Analyse des relaxations enthalpiques, mécaniques et diélectriques pour l'étude du vieillissement d'assemblages collés structuraux

L'objectif de ce travail de thèse est d'analyser l'influence du vieillissement simulé en cyclage thermique sous vide sur des assemblages collés structuraux impliqués dans les applications spatiales. Le système étudié est un adhésif polyépoxy de formulation complexe, associé à des substrats aluminium. L'état initial de l'adhésif est dans un premier temps analysé par des techniques mécaniques, calorimétriques et diélectriques. Les réponses expérimentales complexes de l'adhésif en configuration opérationnelle sont résolues par une étude analytique avec séparation des principaux constituants de l'adhésif. Cette analyse de l'état initial indique une forte hétérogénéité de la structure aux échelles localisée et délocalisée. Elle montre une métastabilité de cette structure sous l'effet d'une première rampe en température au-dessus de la transition vitreuse du réseau. Par contre, une approche phénoménologique du vieillissement physique de l'adhésif montre que son incidence sur les propriétés des assemblages collés est limitée. L'influence de différents protocoles de vieillissement statiques et dynamiques est ensuite étudiée. Les effets du vide et de la température sont analysés lors de la comparaison entre les protocoles de vieillissement isotherme et en cyclage thermique. Le vide n'est pas identifié comme un facteur de vieillissement. La température maximale des cycles en température conditionne les effets du vieillissement sur les propriétés de l'adhésif et de l'assemblage. Les évolutions constatées de la structure de l'adhésif sont reliées à celles des assemblages collés. Elles restent toutefois limitées à une variabilité acceptable des propriétés fonctionnelles.The aim of this work is to analyse the influence of simulated ageing during thermal cycling under vacuum on structural adhesive joints used in space applications. An epoxy adhesive showing a complex formulation is chosen to bond aluminium substrates. The initial state is first analysed by mechanical, calorimetric and dielectric techniques. The complex experimental signals of the adhesive studied in representative configuration are solved by an analytical study based on separation of the adhesive constituents. This analysis of the initial state indicates a significant structural heterogeneity at localized and delocalized scales. The physical structure exhibits a metastability during the first temperature ramp above the network glass transition. Contrarily, a phenomenological approach of adhesive physical aging shows that its influence on the bonded joints properties is limited. The influence of static and dynamic ageing protocols is then studied. The effects of vacuum and temperature are considered thanks to the comparison between isothermal ageing and thermal cycling. The vacuum is not identified as an ageing factor. Considering adhesive behaviour, the maximum temperature determines the effects of thermal cycling. The adhesive structural evolutions are correlated with the assembly behaviour. However, they remain limited to an acceptable variability for the functional properties

Isabelle Graci, Marielle Rispail, Marine Totozani, L’Arc-en-ciel de nos langues. Jalons pour une école plurilingue

Le constat d’une inadéquation entre la formation des enseignant.e.s et la réalité des classes comme des espaces plurilingues et pluriculturels est à l’origine de l’ouvrage, L’arc-en-ciel de nos langues, dirigé par Isabelle Graci, enseignante de français langue seconde et coordinatrice du Dispositif d’Accueil et de Scolarisation des Élèves Allophones de la Loire, Marielle Rispail et Marine Totozani, enseignantes-chercheures à l’Université Jean Monnet Saint-Étienne. Il s’adresse aux enseignant...

New bonded assembly configuration for dynamic mechanical analysis of adhesives

A new sample configuration has been developed in order to study molecular mobility of an adhesive in a bonded assembly configuration by dynamic mechanical analysis. The torsional rectangular mode is used to provide a shear solicitation all along the adherend/adhesive interface. The initial mechanical properties of each assembly's constituent are first investigated as reference. The modulus of aluminum foils used as substrates exhibits a classic elastic component and a slight viscous part due to microstructural changes or stress relaxation. Four relaxation modes are highlighted and identified for epoxy adhesive tested as a bulk material. Its viscoelastic behavior is compared to the one of adhesive tested in assembly configuration. The relaxation modes of the adhesive remain visible in spite of the sample stiffening by aluminum foils. Relaxation modes comparison shows that the temperature of loss modulus associated with the mechanical manifestation of glass transition slightly increases for the assembly configuration. Energy losses during this relaxation are much higher in the assembly configuration. Influence of rigid aluminum substrates is discussed in terms of the adhesively bonded joint solicitation mode

Specific heat capacity and thermal conductivity of PEEK/Ag nanoparticles composites determined by Modulated-Temperature Differential Scanning Calorimetry

The thermal conductivity accurate measurement of polymer based composites is a challenge: it would allow us to understand the mechanisms of thermal transport in such materials. Silver nanoparticles were introduced in Polyetheretherketone matrix and their influence on thermal properties was studied. Thermal conductivity and specific heat capacity of composites were determined by Modulated-Temperature Differential Scanning Calorimetry and analysed as a function of particles volume content and temperature. The specific heat capacity of the composites decreases with increasing silver particles content below the electrical percolation threshold. Above the electrical percolation threshold the specific heat capacity decreases more slowly and converge toward the specific heat capacity of compressed silver nanoparticles. The evolution of the thermal conductivity with filler content exhibits a non-linear profile. Experimental data are coherent with the Maxwell model suggesting continuity of the polymer matrix and a contribution of the silver particles to the effective thermal conductivity greater than volume effect. The temperature dependence of the composites thermal conductivity is characteristic of amorphous phase, while a transition from vitreous-like to crystalline-like behaviour of the specific heat capacity is observed with the introduction of metallic particles

Influence of the architecture on the molecular mobility of synthetic fragments inspired from human tropoelastin

This work deals with the vibrational, thermal and dielectric characterization of a synthetic peptide (S4) released during the proteolysis of human tropoelastin. This peptide was shown to form amyloid-like fibers implied in neurodegenerative pathologies. The comparison between the linear peptides and the associated amyloidlike fibers evidences the strong influence of the secondary structures on the physical structure and chain dynamics of these polypeptides

Corrosion rate determination of rare-earth Mg alloys in a Na2SO4 solution by electrochemical measurements and inductive coupled plasma-optical emission spectroscopy

The corrosion resistance of three Mg alloys containing rare-earth elements (WE43, EV31 and ZE41) was studied and compared to that of two Mg– Al alloys (AZ31 and AZ91) and of pure Mg (99.95 wt.%). Current-voltage curves and electrochemical impedance measurements were performed with rotating disk electrodes in an aerated 0.1 M Na 2 SO 4 solution. For all the alloys, it was confirmed that the intermetallic particles acted as local cathodes and that more protective films were formed on the alloys surface by comparison with the pure Mg. Corrosion rates were determined from inductive coupled plasma-optical emission spectroscopy measurements and from the electrochemical measurements. Higher corrosion rates were observed for the rare-earth Mg alloys compared to the AZ series alloys. These data allowed the corrosion mechanisms to be discussed

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

Thermal cycling in space environment can cause physical aging of polymers used in structural adhesive bonded joint. Later, they can initiate failure. A methodology to follow physical aging effects on their thermal, mechanical and dielectric properties is applied to a commercial epoxy adhesive. The analytic description, using Tool, Narayanaswamy and Moynihan model gives a good description of the enthalpy relaxation. It is completed by a phenomenological analysis of the evolution of the adhesive thermal transitions, mechanical properties and molecular mobility. Testedsamples with bondedassembly are representative ofin service configurations. The influence of physical aging on the adhesive and the associated bonded assemblies is analyzed

Determination of water uptake in organic coatings deposited on 2024 aluminium alloy: Comparison between impedance measurements and gravimetry

Electrochemical impedance spectroscopy (EIS) and gravimetric measurements on two water-based coatings, containing either SrCrO4 or a mixture of Cr(VI)-free pigments and deposited on 2024 aluminium alloy, were performed to follow the water uptake in a 0.5 M NaCl solution as a function of exposure time. To account for the observed non-ideal capacitive behaviour, the coating capacitance and dielectric constant values were extracted from the EIS data in two ways: (i) by using a complex-capacitance representation and (ii) by fitting to the EIS data a model that assumed an exponential distribution of coating resistivity. The agreement of values obtained by these independent methods served to validate the model used to account for the observed pseudo constant-phase-element (CPE) behaviour of the coatings. The water uptake calculated from dielectric constant values, employing a linear combination formula, was in good agreement with that directly measured by gravimetry, using supported-films

Liquid antimony pentachloride as oxidant for robust oxidative chemical vapor deposition of Poly(3,4-ethylenedioxythiophene) films

The oxidative chemical vapor deposition (oCVD) process is investigated to produce Poly (3,4-ethylenedioxythiophene) (PEDOT) thin films on 10 cm diameter Si wafers, involving the SbCl5 liquid oxidant. Process/structure/properties correlations are thoroughly studied, including the influence of the SbCl5/EDOT ratio, substrate temperature (Tsub), total pressure and deposition duration, on the thickness, composition, morphology, spatial uniformity of the films, and on their electrical conductivity and optical transmittance. The electrical conductivity of the films increases by decreasing the oxidant/EDOT ratio and by increasing the substrate temperature. Such increase of substrate temperature results in the decrease of the deposited mass and thickness. Films uniformity and electrical conductivity are improved by decreasing the total pressure. Operating at 75 Pa results in films of equal and uniform thickness deposited on substrates located all over the deposition chamber, proving the potential of the PEDOT-SbCl5 oCVD process to be scaled up over larger surfaces. The absence of post deposition liquid rinsing allows developing a full dry process, which is of interest to coat sensitive substrates and can thus be implemented for the processing of devices requiring durable, transparent, conductive PEDOT thin films, in particular in the field of optoelectronics

An out of the box vision over oxidative chemical vapor deposition of PEDOT involving sublimed iron trichloride

Oxidative chemical vapor deposition (oCVD) is an efficient technique to produce highly conductive films of Poly (3,4-ethylenedioxythiophene) (PEDOT). Despite numerous studies on the oCVD of PEDOT films, there is limited information on the stability of the sublimation of solid oxidants and on their impact on the polymerization reactions. In this work, we use an in situ Quartz Crystal Microbalance to monitor film formation over time. Through a series of deposition experiments between 20 °C and 100 °C and for FeCl3/EDOT molar gas ratios between 17.3 and 75.3, we analyze in detail the correlations between process parameters and film morphology,composition, surface topography and electrical conductivity on 10 cm silicon wafers. By using multiple substrates at different positions into the reactor, we demonstrate that the formation of PEDOT occurs uniformly through purely surface reactions, following step growth polymerization principles. These results pave the way towards highly conductive oCVD PEDOT films processed from convenient solid oxidants