Composite materials, comprising nanoparticles (NPs) dispersed in a matrix, are of great interest, since the NPs can enhance the matrix properties or impart new functionalities and because the matrix can act as a template that structures the particles at the nanometer scale. However, controlling the spatial distribution of NPs remains a challenging task, as it usually depends crucially on the particles surface chemistry. We present here a general approach to confine NPs in colloidal materials in a controlled fashion. We design nanocomposite material obtained by dispersing small quantities of NPs (at most 2%) in a colloidal crystalline matrix composed of thermosensitive copolymer micelles. The volume fraction of the micelles increases with temperature T, until crystallization occurs due to entropic reasons. Hence our system allows crystallization to be induced at the desired rate simply by varying T. By analogy with atomic crystals, we expect the NPs, which act as impurities, to be partially expulsed from the growing lattice and to segregate in the grain boundaries. We use scattering techniques and confocal microscopy to probe the microscopic and mesoscopic structures of the composite materials. We show that the NPs do not perturb the crystalline structure of the micelles but that different crystallization rates lead to different NPs rearrangements. We show that the NPs segregate in the grain-boundaries of the copolymer polycrystal. We demonstrate that the texture of the polycrystal can be tuned by varying independently the nanoparticle volume fraction and the crystallization rate, and quantify our findings using classical nucleation theory. Remarkably, we find that the efficiency of the segregation of the NPs in the grain-boundaries is determined solely by the typical size of the crystalline grains.

The evolution of the polymer structure during nanocomposite formation and annealing of silica-latex nanocomposites is studied using contrast-variation small angle neutron scattering. The experimental system is made of silica nanoparticles (Rsi ≈ 8 nm) and a mixture of purpose-synthesized hydrogenated and deuterated nanolatex (Rlatex ≈ 12.5 nm). The progressive disappearance of the latex beads by chain interdiffusion and release in the nanocomposites is analyzed quantitatively with a model for the scattered intensity of hairy latex beads and an RPA description of the free chains. In silica-free matrices and nanocomposites of low silica content (7%v), the annealing procedure over weeks at up to Tg + 85 K results in a molecular dispersion of chains, the radius of gyration of which is reported. At higher silica content (20%v), chain interdiffusion seems to be slowed down on time-scales of weeks, reaching a molecular dispersion only at the strongest annealing. Chain radii of gyration are found to be unaffected by the presence of the silica filler.

Model systems of nanocomposites are important for our understanding of the structural and dynamical contributions of the constituents to macroscopic properties, and in particular mechanical reinforcement. Systems in which the filler aggregation can be finely tuned thanks to an aqueous casting route were developed1. The control of the initial colloidal solution stability (mixture of nanolatex and colloidal silica) by the pH enabled to obtain various microstructures for given silica fractions. Using Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy (TEM) data, aggregation diagrams were obtained and enabled to study separately the effect of the silica fraction and its aggregation state on mechanical properties2 (see left side of the figure). On the other hand, we studied polymer structure and dynamic in such nanocomposites. SANS experiments were performed on samples prepared with a mixture of hydrogenated (H) and deuterated (D) chains in order to match the silica signal and get independently the polymer signal. A first system made with incompatible H and D latex beads enabled to follow the demixing kinetic for various silica fractions. The evolution of H zone size with annealing indicated that the polymer was immobilized in 15% v silica samples (see right side of the figure)2. A second system made with compatible beads highlighted the dissolution kinetic of latex beads with annealing. With sufficient annealing conditions a molecular mixture of H and D chains was obtained and the effect of silica fraction on chain radius of gyration was studied.

The tunable structure of silica-latex nanocomposites made of silica nanoparticles (radius ≈ 80 Å) and a copolymer of methyl methacrylate and butyl acrylate-latex beads (radius≈210 Å) has been studied by small-angle neutron scattering and transmission electron microscopy. An aggregation diagram as a function of the control parameters--silica volume fraction and precursor solution pH--has been established. In this aggregation diagram, isoaggregation lines have been identified. It was used to express the small-strain reinforcement factor measured with stress-strain isotherms as a function of volume fraction at fixed aggregation number in the range from 50 to 100. The large-strain properties have been rationalized using the energy needed to rupture samples, and this quantity has been found to present an optimum at intermediate volume fractions (15%). In order to understand the striking rheology of the system, a neutron contrast-matching study has been undertaken by adding deuterated polymer beads. The bead demixing kinetics during annealing has been used to characterize the dynamics in various environments defined by the hard silica structure. In particular, in nanocomposite samples containing 15 vol % of silica the dynamics is found to be blocked.

Nous utilisons des nanocomposites silice-latex comme système modèle du renforcement qui dépend de la structure des charges et des chaînes. Ces nanocomposites sont produits par filmification du latex à partir de solutions colloïdales, ce qui permet de jouer sur les interactions entre les particules via le pH des solutions mères. Nous avons pu montrer par Diffusion de Neutrons au Petits Angles (DNPA) que la variation du pH et de la fraction volumique de silice permettent de contrôler le degrè d'aggrégation final des particules, sans modification de l'interface charge-matrice. Nous nous sommes ensuite intéressés à la structure des chaînes (copolymère statistique PMMA/PBuA). La DNPA et la méthode du Contraste Moyen Nul permettent d'avoir accès au signal de la chaîne en mélangeant des chaînes H et D dans des proportions telles que le signal de la silice est effacé. Deux systèmes ont été étudiés : ● Lorsque les nanolatex H et D ont un système de stabilisation différent, une démixtion est observée. L'analyse de la cinétique de démixion donne des informations sur la dynamique des billes de latex en présence de charges. ● Avec la même stabilisation, les deux latex deviennent compatibles et nous avons pu suivre la dissolution progressive des billes de latex et l'interdiffusion des chaînes pour des recuits de plus en plus importants. Une modélisation originale des données de DNPA a été mise en place pour les structures intermédiaires. Notre modèle est basé sur une analyse quantitative de ces structures, avec un facteur de forme pour des objets sphériques de type coeur-couronne (billes de latex en cours de destruction) et une description RPA (Random Phase Approximation) pour les chaînes libres. Dans le fondu et les composites faiblement chargés, la procédure de recuit sur plusieurs semaines jusqu'à Tg + 70 K conduit à une dispersion moléculaire des chaînes permettant d'obtenir leur rayon de giration. Dans les composites avec 20%v de silice, l'interdiffusion des chaînes apparaît complètement bloquée à l'échelle de temps étudiée. Dans les deux systèmes étudiés, la présence de silice dans les composites fortement chargés réduit significativement la mobilité des chaînes.

The Watson-Crick binding of DNA single strands is a powerful tool for the assembly of nanostructures. Our objective is to develop polymer nanoparticles equipped with DNA strands for surface-patterning applications, taking advantage of the DNA technology, in particular, recognition and reversibility. A hybrid DNA copolymer is synthesized through the conjugation of a ssDNA (22-mer) with a poly(ethylene oxide)-poly(caprolactone) diblock copolymer (PEO-b-PCl). It is shown that, in water, the PEO-b-PCl-ssDNA(22) polymer forms micelles with a PCl hydrophobic core and a hydrophilic corona made of PEO and DNA. The micelles are thoroughly characterized using electron microscopy (TEM and cryoTEM) and small-angle neutron scattering. The binding of these DNA micelles to a surface through DNA recognition is monitored using a quartz crystal microbalance and imaged by atomic force microscopy. The micelles can be released from the surface by a competitive displacement event.

This work aims at demonstrating the interest of gradient copolymers in supercritical CO2 in comparison with block copolymers. Gradient copolymers exhibit a better solubility in supercritical CO2 than block copolymers, as attested by cloud point data. The self-assembly of gradient and block copolymers in dense CO2 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 CO2.