Plastic deformation of spherulitic semi-crystalline polymers: An in situ AFM study of polybutene under tensile drawing.

International audienceThe plastic deformation of semi-crystalline polybutene (PB) has been studied at the micrometric and nanometric scales by Atomic Force Microscopy (AFM). Owing to a movable tensile drawing stage, capturing images from the same locus of the sample allowed for quasi in situ observations of the plastic processes. In the case of PB films having an average spherulite diameter of about 20 μm, the macroscopic deformation was homogeneous over the whole gauge length of the sample, up to rupture. In parallel, the local deformation at the scale of the spherulites was very close to homogeneous and obeyed an affine deformation law over the whole strain range: the shape of the deformed spherulites was kept roughly elliptical up to rupture without clues of fibrillar transformation. The inter-spherulitic boundaries displayed very high cohesion. Fragmentation of the crystalline lamellae proved to be a predominant process, while crystal slip could not be detected at the scale of the AFM resolution, i.e. a few nanometers. Wide-angle and small-angle X-ray scattering yet revealed the occurrence of crystal plastic shear. Similar observations have been made in the case of PB films having an average spherulite diameter of about 5 μm. In the conclusion, a comparison is made with a previous study regarding the deformation mechanisms of a PB sample having 200 μm wide spherulites which displayed brittle behavior

Modèlisation de la croissance et étude des propriétés élastiques des agrégrats de sphérolites

International audienceLes matériaux polymères semi-cristallins jouent un rôle essentiel dans l’industrie. La modélisation et la prédiction des microstructures polymères ont des intérêts environnementaux et économiques évidents. Dans cette étude, on construit une modélisation à l’échelle mésoscopique des agrégats de sphérolites. La croissance des sphérolites est simulée par un modèle de champs de phases et par un automate cellu- laire. La nucléation des sphérolites est modélisèe en utilisant le concept de temps de nucléation locale aléatoire. Les paramètres cinétiques de la nucléation et du taux de croissance des cristaux sont identi- fiés à partir de données expérimentales sur le PESU. Les microstructures générées sont utilisées pour étudier l’anisotropie élastique à l’échelle mésoscopique

DTS : un logiciel d'aide à l'élaboration de modèles d'écoulement dans les réacteurs Dts: a Software Package for Flow Simulation in Reactors

Cet article a pour but la présentation d'un logiciel d'aide à la construction de modèles d'écoulement à partir de la méthode des distributions de temps de séjour (DTS). La théorie des distributions de temps de séjour ainsi que des méthodes de traçage couramment utilisées sont rappelées. Une description du logiciel est ensuite effectuée. Le modèle est basé sur l'association de modules élémentaires (réacteur parfaitement agité, réacteur piston. . . ), la détermination de la fonction de transfert étant effectuée en résolvant les bilans de matière dans l'espace de Laplace. Le logiciel peut donner la réponse à un signal d'entrée quelconque et plusieurs paramètres du modèle peuvent être optimisés. Différents couples de traceurs et de détecteurs sont donnés et les précautions à prendre lors des traçages sont décrites. Finalement, différentes études effectuées à l'aide du logiciel sont présentées afin de montrer les utilisations possibles. <br> Improving the performance of an existing reactor or studying a new design requires modeling the flow of the different phases. Computational fluid dynamics can give an accurate a priori description of the velocity and concentration fields. However, this approach is too complicated in some cases (complicated geometry, random flow media, etc. ), and this results in an abundance of information. A simpler approach relies on the theory of Residence Time Distribution (RTD), which is a theoretical interpretation framework for tracer experiments. This paper describes a software package that simulates hydrodynamic models derived from RTD experiments. The concept of RTD, introduced by Danckwerts in 1953, is briefly reviewed. Then tracer experiments, commonly used tracers and detectors are dealt with. Finally, the care required in tracer experiments is described in detail. Visual inspection of the tracer response makes it is possible to guess the main characteristics of the flow pattern, such as dead zones, bypasses or recirculations (see Fig. 4), which must be represented by the reactor geometry. Based on these observations and also on the knowledge of the reactor studied, the flow pattern can be modeled with an association of elementary units such as a perfect mixing-cell, plug-flow reactor, perfect mixing-cell in series and perfect mixing-cell in series with exchange to a dead zone. Fig. 2 gives the typical outlet responses of these four elementary units to an ideal inlet pulse of a tracer. Complicated flow patterns can generally be modeled by a network of properly interconnecting elementary units. The software needs to be fed only with the description of this network. Then it derives and solves the corresponding mass-balance equations in the Laplace domain. Finally, it gives the impulse or step response at any node of the network. A convolution with an experimental inlet curve can also be made, and some model parameters can be fitted so as to recover an experimental outlet response at a given node. Various studies carried out using this software package are described in this article in order to point out the possible utilisations :1. An industrial crystalizer2. A new membrane reactor for large-scale mammalian cell cultures3. A waste-water treatment plant primary clarifier4. A ventilation system in environment engineering5. A concrete mixer in civil engineeringIn all these examples, good agreement between experimental and theoretical results was obtained. However, whatever the application, the agreement is meaningful only when :1. The geometry of the flow system is effectively reflected in the structure of the network2. Appropriate tracer experiments are performed in subparts of the flow system when numerous parameters have to be adjusted