Search

Natural rubber (NR) is an elastocaloric (eC) material that displays a significant temperature variation under dynamic mechanical loading at high elongations (λ). Its high elastocaloric response and fatigue resistance make it an ideal candidate for the development of robust refrigeration systems. Two phenomena are responsible for its eC performance: orientation/disorientation of macromolecules and crystallization/melting, induced by loading/unloading. In order to better understand the parameters governing the eC response of NR, it was important to first study its crystallization kinetics under deformation (SIC) via thermal, mechanical, and WAXD in-situ measurements. The coupling of these techniques showed that the heat generated mainly by the crystallization delays its own kinetics. Then, in order to evaluate the impact of this SIC on the thermal and mechanical responses of NR under conditions close to those of an eC system, this material was subjected to two types of cyclic solicitations (rectangular/triangular) at 4≤λ≤6. The main difference between them is that under rectangular loading, crystallization occurs mainly after adiabatic elongation while in the triangular case, it occurs during deformation. For f≤0.01Hz, the mechanical hysteresis is three times higher under rectangular loading than under triangular one for the same amount of heat that can be exchanged (Qtotal), consequently the eC efficiency of the material is significantly reduced. For 0.01Hz≤f≤0.5Hz, this heat is certainly lowered under triangular conditions, however, this type of loading is still more advantageous due to its low mechanical losses. Finally, a comparison of the eC performance (Qtotal and coefficient of performance COP) of different polyisoprene samples under triangular deformation shows that synthetic rubber and non-crosslinked NR are less efficient from an eC point of view. The former because its Qtotal is very low and the latter because of its large plastic deformation. The sulfur-crosslinked NR sample, whose active chain density is close to 1.5x10-4mol/cm3, shows the highest eC effect (Qtotal≈11MJ/m3 and COP≈30)., Le caoutchouc naturel (NR) est un matériau élastocalorique (eC) dont la température peut varier significativement sous l’effet d’une sollicitation mécanique dynamique à des élongations (λ) élevées. Son potentiel élastocalorique important et sa résistance à la fatigue en font un candidat idéal pour développer des systèmes de réfrigération solide. Deux phénomènes sont à l'origine de ses performances eC: l’orientation/désorientation des macromolécules et la cristallisation/fusion, induites par l’élongation/rétraction du matériau. Pour mieux comprendre les paramètres qui régissent cette réponse eC du NR, il était primordial d'étudier d’abord sa cinétique de cristallisation sous déformation (SIC) via des mesures thermiques, mécaniques et par WAXD in-situ. Le couplage de ces techniques a montré que la chaleur générée majoritairement par la cristallisation retarde sa propre cinétique. Ensuite, afin d’évaluer l’impact de cette SIC sur les réponses thermiques et mécaniques du NR dans des conditions proches de celles du fonctionnement d’un système eC, ce matériau a subi deux types de sollicitations cycliques (créneaux/triangulaires) à 4≤λ≤6. La principale différence entre ces dernières est qu’en créneau, la cristallisation a lieu majoritairement après la charge adiabatique tandis qu’en triangulaire, elle se fait pendant la déformation. Pour f≤0.01Hz, cette différence de comportement implique l’apparition d’une hystérèse mécanique trois fois plus importante en créneau qu’en triangulaire pour une même quantité de chaleur pouvant être échangée (Qtotale), réduisant ainsi l’efficacité eC du matériau. Pour 0.01Hz≤f≤0.5Hz, cette chaleur est certes plus faible en triangulaire, cependant, cette sollicitation reste plus avantageuse d’un point de vue énergétique grâce à ses faibles pertes mécaniques. Enfin, une comparaison des performances eC (Qtotale et coefficient de performance COP) de différents échantillons de polyisoprène sous déformation triangulaire montre que le NR non-réticulé et le Caoutchouc Synthétique sont moins performants d’un point de vue eC. Le premier, à cause de sa grande déformation rémanente et le deuxième, car son Qtotale est très faible. L’échantillon NR réticulé au soufre avec une densité de chaînes actives proche de 1.5x10-4mol/cm3 présente quant à lui l’effet eC le plus important (Qtotale≈11MJ/m3 et COP≈30).

The development of cut and chip (CC) resistant rubber articles, composed of rubber blends, requires a detailed understanding and a controlled estimation of the CC behavior of each separate rubber component within the blend in a wide range of severity conditions. This study is focused on comparative CC investigations of NR, SBR and NR/SBR (50:50) rubber blends using an Instrumented Chip and Cut Analyser (ICCA, Coesfeld GmbH, Germany) in a broad range of loading conditions. We show the results for the CC effects dependant on the applied normal forces from 90 to 200 N during cyclic impact damaging and the evolution of the temperature on the surface of the damaged specimen. We find significant differences between the used rubbers regarding dependence on the damage parameters and temperature on the normal load which determines the severity to which the rubber is exposed. In the case of NR evolving the CC damage and temperature goes through a maximum at critical values of the impacting normal load. This effect is briefly discussed in the context of the appearance of strain-induced crystallization (SIC) in the NR during cyclic impacts above a critical level. The results impressively explain the empirical preference for NR or NR-blends in practice when it comes to minimizing CC wear.

An amorphous poly(urethane-urea) copolymer composed of 70 wt% poly(ethylene oxide) (PEO) soft segments (SS) (Mw = 2000 g mol-1) and 30 wt% cycloaliphatic hard segments (HS) was subjected to in-situ X-Rays during tensile deformation. Mechanical hardening at room temperature was attributed to strain induced crystallization (SIC) of the PEO SS through the multiplication of aligned crystallites. The permanent nature of these crystals after stress removal indicates a certain mechanical stability, which we related here to the concomitant effect of superstraining of the SS crystallites and the HS reorganization upon deformation. This is in marked contrast to previous studies which reported the crystalline phase to be temporary upon unloading. A manifestation of such enhanced stability is the memory effect evidenced by an increased crystallizability of PEO segments during incremental cyclic loading. These results offer a way to (i) tune the mechanical properties of TPUs via the formation of mechanically stable pre-oriented SS crystals and to (ii) tune the thermally/water activated shape memory properties of TPUs (shape fixity, kinetics of shape memory recovery).

Recent experimental results show that strain-induced crystallization can substantially improve the crack growth resistance of natural rubber. While this might suggest superior designs of tires or other industrial applications where elastomers are used, a more thorough understanding of the underlying physics of strain-induced crystallization in natural rubber has to be developed before any design process can be started.The objective of this work is to develop a computationally-accessible micro-mechanically based continuum model, which is able to predict the macroscopic behavior of strain crystallizing natural rubber. While several researchers have developed micro-mechanical models of partially crystallized polymer chains, their results mainly give qualitative agreement with experimental data due to a lack of good micro-macro transition theories or the lack of computational power. However, recent developments in multiscale modeling in polymers provide new tools to continue this early work. In this thesis, a new model is proposed to model strain-induced crystallization in natural rubber. To this end, a micro-mechanical model of a constrained partially crystallized polymer chain with an extended-chain crystal is derived and connected to the macroscopic level using the non-affine micro-sphere model. On the macroscopic level, a thermodynamically consistent framework for strain-crystallizing materials is developed, and a description of the crystallization kinetics is introduced. For that matter, an evolution law for crystallization based on the gradient of the macroscopic Helmholtz free energy function (chemical potential) in combination with a simple threshold function is used. A numerical implementation of the model is proposed and its predictive performance assessed using published data.

International audience; A Haigh diagram is built for a carbon black filled rubber blend that exhibits Strain Induced Crystallization (SIC) for a wide range of positive displacement ratios. A strategy for the initiation detection, which becomes difficult for high displacement ratios, is proposed and validated thanks to regular visual follow-up. Some experimental cautions are taken to avoid any temperature and strain rate effects on the results, more specifically on the strain induced crystallization phenomenon. It is found that a reinforcement related to strain induced crystallization is present for load ratios (up to displacement load ratio of 0.35). For higher load ratios, the reinforcement effect reduces leading to a Haigh diagram that looks like a bell, as already shown by Cadwell et al. (S. Cadwell, R. Merrill, C. Sloman, F. Yost, Dynamic fatigue life of rubber, Industrial and Engineering Chemistry 12 (1940) 19–23).

The phase behavior of ethylene/propylene(/diene) (EP(D)M) co- (and ter-)polymers characterized by ethylene content in the range 64–78 wt%, and comparatively low level of crystallinity, in the range 1–15%, is analyzed during stretching, through collection of wide angle X-ray scattering data. All samples experience crystallization by effect of stretching, but whereas for the samples with ethylene content ≥71% crystallinity starts increasing already after yielding, the nearly amorphous sample, with 64 wt% ethylene content, experiences crystallization only at strain above a threshold (≈170%). In all cases, a high degree of orientation of the amorphous phase is achieved already at low deformations. It is shown that for the samples with high ethylene content, the pre-existing crystals undergo initial fragmentation and/or mechanical melting at low deformations, and successive oriented recrystallization at higher deformations. The role of oriented amorphous phase, along with the role of pre-existing crystals as nuclei able to foster further crystallization is highlighted.

International audience; Natural Rubber (NR) exhibits a remarkable fatigue resistance, especially for non-relaxing loadings under which a strong lifetime reinforcement is observed (Cadwell et al. 1940). Such a resistance is classically attributed to strain-induced crystallization (SIC). At the microscopic scale, it has been shown that SIC induces striations on the fracture surface of NR samples tested under fatigue loadings (Le Cam and Toussaint 2010, Muñoz-Mejia 2011, Le Cam et al. 2013, Ruellan et al. 2018). In order to provide additional information on the role of SIC in the fatigue crack growth resistance of NR, striations are investigated through post-mortem analysis after fatigue experiments carried out under both relaxing and non-relaxing loadings. Results show that two striation regimes take place. Regime 1 corresponds to small striation patches with different orientations and Regime 2 induces zones with large and well-formed striations. As fatigue striations are observed for all the loading ratios applied, they are therefore not the signature of the reinforcement. Nevertheless, increasing the minimum value of the strain amplified the striation phenomenon and the occurrence of Regime 2. The analysis carried out unifies the results obtained in the literature for relaxing and fully relaxing loadings in the sense that increasing the loading, i.e. the tearing energy, leads to an increase in the crack growth rate Lindley (1973) and to a striation typology evolution, especially the striation size (Ruellan et al. 2018).

International audience; A mathematical modeling, the Finsler geometry (FG) technique, is applied to study the rubberelasticity. Existing experimental data of stress-strain (SS) diagrams, which are highly non-linear,are numerically reproduced. Moreover, the strain induced crystallization (SIC), typical of somerubbers like Natural Rubber (NR), which is known to play an important role in the mechanicalproperty of rubbers, is partly implemented in the model. Indeed, experimentally observedhysteresis of SS curve can be reproduced if the parameter aof non-polar (or polar) interactionenergy is increased for the unloading or shrinkage process in the Monte Carlo (MC) simulations,and at the same time we find that the order parameter Mof the directional degrees of freedom σof polymer show a hysteresis behavior which is compatible with that of the crystallization ratio.In addition, rupture phenomena, which are accompanied by a necking phenomenon observedin the plastic deformation region, can also be reproduced. Thus we find that the interactionimplemented in the FG model via the Finsler metric is suitable in describing the mechanicalproperty of rubbers.

Natural rubber (NR) is largely used in the tire industry due to its excellent mechanical properties, e.g. its very good resistance to fatigue crack growth at high strain. It is generally accepted that this outstanding behavior is related to its ability to crystallize under strain. Such phenomenon, so called SIC, strongly depends on parameters like temperature, strain rate as well as the architecture of the rubber network. The microstructure of this network is formed during the crosslinking process and depends on the vulcanization system, i.e. “Efficient” or “Conventional”. The former vulcanization recipe consists in the formation of short or monosulfide bridges in the elastomer network whereas the latter (necessary to ensure a good adhesion between metallic and rubber parts in a tire) will mainly create longer polysulfide bridges. During its life, the tire will be submitted to a slow aerobic ageing which will cause structural modifications of the initial network and therefore an evolution of the rubber ability to crystallize under strain and to resist against crack propagation. In general, the structural modifications are caused by complex chemical mechanisms, highly sensitive to temperature, leading to chain scission and chain crosslinking. They can also involve sulfur bridge reorganization when NR is conventionally vulcanized. Nevertheless, most of the literature on NR ageing has been performed on efficiently cross-linked NR, and in thermal conditions which are much too severe to be representative of the material ageing in tire applications. Within this frame, our objective is to study this material when it is aged at 77 °C in air. Such parameters have been identified as capable of reproducing more realistically and over a reasonable duration, the ageing of rubber in some use conditions. After characterization of the evolution of the aged materials microstructure, their crack propagation resistance will be studied at 0.01 Hz for different values of macroscopic deformations. Time resolved Wide Angle X-ray scattering (WAXS) measure-ments, carried out at room temperature, will then provide information on the crystallization process around the crack tip. Based on these results, the relation between the network evolution during ageing, the fatigue properties and the ability to strain crystal-lize in such conditions will be established in this work., Le caoutchouc naturel présente une très bonne résistance à la propagation de fissure. Cette particularité est généralement attribuée dans la littérature à sa capacité à cristalliser sous déformation. A ce jour, l'essentiel des travaux dans ce domaine porte sur des échantillons réticulés par une vulcanisation dite efficace, c’est-à-dire dont les noeuds de réticulation sont principalement composés de ponts monosulfures. Pour certaines applications, et parce qu’elle est réputée conduire à de meilleure propriétés en fatigue, il est intéressant d'utiliser une vulcanisation dite conventionnelle. Le matériau est alors composé majoritairement de ponts polysulfures. Le phénomène de cristallisation sous déformation, exacerbée en pointe de fissure en raison d'une amplification du champ de déformation, semble être l'un des mécanismes responsables de l'accroissement de la durée de vie en fatigue du matériau vulcanisé de manière efficace. Cependant, sur matériaux vulcanisés de manière conventionnelle et thermo-oxydés (77°C), cette corrélation doit être confirmée en raison d’une évolution importante de l'architecture du réseau élastomère pendant son vieillissement pouvant en effet avoir un impact important sur la capacité du matériau à cristalliser sous déformation. Cette étude se propose donc de caractériser l’évolution de réseaux élastomères vulcanisés de manières conventionnelles pendant leur vieillissement thermo-oxydant, puis d’évaluer leur résistance à la propagation de fissure à différents niveaux de déformation macroscopique. Des analyses in situ WAXS sous rayonnement synchrotron en fond de fissure permettent alors de relier ces résultats à l'évolution de l’aptitude de ces matériaux à cristalliser sous déformation.

Caloric effects (CEs), which are the phenomena that temperature variation is caused by entropy change, have been investigated for the novel system which might be able to replace conventional vapor compression refrigeration system. In the present thesis, the elastocaloric effect (ElCE) of natural rubber (NR) and terpolymer, poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)), was focused. First of all, NR, which is an excellent candidate material for ElCE, was evaluated in cyclic deformation with different strain levels. It was found that NR exhibits temperature variation of around 4.0 °C. In general, the relation between stress/strain versus temperature variation is used to evaluate ElCE. The unsuitability of such evaluation method for NR was demonstrated. The evaluation method for ElCE which uses energy balance was then proposed. A linear relation between the temperature variation caused by ElCE and the applied mechanical energy by deformation was experimentally found. This fact verifies the suitability of the proposed method. Using the energy balance, besides, not only the conversion efficiency but also the influences of the Mullins effect and the strain-induced crystallization on the ElCE of NR were discussed. ElCE of P(VDF-TrFE-CTFE) was also evaluated in order to find out the potential of polymer. It was found that present terpolymer which is not one of the elastomers can also exhibit a large temperature variation, 2.1 °C, caused by ElCE if a large pre-stretch such as more than 1050 % is applied in advance. By comparison with other materials for ElCE, it was demonstrated that P(VDF-TrFE-CTFE) can be a high potential material for ElCE. It was also shown that P(VDF-TrFE-CTFE) converts most of the applied mechanical energy into the heat energy., Les effets électrocaloriques, qui se traduisent par une variation de température induite par une variation d’entropie ont été étudiés comme alternative aux systèmes de réfrigération utilisant un cycle de compression/détente. Le travail de thèse se focalise sur l’étude de l’effet élastocalorique dans le caoutchouc naturel et le terpolymère (P(VDF-TrFE-CTFE). En premier lieu, l’effet élastocalorique dans le caoutchouc naturel qui compte parmi les meilleurs candidats, a été évalué pour des cycles de déformation réalisés avec différentes valeurs d’allongement. Une variation de température de 4 °C a pu être observée. Il est usuel d’utiliser la relation déformation/contrainte en fonction de la variation de température pour évaluer l’effet élastocalorique. Il a été démontré que cette méthode ne peut pas être utilisée dans le cas du caoutchouc naturel et qu’elle doit être remplacée par la mesure de la variation de l’énergie mécanique en fonction de la température. Et dans ce cas, une variation linéaire entre ces deux dernières grandeurs a été observée. En réalisant un bilan d’énergie pendant l’essai, non seulement, le rendement énergétique a pu être évalué mais il a été aussi possible de prendre en compte l’effet Mullins et la cristallisation induite par la déformation pour le caoutchouc naturel. Dans un second temps, l’effet élastocalorique a été étudié sur le terpolymère (P(VDF-TrFE-CTFE), ce qui a permis de montrer qu’il était possible d’obtenir une variation de température de 2.1 °C sous réserver de pré-déformer le terpolymère à plus de 1050 % avant. Par comparaison avec d’autres matériaux présentant une bonne conversion élastocalorique, le fort potentiel de ce matériau a pu être mis en évidence. Enfin, il a été mis en évidence que la plus grande partie de l’énergie mécanique était bien convertie en énergie thermique.

According to their formulations and forming processes and thanks to the complexity of their induced microstructure, thermoplastic polymers show a wide range of thermomechanical properties. However, the identification of the evolution of the microstructure of these materials during their use remains difficult. To better understand the microstructural changes occurring during thermomechanical loadings, various in situ and non-destructive techniques of characterization have been used. In this context, a Poly (Ethylene Terephthalate) (PET) amorphous and semi-crystalline was studied in order to highlight the effect of the microstructure on the macroscopic properties of the material. This way, different coupling systems combining several experimental characterization techniques have been implemented such as Raman spectroscopy and X-rays diffraction/scattering coupled to the VidéoTraction™ system or Raman spectroscopy coupled with differential scanning calorimetry (DSC) for the characterization of the deformation micro-mechanisms and the thermal behavior of the material respectively. Monitoring specific vibrational bands thoroughly identified allowed the establishment of a new robust criterion which enables to accurately measure the crystallinity ratio of the material and the identification of the characteristic temperatures of its morphology (Tg, Tc, Tcc, Tm). In addition, a relaxational characterization system by coupling dynamic dielectric spectroscopy to a tensile test has been used in order to highlight the effect of molecular mobility on the elasto-visco-plastic deformation of PET. From a mechanical point of view, the main deformation micro-mechanisms have been studied in real time during a tensile test at different temperatures and constant true strain rates: macromolecular orientation, volume damage, development of mesophase and strain induced crystallization were observed and quantified in situ using the coupled characterization technics presented previously at Petra III (Hambourg) and Elettra (Trieste) synchrotrons. In parallel, a study of the molecular mobility (a determining parameter for the predominance of one deformation micromechanism to another) was conducted via relaxational analysis performed during the deformation of the material. In addition to in situ experiments, post mortem analysis by the previously mentioned technics and by X radiography, scanning electron microscopy and X tomography were performed to assess the influence of the mechanical relaxation of the polymer.; Selon leur formulation et leur mise en forme et grâce à leur complexité microstructurale induite, les polymères thermoplastiques bénéficient d’une grande diversité de propriétés thermomécaniques. Cependant, l’évolution de la microstructure de ces matériaux au cours de leur utilisation reste difficile à identifier. Afin de mieux comprendre les modifications microstructurales ayant lieu au cours de sollicitations thermomécaniques, différentes techniques non destructives de caractérisation en temps réel et in situ ont été développées. Dans ce contexte, un Poly (Ethylène Téréphtalate) (PET) amorphe et semi-cristallin a été étudié afin de mettre en évidence l’effet de la microstructure sur les propriétés macroscopiques du matériau. Pour ce faire, plusieurs couplages de techniques expérimentales de caractérisation ont été mis en œuvre tels que la spectroscopie Raman et la diffraction/diffusion des rayons X couplées au système de VidéoTraction™ ou la spectroscopie Raman couplée à la calorimétrie différentielle à balayage (DSC) pour une caractérisation des micromécanismes de déformation et du comportement thermique du matériau respectivement. Le suivi de différentes bandes vibrationnelles judicieusement identifiées a permis d’établir un nouveau critère robuste et capable de mesurer avec exactitude le taux de cristallinité du matériau ou de remonter aux températures caractéristiques de sa morphologie (Tg, Tc, Tcc, Tf) grâce aux informations extraites d’un spectre Raman. De plus, un système de caractérisation relaxationnelle par un couplage de la spectroscopie diélectrique dynamique avec un essai de traction a été utilisé afin de mettre en évidence l’effet de la mobilité moléculaire sur la déformation élasto-visco-plastique du PET. D’un point de vue mécanique, les principaux micromécanismes de déformation ont été étudiés en temps réel pendant un essai de traction à différentes températures et vitesses de déformation vraies constantes : l’orientation macromoléculaire, l’endommagement volumique, le développement de mésophase et la cristallisation induite sous contrainte, ont été observés et quantifiés in situ en utilisant les couplages précédents au synchrotron Petra III de Hambourg et au synchrotron Elettra de Trieste. En parallèle, une étude de la mobilité moléculaire (paramètre déterminant à la prédominance de tel ou tel micromécanisme de déformation) a été menée via des analyses relaxationnelles réalisées au cours de la déformation du matériau. En complément, des expériences en temps réel, des études post mortem par les techniques précédemment citées et par radiographie X, microscopie électronique à balayage et tomographie X ont été réalisées afin d’apprécier l’influence de la relaxation mécanique du PET.

In this thesis, we aimed to study the eC effect of natural rubber (NR) and to prove its potential to act as an eC material primarily. The method for improving the eC effect efficiency and fatigue life of NR were also proposed. The eC effect of NR is characterized directly, and interpretation based on the theory of strain-induced crystallization/crystallite (SIC) is proposed. The eC adiabatic temperature change and isothermal entropy change of NR can be up to 9 K and 50 kJ.m-3.K-1 (56 J.kg-1.K-1), which are larger than most of caloric materials. Two coefficients, eC strain coefficient and eC stress coefficient , are defined for evaluating the eC performance at different strains, where is the specific entropy, is the engineering strain, is the temperature and is the stretching stress. It’s found that both coefficients are maximum for a strain around 4.5, indicating that the highest eC performance occurred at middle strain, which is attributed to the occurrence of SIC. To improve the eC performance, it is proposed to apply a pre-strain, so that the low strain regime where eC performance is low can be skipped. Moreover, the large needed deformation can be reduced by the pre-strain and thus the possibility of a compact cooling system designed based on NR is improved. The fatigue property of eC effect of NR is then investigated. The fatigue life at large deformation strain amplitudes (strain of 1-6) is about 800 cycles for the tested NR, which is too short to be used for a cooling system. Decreasing strain amplitude is necessary to extend fatigue life up to requirement of a cooling device. For the same small strain amplitude of 3, the fatigue property is compared at amorphous strain regime (strain of 0-3), onset strain of melting (strain of 2-5) and high strain of SIC (strain of 4-7). It’s found that a larger eC temperature change and a better fatigue property can be obtained at two SIC strain regimes (strain of 2-5 and 4-7) than amorphous strain regime. Especially, the fatigue property at the onset strain of melting (strain of 2-5) is better than that at high strain of SIC (strain of 4-7). A high-cycle fatigue was applied at the strain of 2-5 (most promising strain regime) up to 1.7×105 cycles. It was observed that there is no crack of the sample, as well as a degradation degree of 12% of the eC temperature change. Furthermore, the eC stress coefficient (4.4 K/MPa) at onset strain of melting is larger than that at high strain of SIC (1.6 K/MPa). As a result, the middle strain regime (onset strain regime of melting) can get a higher eC performance, larger temperature change, and better fatigue life, which should be chosen for eC cooling system., Les effets caloriques représentent la capacité d’un matériau à voir son entropie varier sous l’effet d’une sollicitation externe et peuvent être utilisés pour des systèmes de refroidissement à l'état solide en remplacement (ou complément) des dispositifs traditionnels à base de fluides frigorifiques. Dans cette thèse, nous avons cherché à étudier l'effet élastocalorique du caoutchouc naturel. Après une présentation des différents matériaux caloriques et du caoutchouc naturel, le chapitre 2 détaille la caractérisation élastocalorique du caoutchouc naturel, et les résultats sont interprétés à partir de la notion de cristallisation induite par la déformation. Le changement de température adiabatique élastocalorique et la variation d’entropie associée atteignent 9K et 50kJ.m-3.K-1, ce qui est très important comparé aux autres matériaux caloriques. L’effet élastocalorique étant maximum pour une déformation voisine de 4,5, une pré-déformation peut être appliquée pour éviter la zone moindre activité élastocalorique. La mesure directe de l’effet élastocalorique est ensuite comparée à une méthode indirecte déduite du facteur de Clapeyron, et les divergences sont discutées. Dans le chapitre 3, la contrainte et la température élastocalorique sont simulées par un modèle de Flory modifié sur la base de la cristallisation. Il est possible de prédire le comportement contrainte-déformation à différentes températures, ainsi que les variations de température élastocaloriques à température ambiante. Dans le chapitre 4, les effets de la fatigue sur l’effet élastocalorique du caoutchouc naturel sont ensuite étudiés. La résistance à la fatigue pour de grandes amplitudes de déformation est très faible (< 800 cycles). Trois régimes de déformation intermédiaire sont ensuite testés : 0-3, 2-5, et 4-7, et permet d’établir que le régime 2-5 est le plus performant (jusqu’à 100 000 cycles). Dans le dernier chapitre, un modèle de système régénératif de refroidissement à base de matériaux caloriques est développé afin d’établir des lignes directrices pour le choix des matériaux élastocaloriques

This works deals with the study of the strain-induced structural changes of PLA, PBAT and their blends with/without multi-functional styrene-acrylic oligomers (Joncryl ADR-4368). The films were prepared through a single-screw extrusion. Uni and bi-axially simultaneous stretching, at a temperature above the glass transition temperature, were performed. The experimental conditions were chosen so as not to generate significant thermal crystallization during tests duration. Stretching induced semi-crystalline films with a supposed in-plane mechanical isotropy. Their structural, morphological and thermo-mechanical properties were characterized using dynamic mechanical and thermal analysis (DMTA), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and transmission electron microscopy (TEM). The strain-induced crystals of PLA combined mesophase and alpha'-form crystals. Besides, the stretched PBAT was found to be composed of mixed-crystal structure of BT and BA units in a PBT-like crystal form. The Joncryl addition promoted significant changes on the crystallization kinetics of PLA without affecting the crystallization kinetic of PBAT. In fact, Joncryl acted as a nucleating agent making thus faster the thermal crystallisation of PLA. In parallel, the strain induced crystallisation in PLA was accelerated and stresses increased. The same effect was still observed in the PLA_PBAT blends.

cited By 14; International audience; Strain induced crystallization (SIC) of natural rubber (NR) has been studied in a large range of strain rate (from 5.6 × 10-5 s-1 to 2.8 × 101 s-1) and temperature (from -40 °C to 80 °C) combining mechanical and thermal analysis. Both methods are used to extend the study of SIC from slow strain rates - performed with in situ wide angle X-rays scattering (WAXS) - to high strain rates. Whatever the temperature tested, the stretching ratio at crystallization onset (λc) increases when the strain rate increases. This strain rate effect is strong at low temperature (close to Tg) and weak at high temperature (much higher than Tg). A theoretical approach derived from the Hoffman-Lauritzen equation has been developed and provides a good qualitative description of the experimental results. At low temperature, the strong increase of λc with strain rate is explained by a too long diffusion time compared to the experimental time. At high temperature, SIC kinetics is rather controlled by the nucleation barrier which mainly depends on the strain energy. When the stretching ratio increases, this nucleation barrier strongly decreases, allowing crystallization even for short experimental time. © 2015 Elsevier Ltd. All rights reserved.

cited By 5; International audience; Strain induced crystallization (SIC) in filled and unfilled natural rubbers is investigated through insitu wide angle X-rays scattering (WAXS) experiments at various strain rates. The values of local stretching ratio at SIC onset (λc,local) are found similar between filled and unfilled rubbers stretched at slow strain rate (10-3s-1) and in a large temperature range [21°C, 80°C]. This is consistent with the fact that the calculated 'local' network densities of the first chains involved in SIC are very close in all samples. Conversely, λm,local for tests carried out in a large temperature range [21°C, 80°C] is found higher in filled samples compared to unfilled one due to a lower crystallites size. In order to investigate SIC in filled and unfilled rubbers submitted to conditions of solicitations met in pneumatic tires, insitu WAXS experiments are performed during high strain rates cycles (10-100s-1) thanks to a recent homemade machine. By increasing the frequency, strong self-heating related to filler-rubber interaction causes a dramatic fall of the crystallinity index (CI) in filled rubber. © 2015 Elsevier Ltd.