Julian Oberdisse

Structure of interacting aggregates of silica nanoparticles in a polymer matrix: small-angle scattering and reverse Monte Carlo simulations

Reinforcement of elastomers by colloidal nanoparticles is an important application where microstructure needs to be understood-and, if possible, controlled-if one wishes to tune macroscopic mechanical properties. Here, the three-dimensional structure of large aggregates of nanometric silica particles embedded in a soft polymeric matrix is determined by small angle neutron scattering. Experimentally, the crowded environment leading to strong reinforcement induces a strong interaction between aggregates, which generates a prominent interaction peak in the scattering. We propose to analyze the total signal by means of a decomposition in a classical colloidal structure factor describing aggregate interaction and an aggregate form factor determined by a reverse Monte Carlo technique. The result gives new insight to the shape of aggregates and their complex interaction in elastomers. For comparison, fractal models for aggregate scattering are also discussed.

Surface aggregate structure of nonionic surfactants on silica nanoparticles

The self-assembly of two nonionic surfactants, pentaethylene glycol monododecyl ether (C12E5) and n-dodecyl-β-maltoside (β-C12G2), in the presence of a purpose-synthesized silica sol of uniform particle size (diameter 16 nm) has been studied by adsorption measurements, dynamic light scattering and small-angle neutron scattering (SANS) using a H2O/D2O mixture matching the silica, in order to highlight the structure of the surfactant aggregates. For C12E5, strong aggregative adsorption onto the silica beads, with a high plateau value of the adsorption isotherm above the CMC was found. SANS measurements were made at a series of loadings, from zero surfactant up to maximum surface coverage. It is found that the spherical core-shell model nicely reproduces the SANS data up to and including the local maximum at q = 0.42 nm−1 but not in the Porod region of high q, indicating that the surface area of the adsorbed surfactant is underestimated by the model of a uniform adsorbed layer. A satisfactory representation of the entire scattering profiles is obtained with the model of micelle-decorated silica beads, indicating that C12E5 is adsorbed as spherical micellar aggregates. This behaviour is attributed to the high surface curvature of the silica, which prevents an effective packing of the hydrophobic chains of the amphiphile in a bilayer configuration. For the maltoside surfactant β-C12G2 very weak adsorption on the silica beads was found. The SANS profile indicates that this surfactant forms oblate ellipsoidal micelles in the silica dispersion, as in the absence of the silica beads.

Solid particles in an elastomer matrix: impact of colloid dispersion and polymer mobility modification on the mechanical properties

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.

Pluronics‐Stabilized Gold Nanoparticles: Investigation of the Structure of the Polymer–Particle Hybrid

Hybrid gold-polymer nanoparticles are obtained by self-assembly of amphiphilic copolymers (Pluronics) in solutions containing preformed gold nanoparticles (diameter ca. 12 nm). Dynamic light scattering, TEM, cryo-TEM, and small-angle neutron scattering experiments with contrast variation are used to characterize the structure of the gold-polymer particles. Five Pluronics (F127, F68, F88, F108, P84) with different molecular weights and hydrophilic/hydrophobic balances are investigated. Gold nanoparticles are individually embedded within globules of polymer, even under conditions for which Pluronics micelles do not form in solution. The hybrid particles are several tens of nanometers in size (larger than micelles of the corresponding Pluronics), and the size can be tuned by changing the temperature.

Universal scattering behavior of coassembled nanoparticle-polymer clusters.

Water-soluble clusters made from 7-nm inorganic nanoparticles have been investigated by small-angle neutron scattering. The internal structure factor of the clusters was derived and exhibited a universal behavior as evidenced by a correlation hole at intermediate wave vectors. Reverse Monte Carlo calculations were performed to adjust the data and provided an accurate description of the clusters in terms of interparticle distance and volume fraction. Additional parameters influencing the microstructure were also investigated, including the nature and thickness of the nanoparticle adlayer.

Effect of nanoparticle size on the morphology of adsorbed surfactant layers.

The surface aggregate structure of dimethyldodecylamine-N-oxide (C(12)DAO) in three silica dispersions of different particle sizes (16-42 nm) was studied by small-angle neutron scattering (SANS) in a H(2)O/D(2)O solvent mixture matching the silica. At the experimental conditions (pH 9), the surfactant exists in its nonionic form, and the structure of the adsorbed layer is not affected by added electrolyte. It is found that C(12)DAO forms spherical surface micelles of 2 nm diameter on the 16 nm silica particles, but oblate ellipsoidal surface micelles are formed on the 27 and 42 nm particles. The dimensions of these oblate surface aggregates (minor and major semiaxes R(n) and R(lat)) are similar to those of C(12)DAO micelles in the aqueous solutions. It is concluded that the morphological transition from spherical to ellipsoidal surface aggregates is induced by the surface curvature of the silica particles. A comparison of the shape and dimensions of the surface aggregates formed by C(12)DAO and C(12)E(5) on the 16 nm silica particles demonstrates that the nature of the surfactant head group does not determine the morphology of the surface aggregates but has a strong influence on the number of surface aggregates per particle due to the different interactions of the head groups with the silica surface.

Solubility and Self-Assembly of Amphiphilic Gradient and Block Copolymers in Supercritical CO2

This work aims at demonstrating the interest of gradient copolymers in supercritical CO(2) in comparison with block copolymers. Gradient copolymers exhibit a better solubility in supercritical CO(2) than block copolymers, as attested by cloud point data. The self-assembly of gradient and block copolymers in dense CO(2) has been characterized by Small-Angle Neutron Scattering (SANS), and it is shown that it is not fundamentally modified when changing from block copolymers to gradient copolymers. Therefore, gradient copolymers are advantageous thanks to their easier synthesis and their solubility at lower pressure while maintaining a good ability for self-organization in dense CO(2).

Collective rearrangement at the onset of flow of a polycrystalline hexagonal columnar phase.

Creep experiments on polycrystalline surfactant hexagonal columnar phases show a power law regime, followed by a drastic fluidization before reaching a final stationary flow. The scaling of the fluidization time with the shear modulus of the sample and stress applied suggests that the onset of flow involves a bulk reorganization of the material. This is confirmed by x-ray scattering under stress coupled to in situ rheology experiments, which show a collective reorientation of all crystallites at the onset of flow. The analogy with the fracture of heterogeneous materials is discussed.

The new very-small-angle neutron scattering spectrometer at Laboratoire Léon Brillouin

The design and characteristics of the new very-small-angle neutron scattering spectrometer under construction at the Laboratoire Leon Brillouin are described. Its goal is to extend the range of scattering-vector magnitudes towards 2 × 10−4 A−1. The unique feature of this new spectrometer is a high-resolution two-dimensional image-plate detector sensitive to neutrons. The wavelength selection is achieved by a double-reflection supermirror monochromator and the collimator uses a novel multibeam design.

Structure of nanoparticles embedded in micellar polycrystals.

We investigate by scattering techniques the structure of water-based soft composite materials comprising a crystal made of Pluronic block-copolymer micelles arranged in a face-centered cubic lattice and a small amount (at most 2% by volume) of silica nanoparticles, of size comparable to that of the micelles. The copolymer is thermosensitive: it is hydrophilic and fully dissolved in water at low temperature (T ~ 0 °C), and self-assembles into micelles at room temperature, where the block-copolymer is amphiphilic. We use contrast matching small-angle neuron scattering experiments to independently probe the structure of the nanoparticles and that of the polymer. We find that the nanoparticles do not perturb the crystalline order. In addition, a structure peak is measured for the silica nanoparticles dispersed in the polycrystalline samples. This implies that the samples are spatially heterogeneous and comprise, without macroscopic phase separation, silica-poor and silica-rich regions. We show that the nanoparticle concentration in the silica-rich regions is about 10-fold the average concentration. These regions are grain boundaries between crystallites, where nanoparticles concentrate, as shown by static light scattering and by light microscopy imaging of the samples. We show that the temperature rate at which the sample is prepared strongly influence the segregation of the nanoparticles in the grain-boundaries.