N. D. Alberola

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.

Mechanical modeling of the transcrystalline interphase behavior in commingled PBT/glass fiber composites

In a previous work, a mechanical model was proposed to predict the reinforcement of amorphous polymers by particulates as well as by unidirectional fibers over wide ranges of volume fractions of fillers and temperatures (or frequencies). This model is based on both the definition of a representative morphological pattern (RMP), accounting for the presence of fiber clusters, and a quantitative morphology analysis, based on the percolation concept. In this work, such an approach is extended to describe the viscoelastic properties of a semicrystalline polymer, poly(butylene terephthalate), commingled with 30 and 50 vol % of unidirectional glass fibers. It is found that aggregates constituted by both fiber clusters and a transcrystalline region (TCR) can act as the continuous phase. Based on the use of a mechanical model in a reverse mode, the actual viscoelastic behavior of this TCR is extracted and compared to that displayed by the unfilled polymer.

Uniirectional fibre-reinforced polymers: analytical morphology approach and mechanical modelling based on the percolation concept

Abstract An improved self-consistent scheme accounting for the 21) geometric arrangement of fibres within the matrix is proposed to predict the influence of the reinforcement effect of DGEBA/anhydride matrix by unidirectional raw glass fibres or coated fibres with silicone or DGEBA/silane coupling agents. This model is based on both (i) an analytical morphology approach underlying the percolation concept and (ii) the definition of an original ‘representative morphological pattern’ accounting for local phase inversions owing the heterogeneous morphology of composite materials. It is shown that the nature of fibre sizing can influence the viscoelasticity of composite materials by inducing variations in morphology and then in the mechanical coupling. This could result from the more or less good wettability of fibres by the resin during the process. After removing the reinforcement effect through such a theoretical approach, microstructural changes of the polymer matrix induced by fibres, i.e a decrease in the crosslinking degree of the epoxy network, are quantified but no specific influence of the nature of the interface can be revealed.

Binary and ternary particulate composites. I. Viscoelastic behavior

Dynamic mechanical behaviors of binary and ternary particulate composites are investigated to test for the influence of the nature of a polymeric adduct, i.e., maleated styrene ethylene-co-butylene styrene (SEBS) or styrene-co-methacrylic acid (SAMA) copolymers, at the polystyrene (PS)/glass-beads interface on the mechanical coupling and the adhesion quality between phases. Then, to separate the influence of these two causes, a rigorous modeling of the viscoelastic properties of ternary particulate composites is proposed. Thus, by comparing experimental and theoretical data, it is concluded that the rubbery and rigid adducts both induce a change in the magnitude of the mechanical coupling between phases. It is shown, however, that the SAMA coupling agent exhibits a better compatibility with the PS matrix than that of maleated SEBS. This could result from the difference in the wt % of styrene segments between these two copolymers.

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.

Binary and ternary particulate composites. II. Tensile behavior over a wide range of strain rates

Tensile properties of both a binary material, i.e., polystyrene (PS) reinforced by 15 vol % of glass beads, and ternary composites, i.e., showing either a maleated styrene/ethylene-co-butylene/styrene copolymer or a styrene-co-methacrylic acid copolymer (SAMA) adduct at the PS/glass-beads interface, are analyzed at room temperature and over a wide range of strain rates. Because the poor quality of the adhesion at the PS/glass-beads interface, the fracture toughness of these binary composites is strongly reduced, whatever the strain rate. The presence of the rubbery interlayer does not change the deformation mechanisms of the composite and the work to break is not significantly enhanced. This results from the poor compatibility between PS and the rubbery interphase leading to the debonding of coated glass beads. The good adhesion quality at the interfaces between phases in the ternary composite showing the SAMA adduct, i.e., SAMA/glass-beads and PS/SAMA interfaces, hinders the decohesion phenomenon. This results in an improvement in both the transfer load and the maximum strength.

Spatial distribution of the electrical conductivity in highly filled polymers: Experiment, modeling, and application to bipolar plates

A large variety of composites for electrical applications are developed worldwide on a daily basis. Most of these materials are made from carbonaceous fillers dispersed in polymers. The optimization of the formulations is complex and depends on parameters that are difficult to identify ab initio. The results might also be very sensitive to the processing conditions. There is therefore a need for a fast and accurate method to measure the electrical properties of samples with unfamiliar geometric features and without altering their shape. A four point probes method is incremented that fulfills all the above mentioned requirements. An analytical model is proposed that extends Uhlirs theory and permits to quickly determine the resistivity distribution. Experimental and theoretical approaches were performed to validate the method. An example is proposed with the measurement of samples initially designed for fuel cell-bipolar plates-application.

Evidence for mechanical coupling effects in binary polymer blends: Relationships with morphology

The viscoelastic properties of binary thermoset and thermoplastic polymer blends were investigated in connection with blend morphologies. Christensen and Los model was used to predict mechanical coupling effects in such binary multiphased systems by accounting for the actual morphology of samples. Thus, it was shown that the magnitude of mechanical coupling effects between phases in polymer blends, as in composite materials, depends not only on mechanical properties and relative content of each phase but also on the geometric arrangement of the polymeric phases. Furthermore, based on both theory and experiment, a well-suited probe of blend morphology was also proposed.

Crystallization behavior of PP in a new generation of composite material

The microstructure of isotactic polypropylene (iPP) has been investigated during different steps of the processing of commingled PP/unidirectional glass fiber composites. From wide angle x-ray scattering and differential scanning calorimetry analysis, complemented by density measurements, it can be concluded that the crystalline phase in both un-reinforced PP and composite materials is only constituted of the α phase. The morphology of the crystalline phase and the two-dimensional geometric arrangement of fibers within the matrix of commingled composites reinforced by 45, 60, and 75 wt% of fibers have also been investigated using optical microscopy. The composites exhibit heterogeneous morphology whatever the fiber content is. Moreover, large spherulites can be distinguished, but the presence of a transcrystalline phase around the fibers cannot be detected.