Polymer nanocomposites (PNCs) are made by dispersion of hard fillers in a polymer matrix, where the fillers are usually added to enhance mechanical properties. Thus, such materials attract industrial interest and their investigation is essential for the improvement of their performances.The present study concerns nanocomposites obtained using hydrophilic silica nanoparticles (NPs) dispersed in a soft polymer matrix of styrene-butadiene, commonly used in car tire manufacturing. The mechanical performances of such PNCs are strongly related to the structural properties: the dispersion state of silica particles in the polymer melt is governed by the mixing protocols (e.g., solid phase mixing or solvent casting) and thermodynamics of the system (i.e., particle miscibility) which influence the filler-filler and the filler-polymer interactions.We developed a new model nanocomposite system, equivalent to a simplified industrial one, made by solvent casting technique using colloidal silica NPs. In such model PNCs, the dispersion of colloidal silica is tuned via the surface-modification of silica particles by grafting hydrophobic molecules, i.e., silane coating agents, which promote the filler-polymer compatibility.To describe NP structure in solvents and after casting into polymer matrix, we propose a new method which combines Small Angle X-ray Scattering (SAXS), reverse Monte Carlo (RMC) and aggregate recognition (AR) analysis. This approach allows obtaining a detailed structural characterization of the silica NPs surface-modified with different silane grafting groups (i.e., tri-ethoxy, tri-methoxy or mono-methoxy silane). In particular, we show that silane molecules with trifunctional grafting functions may lead to the formation of polycondensed patches (i.e., reaction of lateral groups with neighboring molecules) on silica surface, favoring attractive interactions. Thus, the lowest aggregation is observed using mono-functionalized silane molecules where patches cannot form.Next, the SAXS-RMC-AR approach will be extended to investigate the structural properties of nanocomposite systems where silica NPs are surface-modified with silane molecules of various alkyl chain.

Tuning local nanoparticle arrangements and dynamical properties in polymer nanocomposites by grafting of small molecules

Contact-resonance atomic force microscopy (CR-AFM) is of great interest and very valuable for a deeper understanding of the mechanics of biological materials with moduli of at least a few GPa. However, sample surfaces can present a high topography range with significant slopes, where the local angle can be as large as ± 50°. The non-trivial correlation between surface slope and CR-frequency hinders a straightforward interpretation of CR-AFM indentation modulus measurements on such samples. We aim to demonstrate the significant influence of the surface slope on the CR-frequency that is caused by the local angle between sample surface and the AFM cantilever and present a practical method to correct the measurements. Based on existing analytical models of the effect of the AFM set-up's intrinsic cantilever tilt on CR-frequencies, we compute the non-linear variation of the first two (eigen)modes CR-frequency for a large range of surface angles. The computations are confirmed by CR-AFM experiments performed on a curved surface. Finally, the model is applied to directly correct contact modulus measurements on a durum wheat starch granule as an exemplary sample.

We investigate the non-linear buckling patterns produced by the elastic Rayleigh-Taylor instability in a hyper-elastic slab hanging below a rigid horizontal plane, using a combination of experiments, weakly non-linear expansions and numerical simulations. Our experiments reveal the formation of hexagonal patterns through a discontinuous transition. As the unbuckled state is transversely isotropic, a continuum of linear modes become critical at the first bifurcation load: the critical wavevectors form a circle contained in a horizontal plane. Using a weakly non-linear post-bifurcation expansion, we investigate how these linear modes cooperate to produce buckling patterns: by a mechanism documented in other transversely isotropic structures, three-modes coupling make the unbuckled configuration unstable with respect to hexagonal patterns by a transcritical bifurcation. Stripe and square patterns are solutions of the post-bifurcation expansion as well but they are unstable near the threshold. These analytical results are confirmed and complemented by numerical simulations.

Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3, and α−FAPbI3 (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus C44. Further, a tendency towards an incipient ferroelastic transition is observed in FAPbBr3. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α−FAPbI3.

Recent experimental results using in particular small-angle scattering to characterize the self-assembly of mainly hard spherical nanoparticles into higher ordered structures ranging from fractal aggregates to ordered assemblies are reviewed. The crucial control of interparticle interactions is discussed, from chemical surface-modification, or the action of additives like depletion agents, to the generation of directional patches and the use of external fields. It is shown how the properties of interparticle interactions have been used to allow inducing and possibly controlling aggregation, opening the road to the generation of colloidal molecules or potentially metamaterials. In the last part, studies of the microstructure of polymer nanocomposites as an application of volume-spanning and stress-carrying aggregates are discussed.