on film with a classical positron source gives informations on the positronium fraction formed and also on the hole size within the film. The Doppler broadening spectra (DBS) of the gamma annihilation rays coupled with a variable energy positron beam allow the microstructural analyses as a function of the film depth. Experimental data were also linked to the chemical structure of the polyimides. It was found that the presence of the fluorine atoms strongly affects the positron annihilitation process and especially the DBS responses.