We present experimental and theoretical results on intrusion-extrusion cycles of water in hydrophobic mesoporous materials, characterized by independent cylindrical pores. The intrusion, which takes place above the bulk saturation pressure, can be well described using a macroscopic capillary model. Once the material is saturated with water, extrusion takes place upon reduction of the externally applied pressure. Our results for the extrusion pressure can only be understood by assuming that the limiting extrusion mechanism is the nucleation of a vapor bubble inside the pores. A comparison of calculated and experimental nucleation pressures shows that a proper inclusion of line tension effects is necessary to account for the observed values of nucleation barriers. Negative line tensions of order 10(-11) J m(-1) are found for our system, in reasonable agreement with other experimental estimates of this quantity.

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International audienceUltrahigh-molecular-weight polyethylene (UHMWPE) has been processed by means of sintering of a nascent powder. Particular attention was paid to the precompaction of the powder just below the melting point (Tm) under vacuum. The particle welding was subsequently carried out under pressure at various temperatures above Tm for various durations. Tensile drawing experiments performed on sintered samples either at room temperature or above Tm were specifically aimed at discriminating the role of chain interdiffusion through the particle interfaces from that of cocrystallization in the mechanism of particle welding. It turned out that efficient welding occurred within a very short time. One of the novel results of the work is the much weaker influence of sintering time as compared with temperature, giving evidence that chain interdiffusion is not governed by a reptation process. The entropy-driven melting explosion over distances much larger than the chain length between entanglements is suggested to be the main mechanism of the fast chain re-entanglement and particle welding in the present case of a nascent powder consisting of nonequilibrium chain-disentangled crystals. Another major aspect of this study is the demonstration of the huge cocrystallization efficiency in the interface consolidation in the solid state that significantly hides the kinetics of chain intertwining occurring in the melt

The phenomenon of cavitation generally appears close to yielding in the high-density polyethylene. It can affect the yield stress and the properties at large strains. The influence of the microstructural and molecular parameters on cavitation is not well established; it is not even clear whether the cavitation is a cause or a consequence of plasticity. In this work, we focus on the initiation of cavitation and on the nucleation rate. Various polyethylenes with a wide range of microstructural and molecular parameters have been obtained. The cavitation is followed up by SAXS in-situ tensile tests. It is found that, depending on the polyethylene, cavitation can be avoided or, on the contrary, appears before or after yielding. The stresses necessary to initiate cavitation and crystallite shearing have been relied respectively on stress transmitters (tie molecules, interphase, etc.) and crystallite thickness. Then the comparison between the materials has allowed predicting the various polyethylene behaviors. All of the latter have been explained by a simple model based on very few microstructural parameters. Surprisingly, our results have shown that all the scenarios of plasticity and cavitation are possible. One is the cause or the consequence of the other in accordance with the molecular topology and the microstructure.

Cellulose whiskers have been used as reinforcement in a copolymer matrix prepared from a latex phase. If a water suspension‐mixing procedure is adopted, the fibril breakage that usually occurs during the mixing with a molten polymer can be avoided, and an enhanced filler dispersion can be expected. In this study, different processing methods have been used to prepare composite films, either by film casting (water evaporation) or by freeze drying, followed by classical compression or extrusion processes. The thermomechanical properties of these nanocomposites have been investigated, and the influence of processing conditions and the effect of whisker content have been considered. Processing conditions have a large influence on the mechanical behavior and can be classified in ascending order of their reinforcement efficiency: It can be attributed to a decrease of the apparent whisker aspect ratio, due to gradual breakage and/or orientation of the whiskers when hot pressing or extrusion is used. Below Tg, good agreement is found between experimental moduli and the theoretical predictions of the Halpin‐Kardos equation. On the other hand, above Tg, a spectacular reinforcing effect is observed, which is widely underestimated by this short fiber composite model. This is related to the presence of a rigid cellulose network, linked by hydrogen bonds, when the whisker content is above its percolation threshold. The quality of this network (i.e., density and homogeneity) and thus, the magnitude of the reinforcing effect, depend on processing conditions.

We study the slow dynamics of water evaporation out of hydrophobic cavities by using model porous silica materials grafted with octylsilanes. The cylindrical pores are monodisperse, with a radius in the range of 1-2 nm. Liquid water penetrates in the nanopores at high pressure and empties the pores when the pressure is lowered. The drying pressure exhibits a logarithmic growth as a function of the driving rate over more than three decades, showing the thermally activated nucleation of vapor bubbles. We find that the slow dynamics and the critical volume of the vapor nucleus are quantitatively described by the classical theory of capillarity without adjustable parameter. However, classical capillarity utterly overestimates the critical bubble energy. We discuss the possible influence of surface heterogeneities, long-range interactions, and high-curvature effects, and we show that a classical theory can describe vapor nucleation provided that a negative line tension is taken into account. The drying pressure then provides a determination of this line tension with much higher precision than currently available methods. We find consistent values of the order of −30 pN in a variety of hydrophobic materials. A remarkable property of water is its ability to form nanosize bubbles, or cavities, on hydrophobic bodies (1). Since their first direct observations through atomic force microscopy about a decade ago (2, 3), surface nanobubbles on hydrophobic surfaces have raised considerable interest, and they are believed to play a major role in surface-driven phenomena, such as boundary slippage of water flows, heat transfer at walls, vaporization and boiling, surface cleaning, etc. (4, 5). In a different context, the evaporation of water in the vicinity of hydrophobic bodies has been studied as a core mechanism for the hydrophobic interaction mediated by water (6-8), which plays a central role in biological matter. The formation of cavities able to bridge hydrophobic units provides a driving force for protein folding and supermolecular aggregation (9). Simulation examples of such drying-induced phenomena include the collapse of a polymer chain, multidomain proteins, and hydrophobic particles (9-13).Despite their direct observation, the easy formation and the high stability of nanobubbles on hydrophobic bodies still raise fundamental questions (5,14,15). Because of significant theoretical work, it is now established that, at the scale of the nanometer, macroscopic concepts apply: hydrophobicity is described by interfacial energies, and the drying transition in hydrophobic confinement is a first-order transition triggered by the nucleation of a critical vapor bubble (1). The energy barrier limiting the kinetics of this transition is a strong signature of nanobubbles properties. Evaporation kinetics has also been pointed out as the most direct measure of the importance of hydrophobic collapse in protein folding (9). However, rate effects in the drying transition have not received much attention. A few numerical studies have addr...

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Intercalated and exfoliated nanocomposites were prepared by the extrusion and injection of polyamide‐6 and highly swollen or slightly swollen montmorillonite, respectively. The microstructure of pure compounds was characterized. The basal spacing was more homogeneous in swollen montmorillonite than in nontreated montmorillonite. This was attributed to the presence of a surfactant. Surfactant crystallites were observed in swollen montmorillonite and in the nanocomposites. Their distribution is discussed. The nanocomposites exhibited anisotropic properties attributed to the orientation of the montmorillonite sheets. The preferential orientation of the montmorillonite sheets was studied in detail by small‐angle X‐ray scattering (SAXS). It was found to be related to the injection direction. An average distance between two sheets was also measured from the SAXS spectra. The crystallographic state of the polyamide matrix was then analyzed. The orientation of the polymer lamellae relative to the montmorillonite sheets is discussed. The orientation of the montmorillonite sheets and the polyamide lamellae is expected to play a major role in the mechanical properties of nanocomposites. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1360–1370, 2001