Identification of finite shear-elasticity at low thickness in the liquid state of molecular (OTP) and polymeric glass formers (PBuA)

International audienceFinite shear elasticity has been identified at low gap in the liquid state of various viscous fluids. The present study is expanded to a van der Waals glass former, the o-Terphenyl and to an ordinary polymer melt, the polybutylacrylate. Elasticity is also identified in these materials at the sub-millimeter scale and far above any phase transition. This macroscopic property is discussed in the frame of the terminal relaxation modes (-process or reptation times) and of their molecular interpretation

Richness of side-chain liquid-crystal polymers: From isotropic phase towards the identification of neglected solid-like properties in liquids

International audienceVery few studies concern the isotropic phase of Side-Chain Liquid-Crystalline Polymers (SCLCPs). However, the interest for the isotropic phase appears particularly obvious in flow experiments. Unforeseen shear-induced nematic phases are revealed away from the N-I transition temperature. The non-equilibrium nematic phase in the isotropic phase of SCLCP melts challenges the conventional timescales described in theoretical approaches and reveal very long timescales, neglected until now. This spectacular behavior is the starter of the present survey that reveals long range solid-like interactions up to the sub-millimetre scale. We address the question of the origin of this solid-like property by probing more particularly the non-equilibrium behavior of a polyacrylate substituted by a nitrobiphenyl group (PANO2). The comparison with a polybutylacrylate chain of the same degree of polymerization evidences that the solid-like response is exacerbated in SCLCPs. We conclude that the liquid crystal moieties interplay as efficient elastic connectors. Finally, we show that the " solid " character can be evidenced away from the glass transition temperature in glass formers and for the first time, in purely alkane chains above their crystallization temperature. We thus have probed collective elastic effects contained not only in the isotropic phase of SCLCPs, but also more generically in the liquid state of ordinary melts and of ordinary liquids

The missing parameter in rheology: hidden solid-like correlations in viscous liquids, polymer melts and glass formers

International audienceBACKGROUND: Determination of dynamic relaxation consists of measuring the viscous and the elastic components of a material by generally applying a small (oscillatory) deformation. The shear stress is transmitted to the material via contact with a substrate. Dating at least back to Stokes, the no-slip boundary condition between the fluid and the substrate is supposed to be fulfilled during this measurement. We show that the viscoelastic parameters of fluids are usually not determined under no-slip boundary conditions and do not originate from the first linear regime. Viscous and viscoelastic fluids (entangled and unentangled polymers, glass formers) measured under no-slip conditions exhibit a fundamentally different response with a dominant terminal solid-like response. RESULTS: We show that the terminal behaviour of fluids such as liquid polymers or glass formers measured at the sub-millimetre scale and far above the glass transition is not viscous but solid-like. Instead of a viscoelastic behaviour scaling as ω and ω 2 (ω is the frequency) for the viscous and the elastic moduli, respectively, the dynamic response is simplified; for low gap thickness, both viscous and elastic moduli are invariant with respect to the frequency (with the elastic modulus being larger than the viscous modulus) and enhanced by two to four orders of magnitude compared to the conventional viscoelastic response. Over a critical strain amplitude, the solid-like response decreases and is progressively replaced by the conventional viscoelastic behaviour. We discuss the implications of this observation and reconsider the assumptions inherent to a rheology measurement. CONCLUSION: The identification of so far neglected macroscopic elasticity in the fluidic state far above the glass transition temperature in entangled and unentangled polymers and glass formers shows that the liquid state is dominated by long range intermolecular interactions. This information is fundamental to understand and to foresee dynamic behaviour; it sheds further light on nonlinear phenomena such as large time scale relaxations, rheo-thinning, violation of the no-slip boundary condition and spectacular shear-induced instabilities (spurt effect, 'shark-skin' instabilities, gross melt fracture, etc.) that are unpredictable in the frame of the conventional viscoelastic approach. It also implies that the viscoelastic times (reptation, Rouse) in polymers are not the longest relaxation times of these materials

New Light on Old Wisdoms on Molten Polymers: Conformation, Slippage and Shear Banding in Sheared Entangled and Unentangled Melts

International audienceThe flow of viscoelastic materials is usually interpreted as resulting from intramolecular properties. Typically, the non-linear flow behaviour and sluggish relaxation dynamics in entangled polymers are interpreted by a disentanglement process. This molecular interpretation has never been validated by direct observation. We report here on in situ observations of polymer melts under steady-state shear flow using neutron scattering and particle tracking velocimetry. It is shown that the chains remain largely undeformed under steady-state shear flow whereas wall slippage and shear-banding are identified in both entangled and unentangled polymer melts. These observations are of prime importance; they reveal that the flow mechanism and its viscoelastic signature reflect a collective effect and not properties of individual chains

Insight into the multiscale structure of pre-stretched recast Nafion (R) membranes: Focus on the crystallinity features

International audienceThe structure of solution cast Nafion (R) membranes that were uniaxially stretched, and then thermally annealed was studied over an extended observation window, from a few angstroms to several hundreds of angstroms, using both Small-Angle X-Ray Scattering (SAXS) and Wide-Angle X-Ray Diffraction (WAXD). This Nafion (R) membrane preparation allowed highlighting new structural features, never reported previously. Indeed, while SAXS data evidenced moderate effect of stretching with different behaviors of the matrix and ionomer peaks, WAXD spectra showed a strong stretching induced anisotropy of the crystalline phase. These results defy the Nafion (R) structural models and call for further theoretical and experimental developments taking into account these new data. (C) 2015 Elsevier Ltd. All rights reserved

Inside the structure of a nanocomposite electrolyte membrane: how hybrid particles get along with the polymer matrix

International audienceHybrid materials remain the target for a fruitful range of investigations, especially for energy devices. A number of hybrid electrolyte membranes consisting of inorganic and organic phases were then synthesized. Mechanical, solvent uptake and ionic transport properties were studied with the key point being the characteristic length scale of the interaction between the phases. A group of nanocomposite membranes made of polystyrenesulfonic acid-grafted silica particles embedded in a Poly(Vinylidene Fluoride-co-HexaFluoroPropene) (PVdF-HFP) matrix was studied by combining neutron and X-ray scatterings on the nanometer to angstrom scale. This approach allows for the variation in the morphology and structure as a function of particle loading to be described. These studies showed that the particles aggregate with increasing particle loading and these aggregates swell, creating a physical interaction with the polymer matrix. Particle loadings lower than 30 wt% induce a slight strain between both of the subphases, namely the polymer matrix and the particles. This strain is decreased with particle loading between 20 and 30 wt% conjointly with the beginning of proton conduction. Then the percolation of the aggregates is the beginning of a significant increase of the conduction without any strain. This new insight can give information on the variation in other important intrinsic properties

Water crystallization inside fuel cell membranes probed by X-ray scattering

International audienceWater properties in ionomer membranes at sub-zero temperatures are investigated as a function of water volume fraction using a micro X-ray beam scanning the membranes along their thicknesses. The influence of both the size of confinement and the membrane nature on water behavior is analyzed. The study is focused on Nafion ® membranes at different water contents to tune the size of water confinement and on water-saturated sulfonated polyimide (sPI) membrane. The scattering curves show that water does not crystallize inside Nafion ® membrane up to a characteristic size of water confinement of 3 nm. Water does crystallize inside Nafion ® for characteristic size of ionic domains larger than 5 nm, i.e. for hyperswollen state. In order to predict qualitatively the freezing occurrence of water in the Nafion ® matrix, the Gibbs-Thomson model was considered; both the effect of confinement and acidity were taken into account. In this context, the water-Nafion ® interfacial tension was determined using a contact angle experiment. The model is in accordance with a water crystallization occurring inside Nafion ® provided that it is highly swollen. However, water behavior is also strongly dependent on the nature of the membrane. In the case of water-saturated sulfonated polyimide, a hydrocarbon based membrane, water does not crystallize upon cooling but does not desorb upon annealing in contrast to Nafion ® membrane exposed to the same external conditions (temperature and relative humidity). Hence, the host-water interaction appears as a crucial parameter to take into account to describe properly the sorption-desorption and freezing phenomena

Récupération électrochimique de métaux précieux contenus dans des déchets d’équipements électriques et électroniques (DEEE) en milieu liquide ionique

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Récupération électrochimique de métaux précieux contenus dans des déchets d’équipements électriques et électroniques (DEEE) en milieu liquide ionique

National audienc

Thermoplastic/thermoset multilayer composites: A way to improve the impact damage tolerance of thermosetting resin matrix composites

International audienceWe present a method to improve the impact damage tolerance of thermoset resin matrix composites by exploiting thermoplastic material impact resistance properties. Two configurations of thermoplastic/thermoset multilayer composites are considered: (a) thermoplastic material is used on sample surface as shock absorber and (b) thermoplastic film is inserted in between thermoset layers to act as interlayer. We aim at increasing adhesion strength between thermoplastic and thermoset resin matrix. We show that mode I critical strain energy release rate, determined by the wedge double cantilever beam fracture test, increases significantly when a third amorphous polymer interlayer compatible with both thermoplastic and thermoset resin is inserted. The analysis of scanning electron microscope fracture surfaces helps us to clarify the adhesion mechanism involved. The high adhesion between thermoplastic and thermoset resins obtained with this method, together with the ability of thermoplastics to dissipate the impact damage in the first plies ensure the reliability of this stacked configuration. Finally, we show that these multilayer composite significantly increases the impact resistance of composite materials