Ce travail est consacré à l’étude par simulation de dynamique moléculaire l’effet d’une perturbation contrôlée, utilisant des molécules photoisomérisables sur les effets de taille finie et de surface dans les matériaux amorphes. Le présent modèle utilise le potentiel de Lennard-Jones et l’algorithme de Verlet pour résoudre les équations du mouvement. Nous avons réalisé plusieurs boîtes de simulation de tailles différentes dans le but d’étudier l’effet de la taille de la boîte sur les propriétés du matériau. Nous avons utilisé ces boîtes pour simuler le matériau à différentes températures, le matériau était avec un chromophore dilué à l’intérieur, le chromophore s’isomérisant ou non,et avec une période courte ou longue. L’étude utilise des simulations à l’échelle atomique par dynamique moléculaire où l’énergie produite par le processus de photo-isomérisation est absorbée par un thermostat,après sa dégradation thermique à l’intérieur de la matrice hôte. Par une modification continue de la forme des photochromes trans vers cis et inversement, une photo isomérisation est introduite de manière périodique. Dans ces conditions, nous avons montré que ce modèle reproduit bien les propriétés statiques et dynamiques d’un matériau amorphe. Nous avons trouvé que des hétérogénéités dynamiques apparaissent dans un liquide surfondu pour une température qui égale 120K. Nous avons constaté que le temps de relaxation diminue en même temps que la force des mouvements coopératifs lorsque la taille du système est devient plus petite. Ces résultats suggèrent la présence de deux mécanismes physiques concurrents différents avec différentes échelles de longueur. / This work is devoted to the study by molecular dynamics simulation the effect of a controlled disturbance, using photo-isomerizable molecules on the finite size effects and surface in amorphous materials. This model uses the potential of Leonard Jones and the Verlet algorithm to solve the equations of motion.We realized several simulation boxes of different sizes in order to study the effect of the size of the box on the material properties. We used these boxes to simulate the material at different temperatures, the material was diluted with a chromophore inside, the chromophore isomerized or not, and with a short or long period. The study uses simulations at the atomic scale molecular dynamics where the energy produced by the photoisomerization is absorbed by a thermostat after thermal degradation within the host matrix. By a continuous change in the form of photochromes trans tocis and vice versa, a photo isomerization is introduced periodically. Under these conditions, we have shown that this model reproduces well the static and dynamic properties of amorphous material. We found that dynamic heterogeneities appear in a supercooled liquid to a temperature equal to 120 K. We found that the relaxation time decreases along with the strength of cooperative movements when the system size is becoming smaller. These results suggest the presence of two different physical mechanisms competitors with different length scales

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La cristallisation sous tension (SIC) du caoutchouc naturel (NR) a fait l’objet d’un nombre considérable d’études depuis sa découverte il y a près d’un siècle. Cependant, il existe peu d’informations dans la littérature concernant le comportement du caoutchouc à des vitesses de sollicitation proches des temps caractéristiques de cristallisation. L’objectif de cette thèse est alors de contribuer à la compréhension du phénomène de cristallisation sous tension grâce à des essais dynamiques à grandes vitesses. Pour répondre à cet objectif, nous avons développé une machine de traction permettant de déformer des échantillons d’élastomères à des vitesses de sollicitation pouvant aller jusqu’à 290s-1. Les essais ont été réalisés sur quatre NR avec des taux de soufre variables, deux NR chargés comportant des taux de noir de carbone différents. Nous avons également étudié un matériau synthétique à base de polyisoprène (IR) afin de comparer ses performances à celle du NR. Les essais dynamiques étant relativement difficiles à interpréter, un travail conséquent a donc été d’abord réalisé à basse vitesse. En outre, l’approche expérimentale proposée a été couplée à une approche thermodynamique de la SIC. Les mécanismes généraux associés à la cristallisation que nous identifions sont les suivants: lors d’une traction, la cristallisation consiste en l’apparition de populations cristallines conditionnée par l’hétérogénéité de réticulation des échantillons. Cette cristallisation semble nettement accélérée dès lors que ce cycle est réalisé au-dessus de la déformation de fusion. Nous attribuons ce phénomène à un effet mémoire dû à un alignement permanent des chaînes. Enfin, l’effet de la vitesse est décrit théoriquement en intégrant un terme de diffusion des chaînes dans la cinétique de SIC. Cette approche couplée à des essais mécaniques suggère que la SIC est essentiellement gouvernée par la cinétique de nucléation. Lors des tests dynamiques, la combinaison de l’effet mémoire et d’une accélération de la fusion pendant le cycle entraine une nette diminution voire une disparition de l’hystérèse cristalline. En outre, l’auto-échauffement, qui augmente progressivement avec la fréquence du cycle, tend à supprimer l’effet mémoire en provoquant le passage du cycle en dessous de la déformation de fusion. Lors de ces essais dynamiques, la SIC semble favorisée pour le matériau le moins réticulé. Nous attribuons cet effet au blocage d’enchevêtrements jouant le rôle de sites nucléants pour la SIC. Le matériau chargé semble avoir une moins bonne aptitude à cristalliser à hautes vitesses, par rapport à l’élastomère non chargé, en raison d’un auto-échauffement important à l’interface entre charges et matrice. Enfin, nous notons une convergence des cinétiques de cristallisation du caoutchouc naturel et synthétique à grande déformation et grande vitesse de sollicitation, que nous attribuons à la prédominance du terme énergétique d’origine entropique dans la cinétique de nucléation. / Strain induced crystallization (SIC) of Natural Rubber (NR) has been the subject of a large number of studies since its discovery in 1929. However, the literature is very poor concerning the study of SIC when samples are deformed with a stretching time in the range of the SIC characteristic time (around 10msec-100msec). Thus, the aim of this thesis is to contribute to the understanding of the SIC phenomenon thanks to dynamic tensile tests at high strain rates. To meet this goal, we have developed a dynamic tensile test machine allowing stretching samples of elastomers at strain rates up to 290 s-1. The tests are carried out on four NR with different sulphur amount, two NR with different carbon black filler amounts. We also studied a synthetic rubber made of polyisoprene chains (IR) able to crystallize under strain. Dynamic tests are relatively difficult to interpret; a significant work has thus been first performed at slow strain rate. Moreover, the experiments are coupled with a thermodynamic approach. First, the general mechanisms associated to the crystallization are identified as follows: during mechanical loading or during cooling in the deformed state, SIC is the result of successive appearance of crystallite populations whose nucleation and growth depend on the local network density. Crystallization is enhanced when the cycle is performed above the melting stretching ratio. This phenomenon is attributed to a memory effect due to a permanent alignment of the chains. Finally, the effect of the strain rate is theoretically described thanks to a diffusion term. This approach, coupled with experiments suggests that SIC is mainly governed by the nucleation kinetics. For the dynamic test, the combination of the memory effect and the acceleration of the melting during the cycle lead to a reduction or even disappearance of the crystalline hysteresis. In addition, self-heating, which progressively increases with the frequency of the cycle, causes the delay of the melting stretching ratio. This well explains why the crystallinity index decreases at the minimum stretching ratio of the dynamic cycles when the frequency increases. We finally compared the ability of our different rubbers to crystallize at high strain rates. SIC is enhanced for the weakly crosslinked rubber. This might be related to the dynamics of its free entanglements, these ones acting as supplementary crosslinks at high strain rates. Then, a filled rubber is compared to the unfilled one. We found that the filled sample has a lower ability to crystallize at high strain rates as compared to the unfilled one. This is likely due to the strong self-heating at the interface between the fillers and the rubbery matrix. Finally, we observe a convergence of crystallization kinetics in natural and synthetic rubbers at high strains and high strain rates. This is attributed to the predominance of the entropic energy in the nucleation kinetics in these experimental conditions.

La cristallisation induite par déformation des élastomères est un processus cinétique qui conduit à la formation de nano-cristallites thermodynamiquement stables. La présence de ces nano-cristallites au sein de la phase amorphe modifie considérablement les propriétés mécaniques des élastomères cristallisables. Ces élastomères ont en effet la propriété intéressante d'être auto-renforçants. L’objectif de ce travail est de développer un modèle physique capable de décrire localement l’évolution de la microstructure sous l’effet d’un champ de contrainte élastique durant la cristallisation. Dans ce but, un modèle de champ de phase est élaboré et mis en œuvre dans le cadre de la mécanique des milieux continus en couplant thermodynamique et mécanique avec une dynamique de transition de phase d’Allen-Cahn. La description thermodynamique de la cristallisation induite par déformation à petite échelle est basée sur la fonctionnelle d’énergie libre du système amorphe-cristal. Les conséquences du choix de cette formulation sont discutées, on étudie en particulier les effets de contraintes élastiques sur l’équilibre des phases en volumes ainsi que sur la cinétique de croissance des domaines cristallins au sein de l’amorphe. L’introduction de l’élasticité du réseau des contraintes topologiques induite par les enchevêtrements et/ou les nœuds de réticulation dans le modèle de champ de phase a permis de mettre en évidence l’existence d’un état stable de cristallites formées (modèle énergétique) mais aussi des instabilités de croissance (modèle cinétique). Sur la base de ces deux modèles, cinétique et énergétique, nous avons étudié systématiquement l’influence des contraintes topologiques sur la cinétique de croissance et nous montrons que cette cinétique est en effet contrôlée par l’accumulation de contraintes élastiques à l’interface. La prise en compte de l’élasticité du réseau des contraintes topologiques dans l’approche thermodynamique de cristallisation prédit une augmentation de la tension de surface et par conséquent un arrêt du mécanisme de croissance en donnant lieu à la formation de cristallites stables. Enfin, nous avons adopté le modèle énergétique pour modéliser le couplage entre nucléation, croissance et déformation cyclique. Pour valider le modèle local proposé une comparaison entre les résultats des simulations par champ de phase et les données expérimentales issues de la caractérisation d’un caoutchouc naturel réticulé est effectuée et nous montrons qualitativement l’accord entre l’expérience et le modèle. / Natural rubber NR and more generally elastomer presents unique physical properties that are very important for many engineering applications. Strain induced crystallization of elastomer presents a major interest because it improves considerably the mechanical properties. In fact, the presence of crystallites within the amorphous phase in a polymer network induces a strengthening of this material, giving NR a self-reinforcement character. In this thesis, we develop a mesoscopic model to describe the crystallization of elastomers under strain. In this context, we present a kinetic model using a new physical approach: a phase field model. This model combines the crystallization thermodynamics with the local stress field. The thermodynamic description of the phase transition is based on a Gibbs free energy functional F which contains all energy contributions of the system: the bulk contributions (enthalpy and entropy) and surface tension. To understand the experimental observation of nanometer size crystalites, an explicit account of the topological constraints induced by both entanglements and/or crosslinks is necessary. We investigated two limiting mechanisms, a kinetic limitation of the growth, and an energetic limitation. Based on both the kinetic and the energetic approaches, we have systematically studied the influence of topological constraints on the growth process. We have shown that the growth process is affected by the accumulation of elastic stress at the interface. The kinetic model predicts the existence of instabilities during the growth. These instabilities induce a heterogeneous dynamical growth which leads to the formation of dendrite like structures. On the contrary, the energetic approach predicts an exponential increase of the surface tension during the growth that limits the size of the crystallites very efficiently. In the last part we investigated elastomer crystallization under cyclic deformation. To this end, we coupled the previous energetic model with the nucleation process. Finally the simulation data are compared with experimental measurements.

This thesis describes the study on structure and dynamics of various kinds of molecular membranes in general. We have studied the morphological transition of colloidal as well as biologically relevant membranes and qualitatively argued regarding the interplay between structure and dynamics. Systematic measurements have been performed to address the issue of ambiguous behavior of molecules under stress when its confined at the interface. The structural and dynamical effect on interfacial membranes have been studied for soft colloidal free standing langmuir monolayer as well as for the quasi two dimensional lipid membranes on solid supports. For organic nanoparticle monolayer we have observed a correlation between the nanoparticle raft dynamics and the underlying morphological transition. In this study we have also found a non-monotonic behavior of dynamical heterogeneity with time which is unusual for a colloidal system in common and beyond the prediction of Mode Coupling Theory. In the case of lipid membrane, we have given an experimental evidence of lipid molecular rearrangement process at molecular level when its perturbed by foreign entities. Using sophisticated X-Ray scattering techniques, we were able to capture the subtle changes happening in the assembly of lipid molecules in a planar bilayer structure when it interacts with molecules having biological relevance. In the next level we have used lipid membranes as an active plat-form to study the physical interaction with several kinds of nanoparticles and explored the mechanism of active participation of lipid molecules in self assembly process. Besides with the help of Fluorescence Correlation Spectroscopy, we have also studied the effect of nanoparticles assemblies on the dynamics of lipid molecules itself. In Chapter 1, we have provided the background along with a brief review of the existing literature for understanding the results represented in the subsequent chapters. This includes discussion on the various physical properties of our systems of interest, including dynamic behavior of colloidal particles in different concentration regime and a detailed theoretical understanding regarding the glass transition and jamming transition for a highly dense colloidal packing. In this section we have also discussed the advantages of interfacial microrheology technique over conventional bulk rheology in terms of efficiency and sensitivity. Here we have also pointed out the formulation of the multi-particle tracking method for achieving different parameters which are correlated in space and time for a given system. Followed by that the Dynamical Susceptibility and the anomaly in Van Hove correlation function, for a heterogeneous system has been argued thoroughly. Towards the end we have discussed about the general features of another type of two dimensional membrane i.e. the lipid membrane at interface. Using raft theory we have also tried to give a plausible explanation of the dynamical heterogeneity of the real cell membrane which is mimicked by the model supported lipid membrane. Here we have argued about the structural six fold symmetry of a compact monolayer. Finally in the last part we have summarized the theoretical aspects of the lipid molecule mediated self assembly process and the how the lipid diffusion plays a vital role in it. Chapter 2 deals with the aspect of measuring the morphological transition and its effect on the dynamics for a two dimensional membrane at air/water interface. It starts with the discussion on the synthesis method for various types of organic molecule grafted nanoparticles like Cadmium Selenide(CdSe Quantum Dots) and Gold Nanoparticle(Au NPs) of different size and properties and followed by a preparation method of 2D film at air/water interface and on solid substrate using Langmuir-Blodgett method. In this chapter we have discussed about the basic principles of several experimental tools like Brewster Angle Microscopy(BAM), Laser Scanning Confocal Microscopy(LSCM), Atomic Force Microscopy(AFM), Thermogravimetric Analysis(TGA), X Ray Reflectivity(XRR), Grazing Incidence Diffraction(GID), Fluorescence Correlation Spectroscopy(FCS) etc. Chapter 3 explains the main aspects of the microscopic dynamics in dense amorphous nanoparticle monolayer at the air-water interface. In this study we have found a transition in mechanical properties, tracked down through the systematic variation of isothermal compressibility(�) with increasing two dimensional packing fraction of nanoparticle rafts up to the area fraction of Φ∼0.82 using Laser Scanning Confocal Microscope. Here we have used multi particle tracking method for a close packed gold monolayer with CdSe tracer to estimate different dynamical properties like Mean Square Displacement(MSD), Dynamical Heterogeneity etc. These calculations indeed point out the non-monotonic variation of the amplitude in the four-point dynamic susceptibility (χ4), a signature of spatio-temporal extension of correlated domains. Along with that we have also observed the anomaly in trend for the inherent relaxation time τ∗with increasing area fraction(Φ). Interestingly the variation in χ4exactly follows the systematic we found for the isothermal compressibility( �) with increasing Φ and that indicates the connection between the observed macroscopic transitions in mechanical properties and the microscopic dynamical phase transitions. Finally we have given a possible explanation of these kind of events in terms of the interaction between this sterically stabilized nanoparticle domains with the help of interpenetration of the capping long chain polymers of the neighboring nanoparticle. Chapter 4 opens up the possibilities of probing the hidden features of biomembranes at molecular scale with the help of very precise techniques based on synchrotron X ray diffraction. Here we have studied the rearrangement of the lipid molecules of an artificial membrane on a solid support as an effect of ad-sorption of organic branched molecules. In this work we have used non toxic PETIM dendrimers of two different generations, i.e. G3and G4which differs a lot in terms of size, no of termination groups, molecular weights and protonation states. Our initial measurements shows quantitatively the in-plane and out of plane symmetry breaking of the lipid bilayer as a result of the interaction with these two types of molecules. The molecular adsorption effect was quantified in terms of thickness reduction and the change in the scattering length density(SLD) or the electron density of the top layer in out of plane reflectivity model. Interestingly both the dendrimers showed different behavior and the interaction reflected in terms of membrane penetration was found stronger for higher generation. On the other hand the GID measurement indicates an enhancement of the in plane unit cell dimension and associated parameters of the arrangement of lipid molecules as a result of interaction with dendrimers. The combined XRR and GID measurements indicate a local fluidization of lipid packing as an outcome of charged branched molecules adsorption on the membrane surface. Chapter 5 is summarizes the lipid mediated self assembly process of nanoparticles on a bilayer and how the interaction changes the local properties of the bilayer represented by the molecular diffusivity. In this study we have used particles of wide variety of features in terms of size, charge, functionality, polarity etc and found a quite dramatic effect in the nanoparticle adsorption event on a solid supported Lαphased DMPC lipid bilayer. We have also seen that de-pending on the concentration and amount of surface charge the nanoparticles form two dimensional regular self assembled patterns on the bilayer surface. In FCS measurement, we have also found a second group of dynamics ( distribution of diffusivity) along with the normal bilayer diffusion which has been identified as the diffusion of the lipid molecules where nanoparticles are adsorbed. The inherent increment in diffusivity supports the argument of local fluidization in lipid membrane in presence of charged nanoparticle as we have observed in our XRR and GID data described in chapter 4. Chapter 6 contains the summary and the future perspective of the work presented here.