Model systems of nanocomposites are important for our understanding of the structural and dynamical contributions of the constituents to macroscopic properties, and in particular mechanical reinforcement. We study silica-latex nanocomposites obtained from drying an aqueous colloidal mixture of polyacrylate nanolatex and silica nanoparticles. In these systems, the filler microstructure is controlled by the precursor solution pH and it is possible to work at constant aggregation number for different filler fractions [1]. We have followed the evolution of the polymer structure during nanocomposite formation and annealing using small angle neutron scattering under zero-average contrast conditions for the silica nanoparticles. The progressive disappearance of the latex beads by chain interdiffusion and release in the nanocomposites is analyzed with a model for the scattered intensity of hairy latex beads and an RPA description of the free chains. In silica-free matrices and nanocomposites of low (7%v) and high (20%v) silica content, the annealing procedure results in a molecular dispersion of chains. However, the interdiffusion of individual chain is delayed in presence of 20% of silica. The radius of gyration was not found to be affected by the silica loading in these samples with similar aggregation state (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] M. Tatou et al., Macromolecules 44, 9029 (2011) [2] A.C. Genix et al., Macromolecules 45, 1663 (2012)

nanocomposites: structure and rheology

Scattering from self-assembled systems

Distribution de particules et conformation de chaînes dans des matériaux nanocomposites

Simplified silica (Zeosil 1165 MP) and SBR (140k carrying silanol end-groups) nanocomposites have been formulated by mixing of a reduced number of ingredients with respect to industrial applications. The thermo-mechanical history of the samples during the mixing process was monitored and adjusted to identical final temperatures. The filler structure on large scales up to micrometers was studied by transmission electron microscopy (TEM) and very small-angle X-ray scattering (SAXS). A complete quantitative model extending from the primary silica nanoparticle (of radius ≈10 nm), to nanoparticles aggregates, up to micrometer-sized branches with typical lateral dimension of 150 nm is proposed. Image analysis of the TEM-pictures yields the fraction of zones of pure polymer, which extend between the branches of a large-scale filler network. This network is compatible with a fractal of average dimension 2.4 as measured by scattering. On smaller length scales, inside the branches, small silica aggregates are present. Their average radius has been deduced from a Kratky analysis, and it ranges between 35 and 40 nm for all silica fractions investigated here (Φsi = 8−21% vol.). A central piece of our analysis is the description of the interaggregate interaction by a simulated structure factor for polydisperse spheres representing aggregates. A polydispersity of 30% in aggregate size is assumed, and interactions between these aggregates are described with a hard core repulsive potential. The same distribution in size is used to evaluate the polydisperse form factor. Comparison with the experimental intensity leads to the determination of the average aggregate compacity (assumed identical for all aggregates in the distribution, between 31% and 38% depending on Φsi), and thus aggregation number (ca. 45, with a large spread). Because of the effect of aggregate compacity and of pure polymer zones, the volume fraction of aggregates is higher in the branches than Φsi. The repulsion between aggregates has a strong effect on the apparent isothermal compressibility: it leads to a characteristic low-q depression, which cannot be interpreted as aggregate mass decrease in our data. In addition, the reinforcement effect of these silica structures in the SBR-matrix is characterized with oscillatory shear and described with a model based on the same aggregate compacity. Finally, our results show that it is possible to analyze the complex structure of interacting aggregates in nanocomposites of industrial origin in a self-consistent and quantitative manner.

Diblock copolymers of styrene and gelable acryloxy propyl triethoxysilane (APTES) were successfully prepared through Nitroxide-Mediated Polymerization (NMP). Their micellar behaviour in dioxane-methanol solutions was investigated by Small Angle X-ray Scattering (SAXS) and Neutron Scattering (SANS) measurements. The micelles were found to be spherical whatever the copolymer composition with a PS core size varying from 13.5 nm to 37.8 nm depending on the PS chain length and the swelling ratio. The thickness of the PAPTES corona increased from 6.0 nm to 10.8 nm when increasing the PAPTES polymerization degree from 91 to 336. Two micellar structural regimes were identified, depending on the chain length of the PAPTES soluble block. For PAPTES polymerization degrees higher than 100, the PS core radius in dried state RDC can be consistently described by a scaling relationship RDC α (NPS0.34NPAPTES−0.069) and the aggregation number Nagg obeys the relationship Nagg α (NPS0.021NPAPTES−0.21), consistent with previous studies. For polymerization degrees of the soluble block lower than 100, it was shown that RDC and Nagg do not follow the expected scaling relationships with higher values of Nagg, possibly due to complete chain incorporation in the micelle core. After the cross-linking process, a decrease of the hydrodynamic diameter of 1.6 to 4.7 nm was observed. This is mainly due to the corona shrinkage, the aggregation number and the core radius being directly dependent of the parent non-crosslinked micelles.

The reinforcement of elastomers by inorganic fillers, a concept of very high technological importance, is commonly understood to result from the presence of a mechanical network of partially aggregated filler particles. The non-linear mechanical properties, in particular the decrease of the modulus at high strain (Payne effect), are further interpreted to be a consequence of the breakdown of this filler network. There are, however, many open questions concerning the actual nature of the interparticle connections, where a modified polymer layer forming "glassy bridges" constitutes one possibility. In this work, we address this issue with a suitable silica-filled model elastomer, where we characterize the silica dispersion by SANS in combination with reverse Monte-Carlo modeling, and the mobility modification of the polymer by low-field proton NMR spectroscopy. In our samples, we identify a glassy layer as well as a region of intermediate mobility (possibly modified Rouse modes). Based on the structural information from SANS, we are able to quantify the amount of interparticle connections, and correlate it with the magnitude of the Payne effect taken from shear rheology. This works only if we assume that these connections comprise both, the glassy layer as well as the region of intermediate mobility. The amount of glassy immobilized polymer only does not suffice to explain the mechanical properties.