Catherine Gauthier

Nanofillers in polymeric matrix: a study on silica reinforced PA6

Abstract The study was carried out on nanocomposites consisting of nanoscopic silica fillers embedded in polyamide 6. Various composite systems were prepared through in situ polymerisation, with different elementary particle diameters and filler contents as variables. The morphological investigation demonstrated the non influence of the particle presence on the crystalline phase of such composites. The introduction of filler leads to an obvious reinforcement of the matrix elastic modulus: the observed increase depends on the modulus difference between the various phases present, the filler content and its dispersion state. In the same way, the yield point, in both compressive and tensile tests, is found to be sensitive to the latter parameters. Complementary experiments enable to suggest possible local events leading to the rupture of these composite systems.

Influence of the β crystalline phase on the mechanical properties of unfilled and CaCO3-filled polypropylene. I. Structural and mechanical characterisation

Abstract The influence of β crystals on the mechanical properties of isotactic polypropylene is studied on compression-moulded sheets without filler or filled with stearate-coated calcium carbonate particles. A crystallisation procedure is setup for producing sheets with various amounts of β crystals, trying to keep constant crystallinity, spherulite size and crystal thickness. An optimum of β crystals among α crystals is first produced by isothermal crystallisation at 110°C. Then gradual transformation of β into α crystals is carried out through thermal annealing at 152°C after an intermediate cooling down to room temperature. Due to the thermal inertia of the large sheet thickness, the amount of β crystals does exceed 10% for unfilled sheets and 50% for filled sheets. Dynamic mechanical behaviour shows that molecular mobility is higher in the β crystals than in the α crystals, as judged from the temperature of the crystalline relaxation. Loss modulus in the temperature range of the crystalline relaxation also indicates greater damping capacity for the β crystals. This is discussed in terms of activation of conformational defects moving along the chain stems in the crystal. Plane strain compressive testing reveals better ductility for β rich samples. Interpretation is provided using an approach of semi-crystalline polymer plasticity based on dislocation-governed crystallographic slip. Correlation is made with the viscoelastic behaviour through the concept of conformational defects.

In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particle Suspensions—Part I: Initial Instants

This paper investigates the behaviour of colloidal suspensions of alumina particles during directional solidification, by in situ high-resolution observations using X-ray radiography and tomography. This second part is focussed on the evolution of ice crystals during steady state growth (in terms of interface velocity) and on the particles redistribution taking place in this regime. In particular, it is shown that diffusion cannot determine the concentration profile and the particles redistribution in this regime of interface velocities (20-40 microns/s); constitutional supercooling arguments cannot be invoked to interpret particles redistribution. Particles are redistributed by a direct interaction with the moving solidification interface. Several parameters controlling the particles redistribution were identified, namely the interface velocity, the particle size, the shape of the ice crystals and the orientation relationships between the crystals and the temperature gradient.

Nanophases in polymers

Polymers have been reinforced by nanophases for a long time. Nevertheless recent technological progress have allowed to put forward new materials, confirming the reinforcing effect of nanoparticles in polymer. They also emphasize the importance of three main parameters in these nanostructured systems : the mechanical coupling between the two phases, the possible cross-linking of the macromolecules by the nanoparticles and the connectivity between reinforcing particles.

Influence of Particle Size on Ice Nucleation and Growth During the Ice‐Templating Process

The solidification behavior of suspensions of alumina particles during directional solidification is investigated here by in situ observations using X-ray radiography and tomography. The objective of this study was to assess the influence of particle size on the solidification behavior of the suspensions during the early stages of solidification. Four powders with particle size in the range of 0.2–3.4 μm (median size) were investigated. Solidification is obtained by cooling at a constant rate, starting from room temperature. Attention is specifically paid to the nucleation and growth behavior of the ice crystals in these suspensions. We propose that the nucleation of ice crystals is controlled by the particle size, the surface of the particles acting as nucleation sites. Smaller particle size leads to a lower degree of supercooling because nucleation and growth can proceed at a higher temperature than with larger particles. The initial interface velocity is dependent on the degree of supercooling, and controls the extent of the initial structural gradient in the resulting porous materials.

Mechanical properties and cytocompatibility of poly(ε-caprolactone)-infiltrated biphasic calcium phosphate scaffolds with bimodal pore distribution

Biphasic calcium phosphate scaffolds have attracted interest because they have good osteoconductivity and a resorption rate close to that of new bone ingrowth, but their brittleness limits their potential applications. In this study, we show how the infiltration of biphasic calcium phosphate scaffolds with poly(ε-caprolactone) improves their mechanical properties. It was found that the polymer effectively contributes to energy to failure enhancement in bending, compressive and tensile tests. The main toughening mechanism in these composites is crack bridging by polymer fibrils. The presence of fibrils at two different size scales--as found in scaffolds with a bimodal pore distribution--results in a more effective toughening effect as compared to scaffolds with a monomodal pore size distribution, especially in the early stage of mechanical deformation. An optimized infiltration process allowed the preservation of micropore interconnection after infiltration, which is beneficial for cells adhesion. In addition, it is shown that biphasic calcium phosphates infiltrated with poly(ε-caprolactone) are cytocompatible with human bone marrow stromal cells, which makes them good candidates for bone substitution.

Relaxation of non-crystalline solids under mechanical stress

Non-crystalline solids submitted to a thermomechanical stimulus have similar properties in materials as different as amorphous polymers, molecular, metallic or oxide glasses. The purpose of the present paper is to give some insights into the mechanical response of glassy or liquid media, in relation to their state of disorder. To describe thermomechanical response of non-crystalline solids, a molecular theory has been developed. We show how the concept of quasi-point defects (QPD) introduced in the theory is consistent with small angle X-ray scattering results. The calculations based on the QPD model are also compared to experimental data for various glass-forming systems. The magnitudes of the parameters, introduced in the model for these different systems, are discussed.

Films from styrene–butyl acrylate lattices using maleic or succinic surfactants: mechanical properties, water rebound and grafting of the surfactants

Abstract A series of anionic and zwitterionic maleic surfactants have been synthesized and engaged in styrene–butyl acrylate emulsion polymerization. Some non-reactive succinic analogs have been synthesized in order to perform their comparison with the maleic surfactants in polymerization experiments. Films from the obtained lattices were cast, and their mechanical properties and the water rebound were tested. The results of water rebound demonstrate significant difference between the films prepared with maleic or succinic surfactants. Water rebound of the films after 34 days in the case of succinic surfactants was found to be between 51 and 95%, while for maleic surfactants it is only 25–40% of the initial weight of samples, which is a significant improvement. Mechanical properties do not present essential difference, but a clear tendency has been observed in the three cases investigated—the films prepared using maleic surfactants are less resistant to the deformation and they are more extended at the rupture of the samples. Both series of results could be explained assuming heterogeneous inclusions of non-reactive surfactant (succinic derivatives) having an ionomer character. Both the results of water rebound and mechanical properties can be considered as an indirect proof of grafting of maleic surfactant on the polymer.

Prediction of the elastic response of polymer based nanocomposites: a mean field approach and a discrete simulation

Abstract The aim of this paper is to better understand mechanisms of reinforcement in particular nanocomposites, by studying the effects of filler ratio (varying from 0 to 45%) and strength of filler–filler interactions (either slightly or strongly bonded) on the viscoelastic behavior. Nanocomposites materials consisting, at room temperature, of submicronic rigid polysytrene (PS) particles randomly dispersed in a soft polybutylacrylate (PBA) polymeric matrix were obtained from mixtures of a film-forming PBA latex and a PS latex. Their viscoelastic behaviors have been characterized (i) after the film formation at room temperature and (ii) after an annealing treatment performed to enhance the interactions between neighboring PS particles (coalescence). The observed increase of elastic modulus and its evolution with temperature depend both on the filler ratio and on the strength of filler–filler interactions. Two micro-mechanical models have been then proposed to account for the short range interactions between filler particles: a self consistent scheme introducing a third rigidifying phase of matrix immobilized on the surface of filler particles, and a discrete model of sphere assembly taking into account the local contacts between filler/filler and matrix/filler. In addition to the good agreement with the experimental results, the discrete model highlighted the importance of filler–filler interactions on the reinforcement above the percolation threshold.

Ice-Templating of Alumina Suspensions: Effect of Supercooling and Crystal Growth During the Initial Freezing Regime

We investigate the ice-templating behaviour of alumina suspensions by in-situ X-rays radiography and tomography. We focus here on the formation and structure of the transitional zone, which takes place during the initials instants of freezing. For many applications, this part is undesirable since the resulting porosity is heterogeneous, in size, morphology and orientation. We investigate the influence of the composition of alumina suspensions on the formation of the transitional zone. Alumina particles are dispersed by three different dispersants, in various quantities, or by chlorhydric acid. We show that the height and the morphology of the transitional zone are determined by the growth of large dendritic ice-crystals growing in a supercooled state, and growing much faster than the cellular freezing front. When the freezing temperature decreases, the degree of supercooling increases. This results in a faster freezing front velocity and increases the dimensions of the transitional zone. It is therefore possible to adjust the dimensions of the transitional zone by changing the composition of alumina suspensions. The counter-ion Na+ has the most dramatic influence on the freezing temperature of suspensions, yielding a predominance of cellular ice crystals instead of the usual lamellar crystals.