The internal structure of nanometric microgels in water has been studied as a function of temperature, crosslinker content, and level of deuteration. Small-angle neutron scattering from poly(N isopropylmethacrylamide) (pNIPMAM, volume phase transition ≈ 44 °C) microgel particles of radius well below 100 nm in D2O has been measured. The intensities have been analyzed with a combination of polymer chain scattering and form-free radial monomer volume fraction profiles defined over spherical shells, taking polydispersity in size of the particles determined by AFM into account. A reverse Monte Carlo optimization using a limited number of parameters was developed to obtain smoothly decaying profiles in agreement with the experimentally scattered intensities. Result are compared to the swelling curve of microgel particles in the temperature range from 15 to 55 °C obtained from photon correlation spectroscopy (PCS). In addition to hydrodynamic radii measured by PCS, our analysis provides direct information about internal water content and gradients, the strongly varying steepness of the density profile at the particle-water interface, the total spatial extension of the particles, and the visibility of chains. The model has also been applied to a variation of the crosslinker content, N,N′ methylenebisacrylamide (BIS), from 5 to 15 mol%, providing insight in the impact of chain architecture and crosslinking on water uptake and on the definition of the polymer-water interface. The model can easily be generalized to arbitrary monomer contents and types, in particular mixtures of hydrogenated and deuterated species, paving the way to detailed studies of monomer distributions inside more complex microgels, in particular core-shell particles.

The dynamics of glass-forming systems shows a multitude of features that are absent innormal liquids, such as non-exponential relaxation and a strong temperature-dependence ofthe relaxation time. Connecting these dynamic properties to the microscopic structure of thesystem is challenging because of the presence of the structural disorder. Here we usecomputer simulations of a metallic glass-former to establish such a connection. By probingthe temperature and wave-vector dependence of the intermediate scattering function wefindthat the relaxation dynamics of the glassy melt is directly related to the local arrangement oficosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponentialrelaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that isreminiscent to the one found in a solid. Our results show that in metallic glass-formers thesetwo types of relaxation processes can coexist and give rise to a dynamics that is surprisinglycomple

One of the problems largely commented in the sol-gel science is how to make large bodies, because gels tend to crack during drying. The drying stresses are attributed to capillary phenomena and differential strain which result from a pressure gradient in the pore liquid. By the supercritical drying (SCD), the capillary stresses are eliminated and monolithic aerogel can be obtained. This chapter focuses on silica aerogel, the most studied aerogel. It presents an overview of the supercritical drying techniques, but also some of the remarkable aerogel properties (optical, mechanical, thermal and acoustical, etc.) with respect to its peculiar microstructure. The chapter briefly presents other kinds of aerogels (oxides and chalcogenideaerogels, composite aerogels, organic aerogels, etc.) and a panel of potential applications.

Aerogels are generally described in terms of brittle and elastic materials, like glasses or ceramics with poor and not predictable mechanical properties. We propose the synthesis of a nanocomposite aerogels with improved mechanical properties by addition of silica particles (aerosil; 20–100 nm) in the monomer solution. The elastic modulus and rupture strength increase, stiffening and strengthening the structure by a factor 5–8. The toughness, the critical flaw size, and the fracture energy increase also versus the silica particle concentration. Moreover, the mechanical strength distribution and the Weibull modulus, m, characterizing the statistical nature of flaw size in brittle materials show a more homogeneous flaw size distribution. Pores are considered as integral part of flaws, and small angle X-ray scattering shows that the usual fractal structure observed in aerogel disappears with the silica particle addition. The fractal structure in aerogel is characteristic of a large pore size distribution, but also of a large flaw size distribution. If nanocomposites behave as brittle material under a tension stress, they exhibit a different response when the structure is submitted to a compression: densification and plastic hardening. These opposite behaviors (brittle and plastic) are surprisingly related to the same kinds of gel features: pore volume, silanol content, and the pore size distribution. Because of improved mechanical properties, these nanocomposites aerogels could be used as host matrices for the synthesis of glass ceramics, doped glasses, and composites.

Une étude sur la caractérisation de bois tropicaux (la grenadille) pour la facture d’instruments à vent est en cours dans le cadre d’une collaboration entre Henri SELMER Paris et le LMGC. Les recherches portent actuellement sur la diffusion de l’eau dans des échantillons de bois de différentes espèces et présentant des formes variées (cylindriques, parallélépipédiques, autres...), la modélisation de la teneur en eau à l’équilibre (EMC) sous diverses conditions atmosphériques, la mesure des coefficients de retrait et gonflement du bois au cours de changement de teneur en eau, et la validation d’un modèle de diffusion en utilisant l’imagerie TéraHertz (THz).

Une introduction à la spectroscopie Raman de résonance

Introduction to Raman spectroscopy of carbon nanostructures