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Structure of highly loaded Polymer-Silica Nanocomposites
Auteur(s): Genix A.-C., Baeza G., Tatou M., Couty Marc, Oberdisse J.
Conference: Macromolecules in Constrained Environments (Les Houches, FR, 2013-03-24)
Résumé: The mechanical reinforcement of polymer matrices by nanoparticles is a fundamental problem with far
reaching applications, e.g., for car tires. From a conceptual point of view, it is generally recognized
that the filler structure has a strong impact on the mechanical properties, accompanied by the effect of
chain structure evolving in the hard filler environment, and the filler-chain interactions. All these
contributions are related to the filler structure, and it is thus important to be able to characterize it in
detail.
In this work, 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 will be 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 (40
nm range) will be determined by Kratky plots. These aggregates are themselves concentrated in largescale
fractal branches (thickness ca. 150 nm, extending over microns). Inside these branches, the small
aggregates repel each other. Within our model, this is described with a hard-sphere excluded volume
interaction potential, which induces a characteristic depression of the scattered intensity at
intermediate angles. This depression is directly related to the local concentration of aggregates, which
is higher than the nominal silica volume fraction due to the confinement in the fractal branches, and
the presence of polymer inside the aggregates. Therefore, a quantitative TEM analysis was used to
estimate the volume fraction of fractal branches. Secondly, we have set up an independent Monte
Carlo simulation in order to calculate the low-q limit of the polydisperse inter-aggregate structure
factor, which quantifies the depression. Using a polydisperse aggregate form factor obeying the same
polydispersity, the mass of the small aggregates (or, equivalently, their internal silica volume fraction,
here called compacity) and their concentration inside the fractal branches can be extracted from the
scattered intensity. 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.
On the other hand, for future work, it may be important to be able to compare the results obtained here
to model systems where the filler is a well-defined nanoparticle. In this case, we were able to follow
the chain conformation in hard filler environments using contrast-variation small angle neutron
scattering (SANS). In polyacrylate latex based nanocomposites where the silica structure is well
characterized, it has been found that the radius of gyration is not disturbed by the silica loading in a
given state of aggregation (typically ten primary silica particles per aggregate, percolating at high
concentration).[2] This important result opens the way for a systematic study of the chain structure in
complex environments.
[1] G.P. Baeza, A.C. Genix, C. Degrandcourt, L. Petitjean, J. Gummel, M. Couty, J. Oberdisse, accepted in Macromolecules.
[2] A.C. Genix, M. Tatou A. Imaz, J. Forcada, R. Schweins, I. Grillo, J. Oberdisse, Macromolecules
2012, 45 (3), 1663.
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