We report a complete analysis of model silica/styrene− butadiene rubber (SBR) nanocomposites including a direct and quantitative correlation between the filler structure and the mechanical reinforcement. We compared two different ways of sample processing: a solvent casting route with well-defined colloidal silica and the manufacturing process of internal mixing with industrial silica powder. The multiscale filler dispersion was characterized with a combination of SAXS/TEM in both reciprocal and direct space. The mechanical properties were determined with oscillatory shear measurements. We evaluated the influence of two polymer-filler interfacial additives on the filler dispersion: a coating agent and a coupling agent for different particle concentrations. Using simple analytical functions, we succeed in modeling the filler dispersion. We obtained surprisingly the same general trend whatever the sample processing, solvent casting, or internal mixing. The primary particles form fractal primary aggregates inside the matrix as a result of a diffusion-limited aggregation process driven by the interfacial additive. The coupling agent, which can form covalent bonds with the matrix chains, leads to smaller and denser primary aggregates while the coating one gives rise to larger and more ramified objects. This can be explained by the restriction of nanoparticle diffusion due to covalent bonds. The primary aggregates arrange into a secondary large scale structure, agglomerates, or by a branched network. The spatial correlations between the primary aggregates follow a Percus−Yevick function allowing us to distinguish between more or less interpenetrated networks in a situation of percolation. The viscoelastic behavior of the composites has been analyzed quantitatively with a percolation model. Below the percolation threshold, the reinforcement is mostly driven by the cluster compactness. We highlight a mechanical percolation whose threshold is dependent on the interfacial additive, but not on the material fabrication process, arising at lower filler volume fraction than the structural percolation. Above the percolation threshold, the network modulus varies as the power three of filler network density which is determined from geometrical assumptions. The filler network density, traducing the degree of interpenetration of the aggregates inside the network, is driven by both the interfacial additive and the samples preparation: the coupling agent as well as the internal mixing process gives rise to a denser network with a resulting improved modulus.
Nucleation and growth of SBA-15 silica nanostructured particles with well-defined morphologies has been followed with time by small-angle X-ray scattering (SAXS) and ultrasmall-angle X-ray scattering (USAXS), using synchrotron radiation. Three different morphologies have been compared: platelets, toroids, and rods. SEM observations of the particles confirm that two key physical parameters control the morphology: the temperature and the stirring of the solution. USAXS curves demonstrate that primary particles with a defined shape are present very early in the reaction mixture, immediately after a very fast nucleation step. This nucleation step is detected at 10 min (56 °C) or 15 min (50 °C) after the addition of the silica precursor. The main finding is that the USAXS signal is different for each type of morphology, and we demonstrate that the difference is related to the shape of the particles, showing characteristic form factors for the different morphologies (platelet, toroid, and rod). Moreover, the size of the mesocrystal domains is correlated directly with the particle dimensions and shape. When stirred, aggregation between primary particles is detected even after 12 min (56 °C). The platelet morphology is promoted by constant stirring of the solution, through an oriented aggregation step between primary particles. In contrast, toroids and rods are only stabilized under static conditions. However, for toroids, aggregation is detected almost immediately after nucleation.
Hard X-ray free electron lasers allow for the first time to access dynamics of condensed matter samples ranging from femtoseconds to several hundred seconds. In particular, the exceptional large transverse coherence of the X-ray pulses and the high time-averaged flux promises to reach time and length scales that have not been accessible up to now with storage ring based sources. However, due to the fluctuations originating from the stochastic nature of the self-amplified spontaneous emission (SASE) process the application of well established techniques such as X-ray photon correlation spectroscopy (XPCS) is challenging. Here we demonstrate a single-shot based sequential XPCS study on a colloidal suspension with a relaxation time comparable to the SACLA free-electron laser pulse repetition rate. High quality correlation functions could be extracted without any indications for sample damage. This opens the way for systematic sequential XPCS experiments at FEL sources.
A successful implementation of in situ X-ray scattering analysis of synthetized particle materials in silicon/glass microreactors is reported. Calcium carbonate (CaCO3) as a model material was precipitated inside the microchannels through the counter-injection of two aqueous solutions, containing carbonate ions and calcium ions, respectively. The synthesized calcite particles were analyzed in situ in aqueous media by combining Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) techniques at the ESRF ID02 beam line. At high wavevector transfer, WAXS patterns clearly exhibit different scattering features: broad scattering signals originating from the solvent and the glass lid of the chip, and narrow diffraction peaks coming from CaCO3 particles precipitated rapidly inside the microchannel. At low wavevector transfer, SAXS reveals the rhombohedral morphology of the calcite particles together with their micrometer size without any strong background, neither from the chip nor from the water. This study demonstrates that silicon/glass chips are potentially powerful tools for in situ SAXS/WAXS analysis and are promising for studying the structure and morphology of materials in non-conventional conditions like geological materials under high pressure and high temperature.
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