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The structure of polymer nanocomposites has important consequences on final properties, like e.g.mechanical reinforcement. The structure of the hard filler phase is usually characterized by electronmicroscopy and small-angle X-ray scattering (SAXS), and we will review recent examples of fillerstructure in model and industrial nanocomposites. The chain conformation can only bemeasured by small-angle neutron scattering (SANS) with isotopic substitution. Over the past 15 years,considerable efforts have been dedicated to such measurements. Results appear to converge towardGaussian statistics of chains, unperturbed by the presence of filler NPs, and exceptions seem to be dueto system-dependent effects. In nearly all studies, however, unexplained pollution of the scatteringdata in the low-q range has been observed, in spite of careful contrast-matching.In this study, the conformation of polymer chains in silica-latex nanocomposites has been studiedunder zero-average contrast (ZAC) conditions using SANS. Samples have been prepared by dryingcolloidal suspensions of silica and polymer nanoparticles (NPs) for two different silica NPs (radius of5 and 15 nm) and two chain masses (17 and 100 kg/mol). By mixing appropriate amounts ofhydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silicascattering is suppressed. However, the SANS scattering intensities displayed distinct deviations at lowangles. Comparison of the SANS and SAXS intensities proves that the measurement of chain structureby SANS is perturbed by unwanted silica contributions, i.e., the silica is ‘mysteriously’ visible. Aquantitative model rationalizing these contributions for the first time is proposed. It is based on astatistical H/D-mismatch in the local environment of the silica NPs. Such a mismatch leads to theexistence of a nanometer-thick polymer shell surrounding silica NPs, which carries H/D contrast andthereby produce an undesired scattering contribution. These shells are presumably maintained by thereduced mobility of polymer close to interfaces or attractive polymer-silica interactions. The effect ofunwanted filler scattering is NP size-dependent − it is pronounced only for smaller beads. This isprobably why ZAC turned out to be so difficult with polymer nanocomposites.

Ref HAL: hal-01408800_v1
Ref Arxiv: 1811.08158
DOI: 10.1016/j.eurpolymj.2016.08.028
WoS: 000390497100051
Ref. & Cit.: NASA ADS
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11 Citations
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A large body of experimental work on the microstructure and dynamics of simplifiedindustrial nanocomposites made of disordered silica filler in a styrene-butadiene matrixby solid-phase mixing is regrouped and critically discussed in this feature article. Recentresults encompass systems with varying polymer mass, grafting functionality, and fillercontent. They have been obtained by simulation-based structural modelling of nanoparticleaggregate size and mass deduced from small-angle scattering and transmission electronmicroscopy. Our model has been validated by independent swelling experiments.Comparison of structurally-close nanocomposites of widely different chain mass led tothe identification of a unique structure-determining parameter, the grafting density, aswell as to a unified picture of aggregate formation mechanisms in complex nanocompositesduring mixing. In addition, low-field proton NMR allowed for the characterization ofdynamically slowed-down (‘glassy’) polymer layers, which were shown not to dominatethe rheological response, unlike the structural contribution. Finally, broadband dielectricspectroscopy was used in an innovative manner to identify filler percolation – also identifiedby rheology – via dynamics along filler surfaces.

Ref HAL: hal-01398746_v1
DOI: 10.1016/j.commatsci.2016.07.041
WoS: WOS:000383827500039
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10 Citations
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We discuss two procedures to obtain empirical potentials from ab initio trajectories. The first methodconsists in adjusting the parameters of an empirical pair potential so that the radial distribution functionsextracted from classical simulations using this potential match the ones extracted from the ab initio sim-ulations. As a case study, we consider the example of amorphous silica, a material that is highly relevantin the field of glass science as well as in geology. With our approach we are able to obtain an empiricalpotential that gives a better description with respect to structural and thermodynamic properties thanthe potential proposed by van Beest, Kramer, and van Santen, and that has been very frequently usedas a model for amorphous silica. The second method is the so-called ‘‘force matching” approach proposedby Ercolessi and Adams to obtain an empirical potential. We demonstrate that for the case of silica thismethod does not yield a reliable potential and discuss the likely origin for this failure.

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La diffusion hyper-Raman (DHR) est une spectroscopie optique des vibrations qui présente des règles de sélection différentes du celles du Raman et de l'infrarouge. Après avoir décrit les principes de la DHR, nous ferons un état de l'art de la dynamique vibrationnelle (IR, Raman, neutrons) du système relaxeur modèle Pb1/3Mg2/3NbO3 (PMN). Nous insisterons sur le comportement des modes mous polaires dont l'observation est uniquement possible par diffusion hyper-Raman. Les résultats ont été capturés dans un modèle qui offre une vision cohérente des propriétés dynamiques de toute une famille de matériaux. Ils relient par exemples les vibrations à la structure polaire locale, et donnent une origine microscopique aux propriétés diélectriques géantes de ces systèmes.

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Une introduction à la théorie de la réponse linéaire nous permettra dans un premier temps de définir les notions de susceptibilité, de spectre des fluctuations, de fonction d’auto-corrélation,… et de relier ces grandeurs à travers le théorème de fluctuation-dissipation. Nous déterminerons ensuite l’origine de la diffusion de la lumière et calculerons l’intensité Raman et Brillouin diffusée dans le cadre d’une théorie macroscopique (classique). Ces calculs feront apparaitre des règles de sélection qui seront mises en pratique à travers quelques exemples simples pris parmi les solides cristallins et les liquides.

Ref HAL: hal-01388588_v1
PMID 27768376
DOI: 10.1103/PhysRevLett.117.155501
WoS: 000385341900016
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15 Citations
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The fluctuations of electric polarization in a disordered ferroelectric substance, relaxor crystal PbMg_{1/3}Nb_{2/3}O_{3} (PMN), were studied using a nonlinear inelastic light-scattering technique, hyper-Raman scattering, within a 5-100  cm^{-1} spectral interval and in a broad temperature range from 20 to 900 K. The split ferroelectric mode reveals a local anisotropy of up to about 400 K. Spectral anomalies observed at higher temperatures are explained as due to avoided crossing of the single primary polar soft mode with a temperature-independent, nonpolar spectral feature near 45  cm^{-1}, known from Raman scattering. The temperature changes of the vibrational modes involved in the measured fluctuation spectra of PMN were captured in a simple model that accounts for the temperature dependence of the dielectric permittivity as well. The observed slowing down of the relaxational dynamics directly correlates with the huge increase of the dielectric permittivity.