Search

International audience

International audience; Nanocomposites made of hard inorganic nanoparticles (NPs) embedded in a soft polymer matrix may have rheological properties of interest for various applications, and in particular car tires. These properties depend crucially on the dispersion of the filler NPs, which has been studied in simplified industrial (i.e., without crosslinker and related chemistry) styrene–butadiene nanocomposites with silica filler NPs. We have developed a quantitative structural analysis based on a combination of electron microscopy, Monte Carlo simulations, and small-angle X-ray scattering. It allowed us to establish a global structural model, where primary silica beads are organized within small aggregates, which themselves fill large-scale fractal branches. Experimental parameters have been varied independently, starting with the silica volume fraction si, the matrix composition (percentage of graftable chains), and the chain mass. The grafting density on the silica NPs was shown to be the structure- and rheology-determining parameter, and we will show that a simple mechanism is able to explain the evolution of aggregate size and mass with grafting density. Moreover, dynamical properties have been followed by broadband dielectric spectroscopy (BDS) recently, and the detailed characterization of two high-temperature silica-related processes provided evidence that one of them can be linked to the structural evolution of these nanocomposites. Our work thus suggests that such high-temperature BDS-measurements can be used to detect reorganizations in structurally complex silica nanocomposites.Baeza et al, Macromolecules 2013, ibid 2013, ACS Macro Letters 2014, Soft Matter 2014

International audience; Mixing inorganic nanoparticles (ca. 10-20 nm) with polymer leads to the formation of nanocomposites. Their main reason of existence is that they combine the best of both worlds, i.e. nanoparticle strength and polymer flexibility, which has important applications, e.g. for the car tire industry. In this talk, a simplified industrial model system based on filler particles of industrial origin but with a highly reduced chemistry will be presented. It is generally recognized that the filler structure has a strong impact on the rheological properties, and it is thus important to be able to characterize it in detail. Here, microscopic observations of the dispersion of the nanoparticles in the polymer matrix by electron microscopy and scattering will be discussed and related to the microscopic structure to their macroscopic rheological properties. The structure of SBR-silica nanocomposites designed to reproduce key features of industrial samples, but of simplified composition, has been studied on length scales extending from the nanometric primary particles to microns. We propose an original method for scattering data analysis of such multi-scale systems by including self-consistent polydisperse form and structure factors of aggregates [1]. The complex structure of the silica within the nanocomposites is quantitatively modeled in a step-by-step manner, starting with the primary silica beads as basic building units (10 - 20 nm range). These beads are found to be aggregated in small clusters, the typical radius of which is in the 40 nm range. These aggregates are themselves concentrated in large-scale fractal branches (thickness ca. 150 nm, extending over microns), cf. the TEM cliché below. A quantitative analysis of the TEM data is used to estimate the volume fraction of fractal branches, which is an input for structure factor calculations of aggregates inside branches using numerical simulations as indicated in the Figure on the right. Then, the influence of polymer grafting is explored, yielding a strong effect on the aggregate mass [2]. In parallel, the rheological properties of these silica structures in the SBR-matrix are characterized with oscillatory shear. The resulting reinforcement curve of the high-frequency storage modulus can be described using a combination of hydrodynamic reinforcement for silica fraction below a critical percolation fraction, and a percolation law above. It is interesting to note that the aggregate compacity obtained from the structural analysis (SAXS and TEM) is fully compatible with the reinforcement data.

Structure de nanocomposites industriels simplifiés fortement chargés

National audience; Nous proposons l'étude de matériaux composites industriels simplifiés constitués de caoutchouc non réticulé (copolymère styrène-butadiène " SBR ") renforcés par des charges nanométriques de silice hautement dispersible. Afin d'identifier les mécanismes physico-chimiques responsables de ce renforcement et être capable de l'optimiser, nous devons comprendre les corrélations existantes entre les propriétés macroscopiques du matériau et la structure des charges à différentes échelles. Pour cela, une large campagne d'expériences de diffusion des rayons-X aux petits angles (DXPA) ainsi que de nombreux clichés de microscopie électronique ont été réalisés. En couplant ces données avec des simulations Monte-Carlo, il a été notamment possible de mettre en avant la présence d'une organisation à trois niveaux en partant de billes élémentaires d'une dizaine de nanomètres formant des agrégats eux-mêmes arrangés selon un réseau tridimensionnel branché existant à travers tout l'échantillon. Afin de déceler les corrélations existantes entre structure et propriétés, nous nous sommes attachés à décrire systématiquement l'influence de paramètres-clés tels que la fraction volumique en silice, le type de polymère employé (greffable sur la silice ou pas) ou leur masse molaire sur la morphologie des charges (par exemple la taille des agrégats) ainsi que sur le comportement mécanique (par exemple leur module d'élasticité) des composites. Ce travail a permis d'identifier la densité de greffage des chaines comme le paramètre définissant la structure des composites et impactant significativement le renforcement. Ces travaux résolument tournés vers la compréhension fondamentale, sont aussi une contribution à la recherche d'une loi de comportement décrivant l'effet de la structure des charges sur les performances des pneumatiques. Cette dernière doit permettre de répondre à des problématiques rencontrées en ingénierie telles que la résistance à l'usure, l'adhérence, ou la résistance au roulement.

Since 1992 and the apparition of highly dispersible silica as a nanofiller in the green-tire formulation, the structure and dynamics of precipitated silica-SBR nanocomposites produced by solid phase mixing is of a great interest for manufacturers. In this work, we make use of SAXS and TEM to investigate the organization of silica nanoparticles (Rsi 10nm) varying the filler fraction while dynamics is studied by broadband dielectric spectroscopy (BDS) and rheology. The structural analysis of such complex materials revealed a multi-scale filler organization based on a 3D fractal network built up from aggregates made of nanoparticles [1]. The characteristics (size, average aggregation number, compacity...) of such aggregates are extracted from a combined analysis of SAXS and TEM data taking into account the aggregate polydispersity (Nagg50, 35%). Jointly with mechanical experiments, this analysis enables us to estimate the critical aggregation volume fraction at which the network fully percolates (Figure 1a). In the same context, the ionic conductivity () measured by BDS at high temperature displays a jump from 12.7% of silica that we associate with the percolation threshold observed in rheology (Figure 1b). Moreover, at lower temperatures, a Maxwell-Wagner-Sillars process related to the charges blocked at the interface between silica and polymer is observed at temperatures higher than the glass transition in the nanocomposites.