Etude du comportement élastique et viscoélastique linéaire des polymères semi-cristallins par approche micromécanique

Le but de la thèse est de prédire les propriétés mécaniques aux petites déformations (d'abord élastiques, puis viscoélastiques) des polymères semi-cristallins isotropes en utilisant les techniques d'homogénéisation. La morphologie microstructurale a été caractérisée par des techniques de microscopie optique et électronique à balayage ainsi que par diffraction des rayons X aux grands angles. Les propriétés élastiques des phases constituantes ont été essentiellement appréhendés à partir de résultats de la littérature. Les propriétés viscoélastiques ont été mesurées par des essais en torsion dynamique. Enfin des essais de traction uniaxiale permis de déterminer le comportement macroscopique des polymères. Deux représentations morphologiques ont été adoptées: matrice-inclusion et agrégat de composite bicouches. Différents modèles d'homogénéisation ont été comparés d'abord en élasticité. Le modèle différentiel associé à schéma matrice-inclusion s'est avéré le plus performant. Tenant compte de la géométrie des lamelles cristallines, il a notamment permis de comprendre pourquoi le PEhD avait un module plus faible que le pp malgré des phases cristalline et amorphe plus rigide et un taux de cristallinité supérieur. Le travail a été étendu au cas de la viscoélasticité linéaire pour le PET Les outils numériques pour la transformée de Laplace-Carson et l 'inversion numérique, utilisant la méthode des collocations, ont été développées. 11 a été possible, en utilisant ces techniques d'avoir une prédiction satisfaisante du comportement macroscopique viscoélastique linéaire du PETThe goal of the thesis is to predict the mechanical properties in the case of small deformations (initially elastics, the en viscoelastic) of isotropic semi-crystalline polymers using the homogenization techniques. Micro structural morphology was characterized losing optical and electronic scanning microscopy techniques, also by the wide angle X-ray diffraction techniques. The elastic praperlies of the constituent phases were primarily Deduced from the Literature. The viscoelastic properties lever measured by tests in dynamic torsion. Finally iniaxial tensile lasts lever done to work up the macroscopic elastics and viscoelastic behavior of polymers. Various morphological representations lever adopted: ma/rix-inclusion and aggregate of double-layered composite. Various models of homogenization were compared initially in elasticity. The differential model associated to matrix inclusion morphology seems at be the more appropriate. Taking on account the geometry of the crystalline lamellae, it was possible, using the differential scheme, to understand why PEhD had a module weaker than pp in spite of Crystalline and amorphous phases more rigid and a higher crystalline rate. The study was extended to linear, viscoelasticity in the case of the PET. Numerical tools for the Laplace-Carson transformation and the numerical inversion, using the collocation method, was developed. It was possible using these techniques to have a good macroscopic linear viscoelastic behavior prediction.PARIS-Arts et Métiers (751132303) / SudocSudocFranceF

In Vitro Bone Cell Response to Tensile Mechanical Solicitations: Is There an Optimal Protocol?

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Conductive Network and β Polymorph Content Evolution Caused by Thermal Treatment in Carbon Nanotubes-BaTiO 3 Hybrids Reinforced Polyvinylidene Fluoride Composites

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Conductive Network and β Polymorph Content Evolution Caused by Thermal Treatment in Carbon Nanotubes-BaTiO₃ Hybrids Reinforced Polyvinylidene Fluoride Composites

A good dispersion of carbon nanotube (CNT) in polyvinylidene fluoride (PVDF) is realized by using CNT and BaTiO₃ (BT) hybrids (H-CNT-BT) with a special core–shell structure. Thus, a high dielectric performance is achieved for the composite (H-CNT-BT/PVDF). Carried by BT, CNT is easy to connect with each other and thus more interface area may be created which helps to achieve an extremely low percolation threshold (<i>f</i>_c). Moreover, the dielectric permittivity of the composite near <i>f</i>_c is increased more than three times after thermal treatment while dielectric loss remains at a low level. In order to study more comprehensively about the influence of thermal treatment, in situ synchrotron X-ray is used to detect recrystalline behavior of PVDF. Results of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) show that after thermal treatment, the content of β polymorph has increased nearly double times at the interface of CNT-PVDF, and the thickness of amorphous layers (<i>L</i>_a) in PVDF’s long periods (<i>L</i>_p) has shrunk around 10 Å. Increased β polymorph at the interface of CNT-PVDF may form an ideal structure with the grading dielectric permittivities from the center to the border which decreases the contrast between CNT and PVDF. Meanwhile, the evolution of CNT’s network possibly occurs in the procedure of <i>L</i>_a shrinkage, where the strong interfacial polarization may be aroused. Besides, an increase in the thickness of crystalline lamella may also arouse more orientational polarization and improve dielectric properties at high frequency. Combining with BT’s buffer role for blocking possible leakage current during the percolative behavior, the dielectric loss of composite can remain at a very low level even after thermal treatment. In addition, experimental results show that prolonging annealing duration or increasing annealing cycles favors stabilization of CNT’s dynamic percolation, which reduces the sensitivity of CNT’s network in the composite and further improves dielectric properties. After thermal treatment, the dielectric permittivity reachs 1172, but dielectric loss remains at 0.55 at 100 Hz. To our best knowledge, this high dielectric performance is really rare, only found in recent reports

The Osteogenic and Tenogenic Differentiation Potential of C3H10T1/2 (Mesenchymal Stem Cell Model) Cultured on PCL/PLA Electrospun Scaffolds in the Absence of Specific Differentiation Medium

The differentiation potential of mesenchymal stem cells (MSC) has been extensively tested on electrospun scaffolds. However, this potential is often assessed with lineage-specific medium, making it difficult to interpret the real contribution of the properties of the scaffold in the cell response. In this study, we analyzed the ability of different polycaprolactone/polylactic acid PCL/PLA electrospun scaffolds (pure or blended compositions, random or aligned fibers, various fiber diameters) to drive MSC towards bone or tendon lineages in the absence of specific differentiation medium. C3H10T1/2 cells (a mesenchymal stem cell model) were cultured on scaffolds for 96 h without differentiation factors. We performed a cross-analysis of the cell–scaffold interactions (spreading, organization, and specific gene expression) with mechanical (elasticity), morphological (porosity, fibers diameter and orientation) and surface (wettability) characterizations of the electrospun fibers. We concluded that (1) osteogenic differentiation can be initiated on pure PCL-based electrospun scaffolds without specific culture conditions; (2) fiber alignment modified cell organization in the short term and (3) PLA added to PCL with an increased fiber diameter encouraged the stem cells towards the tendon lineage without additional tenogenic factors. In summary, the differentiation potential of stem cells on adapted electrospun fibers could be achieved in factor-free medium, making possible future applications in clinically relevant situations

Biomaterials in Tendon and Skeletal Muscle Tissue Engineering: Current Trends and Challenges

Tissue engineering is a promising approach to repair tendon and muscle when natural healing fails. Biohybrid constructs obtained after cells&rsquo; seeding and culture in dedicated scaffolds have indeed been considered as relevant tools for mimicking native tissue, leading to a better integration in vivo. They can also be employed to perform advanced in vitro studies to model the cell differentiation or regeneration processes. In this review, we report and analyze the different solutions proposed in literature, for the reconstruction of tendon, muscle, and the myotendinous junction. They classically rely on the three pillars of tissue engineering, i.e., cells, biomaterials and environment (both chemical and physical stimuli). We have chosen to present biomimetic or bioinspired strategies based on understanding of the native tissue structure/functions/properties of the tissue of interest. For each tissue, we sorted the relevant publications according to an increasing degree of complexity in the materials&rsquo; shape or manufacture. We present their biological and mechanical performances, observed in vitro and in vivo when available. Although there is no consensus for a gold standard technique to reconstruct these musculo-skeletal tissues, the reader can find different ways to progress in the field and to understand the recent history in the choice of materials, from collagen to polymer-based matrices

Monitoring mechanical stimulation for optimal tendon tissue engineering: a mechanical and biological multiscale study

International audienc

Monitoring mechanical stimulation for optimal tendon tissue engineering: a mechanical and biological multiscale study

International audienc

Nanoparticle size and surface chemistry effects on mechanical and physical properties of nano-reinforced polymers: The case of PVDF-Fe3O4 nano-composites

In the present work, PVDF - Fe3O4 nanoparticle (NP) nanocomposite films were produced using the electrospinning method. We investigated the effect of NP size on the film's morphology (fiber size), mechanical properties, and physical properties (β-phase percentage). Surprisingly, while nanoparticle size acts as an enhancer for mechanical properties, it appeared to act as an inhibitor in terms of its effects on the crystallization of the β-polymorph. This result seemed in discordance with many previous results. A focus on local interactions between the NP surface chemistry and PVDF chains revealed the influence of grafted ligands at the nanoparticle surface on the crystallization of the piezoelectric phase of PVDF. The results from the molecular dynamics (MD) simulations for systems of PVDF chains with slabs of –OH and oleic acid-grafted magnetite, showed that the probability of beta phase configuration decreases when the nanoparticles are functionalized with oleic acid and becomes more probable for –OH terminated magnetite. These computational results are in accordance with our experimental results. To verify this hypothesis, we prepared films with washed nanoparticles to eliminate the excess oleic acid that acts as a β-polymorph inhibitor. As a result, the amount of β-phase obtained for washed nanoparticles increased and the difference in the amount of β-phase between the different samples decreased. Moreover, when heated, the films of nanocomposite with washed NP developed more β-phase for smaller sizes of nanoparticles. At 140 °C, isomerization occurred, and oleic acid was converted into elaidic acid, reducing the steric hindrance, and promoting the interaction between PVDF chains and the surface of the nanoparticles. This isomerization reaction seems to be an enhancer of the α- to β-phase transition. Our results prove that optimizing multiple properties in nano-reinforced polymers requires consideration of different aspects, such as NP size, surface chemistry, and processing methods.Our results based on mixed experimental and modeling approach proved the usefulness of simulation in understanding and guiding our experimental results. Our results suggest that for enhancing piezoelectric properties in PVDF magnetite nano-composites, the chemistry and the molecular morphology of the grafted ligands when combined with NP size could lead to multi-properties enhancement simultaneously