Osmotically induced deformation of capsid-like icosahedral vesicles

International audienceWe report the osmotic deformation of micron-size catanionic vesicles with icosahedral symmetry (20 faces, 12 vertices) upon incubation in small solutes (NaCl, glucose). The vesicles remain icosahedral at low osmotic pressure gradients across the bilayer, or spherical for outwards gradients. Above a threshold value of inwards pressure, the icosahedra develop a buckling instability: a depression is initiated at one or two ridges, grows until one vertex snaps into the icosahedra, leading to full collapse of one half of the vesicle into the other. Despite large local inversions in curvature, no release of encapsulated solutes is observed before the residual volume reaches negligible small values. Thin shell models correctly capture the buckling patterns of icosahedra in the low deformation limit. Comparison of experimental results with Monte Carlo simulations provides a first estimate for the conditions of shell disruption, and suggests it is predominantly driven by curvature rather than two-dimensional stretching or compression. The relevance of these results for the mechanics of viral capsids and controlled release applications is discussed

Une brève introduction à la matière molle

A short introduction to soft condensed matter (polymers, colloids, surfactants) is presented, with particular emphasis to recent progres and applications of small angle scattering. The text is in French

Small-Angle Neutron Scattering study of solubilization of tributyl phosphate in aqueous solutions of L64 Pluronic triblock copolymers

We have studied the solubilization behaviour of tributylphosphate (TBP) in aqueous solutions of L64-Pluronics, using light and small angle neutron scattering (SANS). Varying the temperature and the oil-content, the system presents a non trivial phase behaviour. In particular, at 308K, a first solubilization followed by an emulsification failure and a resolubilization is found. We have measured the microstructure by SANS and characterized the microemulsion droplet core-size, corona-thickness, polydispersity, and interactions. It is shown that at low oil content, the system is made of small swollen micelles. After the phase separation, the resolubilization is carried by larger oil droplets decorated by copolymer. From specific surface measurements at large angles, a surprising change in surfactant conformation is found to accompany this morphological evolution which is also supported by previous results obtained from 1H NMR experiments. In independent measurements, our structural modelling is confirmed using contrast-variation SANS

Polystyrene grafting from silica nanoparticles via Nitroxide-Mediated-Polymerization (NMP): synthesis and SANS analysis with contrast variation method,

International audienceWe present a new convenient and efficient “grafting from” method to obtain well defined polystyrene (PS) silica nanoparticles. The method, based on Nitroxide-Mediated Polymerization (NMP), consists to bind covalently the alkoxyamine, which acts as initiator controller agent, at the silica nanoparticles surface in two steps. The first step is a reaction between the aminopropylsilane and the silica particles in order to functionalize the particles surface with amino group. In a second step, the initiating-controlling alkoxyamine moiety is introduced via an over grafting reaction between the amino group and the N-hydroxysuccinimide based MAMA-SG1 activated ester. To simplify both their chemical transformation and the polymerization step, the native silica particles, initially dispersed in water, have been transferred in an organic solvent, the dimethylacetamide, which is also a good solvent for the polystyrene. The synthesis parameters have been optimized for grafting density, conversion rates, and synthesis reproducibility while keeping the colloidal stability and to avoid any aggregation of silica particles induced by the inter-particles interaction evolution during the synthesis. After synthesis, the final grafted objects have been purified and the non-grafted polymer chains formed in the solvent have been washed out by ultra filtration. Then the particles have been studied using Small angle Neutron Scattering (SANS) coupled to neutron contrast variation method. To optimize the contrast conditions, both hydrogenated and deuterated monomers have been used for the synthesis. A refined fitting analysis based on the comparison on two models, a basic core-shell and the Gaussian Pedersen model, enables us to fit nicely the experimental data for both the hydrogenated and deuterated grafted case. Differences are seen between grafting of normal or deuterated chains which can be due to monomer reactivity or to neutron contrast effect variations. The synthesis and the characterization method established in this work constitute a robust and reproducible way to design well defined grafted polymer nanoparticles. These objects will be incorporated in polymer matrices in a further step to create Nanocomposites for polymer reinforcement

Structure and rheological properties of soft-hard nanocomposites: Influence of aggregation and interfacial modification

A study of the reinforcement effect of a soft polymer matrix by hard nanometric filler particles is presented. In the main part of this article, the structure of the silica filler in the matrix is studied by Small Angle Neutron Scattering (SANS), and stress-strain isotherms are measured to characterize the rheological properties of the composites. Our analysis allows us to quantify the degree of aggregation of the silica in the matrix, which is studied as a function of pH (4-10), silica volume fraction (3-15%) and silica bead size (average radius 78 A and 96 A). Rheological properties of the samples are represented in terms of the strain-dependent reinforcement factor, which highlights the contribution of the filler. Combining the structural information with a quantitative analysis of the reinforcement factor, the aggregate size and compacity (10%-40%) as a function of volume fraction and pH can be deduced. In a second, more explorative study, the grafting of polymer chains on nanosilica beads for future reinforcement applications is followed by SANS. The structure of the silica and the polymer are measured separately by contrast variation, using deuterated material. The aggregation of the silica beads in solution is found to decrease during polymerization, reaching a rather low final aggregation number (less than ten).Comment: 17/12/200

Microemulsion nanocomposites: phase diagram, rheology and structure using a combined small angle neutron scattering and reverse Monte Carlo approach

The effect of silica nanoparticles on transient microemulsion networks made of microemulsion droplets and telechelic copolymer molecules in water is studied, as a function of droplet size and concentration, amount of copolymer, and nanoparticle volume fraction. The phase diagram is found to be affected, and in particular the percolation threshold characterized by rheology is shifted upon addition of nanoparticles, suggesting participation of the particles in the network. This leads to a peculiar reinforcement behaviour of such microemulsion nanocomposites, the silica influencing both the modulus and the relaxation time. The reinforcement is modelled based on nanoparticles connected to the network via droplet adsorption. Contrast-variation Small Angle Neutron Scattering coupled to a reverse Monte Carlo approach is used to analyse the microstructure. The rather surprising intensity curves are shown to be in good agreement with the adsorption of droplets on the nanoparticle surface