Small-angle scattering applied to microemulsions and liquid crystals

The shape and interactions between model microemulsion droplets (R = 8.2 nm, polydispersity 20%) either decorated with hydrophilic-hydrophobic diblock (PEO-m: C12H25-(EO)n, MPEO = 5.2 kg/mol), or with telechelic triblock copolymers (PEO-2m: C12H25-(EO)2n-C12H25 , MPEO = 10.4 kg/mol) have been studied by small angle neutron scattering (SANS). The results as a function of droplet and copolymer concentration have been compared to the reference case of the bare microemulsion. Using Porod representations, the average bare droplet size was found to be independent of microemulsion concentration in the range studied here, up to some 13%v. Upon addition of copolymer (from r=0 to 30 hydrophobic stickers per droplet), the average droplet radius was unaffected. The interactions between bare and decorated droplets have been analyzed using the structure factor S(q), at first in a model-free way based on its low-q limit S(q→0). This analysis provides clear evidence on the concentration-dependent repulsive or attractive nature of the contributions to the pair droplet-droplet pair potential of the copolymers. Model pair potentials describing the steric repulsions and attractions by copolymer bridging are used to describe the low-q behavior of the structure factor based on an integral equation approach, giving a quantitative estimate of the range and amplitude of the potentials. Moreover, they provide an explanation for the observed transient clustering in terms of a shallow minimum of the total potential.

The existence of two independent filler-dependent high-temperature Maxwell–Wagner–Sillars (MWS) dielectric processes is demonstrated and characterized in detail in silica-filled styrene–butadiene (SB) industrial nanocomposites of simplified composition using Broadband Dielectric Spectroscopy (BDS). The uncrosslinked samples are made with 140 kg/mol SB-chains, half of which carry a single graftable end-function (50% D3), and Zeosil 1165 MP silica incorporated by solid-phase mixing. While one high-temperature process is known to exist in other systems, the dielectric properties of a new silica-related process – strength, relaxation time, and activation energy – have been evidenced and described as a function of silica volume fraction and temperature. In particular, it is shown that its strength follows a percolation behavior as observed with the ionic conductivity and rheology. Moreover, activation energies show the role of polymer layers separating aggregates even when they are percolated. Apart from simultaneous characterization over a broad frequency range up to local polymer and silanol dynamics, it is believed that such high-temperature BDS-measurements can thus be used to detect reorganizations in structurally-complex silica nanocomposites. Moreover, they should contribute to a better identification of dynamical processes via the described sensitivity to structure in such systems.

The formation of aggregates in simplified industrial styrene–butadiene nanocomposites with silica filler hasbeen studied using a recent model based on a combination of electron microscopy, computer simulations,and small-angle X-ray scattering. The influence of the chain mass (40 to 280 kg/mol, PI < 1.1), which setsthe linear rheology of the samples, was investigated for a low (9.5 vol%) and high (19 vol%) silica volumefraction. 50% of the chains bear a single graftable end-group, and it is shown that the (chain-massdependent) grafting density is the structure-determining parameter. A model unifying all available dataon this system is proposed and used to determine a critical aggregate grafting density. The latter is foundto be closely related to the mushroom-to-brush transition of the grafted layer. To our best knowledge,this is the first comprehensive evidence for the control of the complex nanoparticle aggregate structurein nanocomposites of industrial relevance by the physical parameters of the grafted layer.

L’incorporation de nanoparticules dans des matrices polymériques est couramment utilisée pour améliorer les propriétés des matériaux (mécaniques, thermiques, optiques, électriques, de transfert…). Ces dernières sont fortement dépendantes de la microstructure développée au sein du matériau. Alors que la fraction volumique et la taille des particules sont déterminées par la formulation, la dispersion est plus difficile à contrôler. Dans cet exposé nous nous intéresserons à des nanocomposites modèles formulés à partir de solutions colloïdales de silice et de nanolatex d’acrylates qui permettent d’obtenir des nanocomposites aux dispersions de particules variées en utilisant différents paramètres : la charge de surface des nanoparticules, le rapport de taille entre les particules de latex et de silice (R), et la masse molaire des chaînes de polymères. Nous montrerons que des agrégats répulsifs de taille variable peuvent être obtenus en jouant sur le pH. Une bonne dispersion des particules est obtenue pour des tailles comparables de particules de polymère et de silice, alors qu’un réseau de particules de silice décorant les particules de latex est mis en évidence lorsque le rapport R est grand. Enfin, en diminuant la masse molaire de la matrice, des agrégats fractals de taille croissante sont générés grâce à la mobilité accrue des particules au sein de matrice polymérique de plus faible viscosité2. L’ensemble des dispersions de silice a été étudié par la combinaison de techniques de diffusion aux petits angles (SAXS) réalisées sur des grands instruments (pour la large gamme de vecteur d’onde disponible) et de microscopie électronique à transmission (TEM). Nous verrons que pour de faibles fractions volumiques de particules les résultats SAXS et TEM ont pu être quantitativement corrélés par des simulations Monte Carlo d’agrégats polydisperses. Par ailleurs la structure des chaînes de polymère a pu être suivie par des mesures de diffusion de neutrons (SANS) réalisées dans des conditions de contraste moyen nul (en utilisant des latex hydrogénés et deutérés). Nous verrons que la cinétique de dissolution de billes de latex au sein du matériau est impactée par les nanoparticules de silice alors que le rayon de giration des chaînes demeure constant.

SAXS, TEM, modelisation and rheology