Sciences et Ingénierie de la Matière Molle

Elastomers are widely used because of their large-strain reversible deformability. Most unfilled elastomers suffer from a poor mechanical strength, which limits their use. Using sacrificial bonds, we show how brittle, unfilled elastomers can be strongly reinforced in stiffness and toughness (up to 4 megapascals and 9 kilojoules per square meter) by introducing a variable proportion of isotropically prestretched chains that can break and dissipate energy before the material fails. Chemoluminescent cross-linking molecules, which emit light as they break, map in real time where and when many of these internal bonds break ahead of a propagating crack. The simple methodology that we use to introduce sacrificial bonds, combined with the mapping of where bonds break, has the potential to stimulate the development of new classes of unfilled tough elastomers and better molecular models of the fracture of soft materials.

The effect of reduced size on the elastic properties measured on silver and lead nanowires and on polypyrrole nanotubes with an outer diameter ranging between 30 and 250 nm is presented and discussed. Resonant-contact atomic force microscopy (AFM) is used to measure their apparent elastic modulus. The measured modulus of the nanomaterials with smaller diameters is significantly higher than that of the larger ones. The latter is comparable to the macroscopic modulus of the materials. The increase of the apparent elastic modulus for the smaller diameters is attributed to surface tension effects. The surface tension of the probed material may be experimentally determined from these AFM measurements.

Soft materials are materials with a low shear modulus relative to their bulk modulus and where elastic restoring forces are mainly of entropic origin. A sparse population of strong bonds connects molecules together and prevents macroscopic flow. In this review we discuss the current state of the art on how these soft materials break and detach from solid surfaces. We focus on how stresses and strains are localized near the fracture plane and how elastic energy can flow from the bulk of the material to the crack tip. Adhesion of pressure-sensitive-adhesives, fracture of gels and rubbers are specifically addressed and the key concepts are pointed out. We define the important length scales in the problem and in particular the elasto-adhesive length Γ/E where Γ is the fracture energy and E is the elastic modulus, and how the ratio between sample size and Γ/E controls the fracture mechanisms. Theoretical concepts bridging solid mechanics and polymer physics are rationalized and illustrated by micromechanical experiments and mechanisms of fracture are described in detail. Open questions and emerging concepts are discussed at the end of the review.

The issue of stress in thin films and functional coatings is a persistent problem in materials science and technology that has congregated many efforts, both from experimental and fundamental points of view, to get a better understanding on how to deal with, how to tailor, and how to manage stress in many areas of applications. With the miniaturization of device components, the quest for increasingly complex film architectures and multiphase systems and the continuous demands for enhanced performance, there is a need toward the reliable assessment of stress on a submicron scale from spatially resolved techniques. Also, the stress evolution during film and coating synthesis using physical vapor deposition (PVD), chemical vapor deposition, plasma enhanced chemical vapor deposition (PECVD), and related processes is the result of many interrelated factors and competing stress sources so that the task to provide a unified picture and a comprehensive model from the vast amount of stress data remains very challenging. This article summarizes the recent advances, challenges, and prospects of both fundamental and applied aspects of stress in thin films and engineering coatings and systems, based on recent achievements presented during the 2016 Stress Workshop entitled “Stress Evolution in Thin Films and Coatings: from Fundamental Understanding to Control.” Evaluation methods, implying wafer curvature, x-ray diffraction, or focused ion beam removal techniques, are reviewed. Selected examples of stress evolution in elemental and alloyed systems, graded layers, and multilayer-stacks as well as amorphous films deposited using a variety of PVD and PECVD techniques are highlighted. Based on mechanisms uncovered by in situ and real-time diagnostics, a kinetic model is outlined that is capable of reproducing the dependence of intrinsic (growth) stress on the grain size, growth rate, and deposited energy. The problems and solutions related to stress in the context of optical coatings, inorganic coatings on plastic substrates, and tribological coatings for aerospace applications are critically examined. This review also suggests strategies to mitigate excessive stress levels from novel coating synthesis perspectives to microstructural design approaches, including the ability to empower crack-based fabrication processes, pathways leading to stress relaxation and compensation, as well as management of the film and coating growth conditions with respect to energetic ion bombardment. Future opportunities and challenges for stress engineering and stress modeling are considered and outlined.

Systematic loading and unloading experiments, in uniaxial tension and uniaxial compression, have been performed on a double-network hydrogel exhibiting a very high toughness. We observed a significant hysteresis during the first loading cycle that increased strongly with the applied maximum deformation. A large hysteresis was not observed during a second loading cycle, implying that the initial hysteresis can be attributed to the fracture of covalent bonds in the primary network. We report this type of dissipative mechanism for polymer gels for the first time. Assuming that the entire energy dissipated during the hysteresis cycle can be attributed to the fracture of network strands by a Lake-Thomas mechanism, our results suggest that the fracture and unloading of only 1% of the bonds within the network leads to a decrease of up to 80% of the number of strands. These results also demonstrate the very large degree of heterogeneity within the hydrogel network. If such a dissipative mechanism is active at the crack tip, it will most likely greatly increase the energy necessary to propagate a macroscopic crack, elucidating the origin of the toughness in these interesting materials.

Semiconductor quantum dots (QDs) could significantly impact the performance of biomedical near-infrared (NIR) imaging by providing fluorescent probes that are brighter and more photostable than conventional organic dyes. However, the toxicity of the components of NIR emitting II-VI and IV-VI QDs that have been made so far (Cd, Hg, Te, Pb, etc.) has remained a major obstacle to the clinical use of QDs. Here, we present the synthesis of CuInS(2)/ZnS core/shell QDs emitting in the NIR ( approximately 800 nm) with good quantum yield and stability even after transfer into water. We demonstrate the potential of these QDs by imaging two regional lymph nodes (LNs) in vivo in mice. We then compare the inflammatory response of the axillary LN induced by different doses of CuInS(2)/ZnS and CdTeSe/CdZnS QDs and show a clear difference in acute local toxicity, the onset of inflammation only occurring at a 10 times more concentrated dose for CuInS(2)/ZnS QDs than for their Cd-containing counterparts.

Soft polymer networks have seen an explosion of recent developments motivated by new high tech applications in the biomedical field or in engineering. We present a candid and critical overview of the current understanding of the relation between the structure and molecular architecture of polymer networks and their mechanical properties, restricting ourselves to soft networks made of flexible polymers and displaying entropic elasticity. We specifically review and compare recent approaches to synthesize swollen hydrogels with enhanced toughness, resilient but tough unfilled elastomers and self-healing networks containing dynamic bonds. The purpose is less to draw a comprehensive catalogue of approaches than to identify and unify the underlying principles controlling toughening mechanisms and mechanical self-healing behavior and to point out remaining challenges.

Mechanochemistry provides a unique approach to investigate macroscopic deformation, failure and healing of polymer materials. The development of mechanophores - molecular units that respond to mechanical force - has been instrumental in the success of this endeavor. This review aims to provide a critical evaluation of the large variety of mechanophores reported in literature, and to assess the molecular and macroscopic factors that determine their activation. Applications in materials science are highlighted, and challenges in polymer mechanochemistry are discussed.

Filled elastomers exhibit a complex dependence of their viscoelastic modulus as a function of both temperature and frequency. Otherwise, recent observations on thin polymer films have shown that their glass transition temperature depends on their thickness. On the basis of these recent results and on a recent model, we propose that the mechanical behavior of the filled elastomer is strongly influenced by a gradient of the glass transition temperature in the vicinity of the particles. This allows us to suggest a specific temperature−frequency superposition law for filled rubbers. This law seems to apply very successfully on two systems with different dispersion qualities, revealing the existence of a glass transition temperature gradient in the vicinity of the particles.

In this work Raman spectroscopy was used for extensive characterization of multiwall carbon nanotube (MWNTs) and of MWCNTs/rubber composites. We have measured the Raman spectra of bundled and dispersed multiwall carbon nanotubes. All the Raman bands of the carbon nanotubes are seen to shift to higher wavenumbers upon debundling on account of less intertube interactions. Effects of laser irradiation were also investigated. Strong effects are observed by changing the wavelength of the laser excitation. On the other hand, at a given excitation wavelength, changes on the Raman bands are observed by changing the laser power density due to sample heating during the measurement procedure.

The effect of univalent electrolyte concentration on protein-polyelectrolyte complex formation has been measured by frontal analysis continuous capillary electrophoresis (FACCE) and turbidimetry for the interaction of bovine serum albumin (BSA) with a synthetic hydrophobically modified polyacid, for BSA with (porcine mucosal) heparin (Hp), a highly charged polyanion, and for Hp and insulin. All three highly diverse systems display maxima or plateaus in complex formation in the range of ionic strength 5 < I < 30 mM, confirmed in the case of BSA-Hp by multiple techniques. Similar maxima are reported in the literature, but with little discussion, for BSA-poly(dimethyldiallylammonium chloride), lysozyme-hyaluronic acid, and lysozyme-chondroitin sulfate, always in the I range 5-30 mM. While inversion of salt effect has been discussed specifically for the interaction of gelatin and sodium polystyrenesulfonate with gelatin(28) and with beta-lactoglobulin,(10) the general nature of this phenomenon, regardless of polyelectrolyte origin, molecular weight, and charge sign has not been recognized. The position of the maxima and their occurrence when protein and polyelectrolyte have the same net charge imply that they arise when Debye lengths extend, at low I, beyond half the protein diameter so that addition of salt screens repulsions, as well as attractions. This appears to be a general effect caused by electrostatic repulsions that can coexist simultaneously with hydrophobic interactions. Modeling of protein electrostatics via Delphi is used to visualize this effect for BSA, lysozyme, insulin, and beta-lactoglobulin.

Abstract The adhesion mechanisms of two acrylic Pressure-Sensitive-Adhesives on a stainless steel probe are investigated with a custom-designed probe tack apparatus. Our setup allows the simultaneous acquisition of a nominal stress and strain curve, and the observation of the adhesive film from underneath the transparent substrate. The temperature was varied in the range -20°C to 50°C and the debonding rate in the range 1–10000 μm/s. For all conditions we observed, upon debonding, the formation of cavities at or near the interface between the probe and the film. These cavities initially grew predominantly in the plane of the film but, at higher values of nominal strain, the walls between the cavities were stretched in the direction normal to the plane of the ifim to become a fibrillar structure. The transition from a cavitated structure to a fibrillar one was only found within a time-temperature window of rheological properties of the adhesive, while the adhesion energy was found to be mainly related to the elongational properties of the adhesive. The maximum tensile stress observed in the probe tack experiment was directly related to the appearance of the cavities and showed a good correlation with the shear modulus of the adhesive, while the adhesion energy was found to be mainly related to the elongational properties of the adhesive. The presence of 2% acrylic acid as a comonomer had a negligible effect on the maximum stress but a very important one on the formation of a fibrillar structure and on the locus of failure.

The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer—most probably due to a gradient of glass transition temperature around the solid particles—is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.

This review compares conventional and controlled radical polymerization techniques and processes in preparing nano-/microgels. Special focus is made on the synthetic parameters that allow controlling their size, morphology, composition, and structural homogeneity.

We have developed a novel surface coating for semiconductor quantum dots (QDs) based on a heterobifunctional ligand that overcomes most of the previous limits of these fluorescent probes in bioimaging applications. Here we show that QDs capped with bidentate zwitterionic dihydrolipoic acid-sulfobetaine (DHLA-SB) ligands are a favorable alternative to polyethylene glycol-coated nanoparticles since they combine small sizes, low nonspecific adsorption, preserved optical properties, and excellent stability over time and a wide range of pH and salinity. Additionally, these QDs can easily be functionalized with biomolecules such as streptavidin (SA) and biotin. We applied streptavidin-functionalized DHLA-SB QDs to track the intracellular recycling of cannabinoid receptor 1 (CB1R) in live cells. These QDs selectively recognized the pool of receptors at the cell surface via SA-biotin interactions with negligible nonspecific adsorption. The QDs retained their optical properties, allowing the internalization of CB1R into endosomes to be followed. Moreover, the cellular activity was apparently unaffected by the probe.

To predict the properties of superamphiphobic layers we analyzed the wetting of a square and a hexagonal array of vertical pillars composed of spheres (radius R) partially sintered together. Apparent contact angles above 150° are obtained by pinning of a non-polar liquid surface at the underside of the top sphere resulting in a Fakir or Cassie state. Analytical equations are derived for the impalement pressure in the limiting case A0 ≫ R2, where A0 is the area of the regular unit cell containing a single pillar. The case of close pillars is investigated numerically. By balancing forces at the rim of a drop, we calculate the apparent receding contact angle. To describe drag reduction of a flowing liquid we calculate the apparent slip length. When considering pressure-induced flow through cylindrical capillaries of radius rc, significant drag reduction occurs only for thin capillaries. The mechanical stability with respect to normal forces and shear is analyzed. Nanoscopic silica glass pillars would be able to sustain the normal and shear stresses caused by capillary and drag forces. For a high impalement pressure and good mechanical stability A0 should be small and R (respectively the neck diameter) should be large, whereas a large A0 and a small R imply low contact angle hysteresis and high slip length.

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the samples have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics, whereas moderate shear over-ages the system. We analyze these phenomena within the frame of the Bouchaud's trap model.

Silicate hydration is prevalent in natural and technological processes, such as, mineral weathering, glass alteration, zeolite syntheses and cement hydration. Tricalcium silicate (Ca3SiO5), the main constituent of Portland cement, is amongst the most reactive silicates in water. Despite its widespread industrial use, the reaction of Ca3SiO5 with water to form calcium-silicate-hydrates (C-S-H) still hosts many open questions. Here, we show that solid-state nuclear magnetic resonance measurements of (29)Si-enriched triclinic Ca3SiO5 enable the quantitative monitoring of the hydration process in terms of transient local molecular composition, extent of silicate hydration and polymerization. This provides insights on the relative influence of surface hydroxylation and hydrate precipitation on the hydration rate. When the rate drops, the amount of hydroxylated Ca3SiO5 decreases, thus demonstrating the partial passivation of the surface during the deceleration stage. Moreover, the relative quantities of monomers, dimers, pentamers and octamers in the C-S-H structure are measured.

Abiomimetic motility assay is used to analyze the mechanism of force production by site-directed polymerization of actin. Polystyrene microspheres, functionalized in a controlled fashion by the N-WASP protein, the ubiquitous activator of Arp2/3 complex, undergo actin-based propulsion in a medium that consists of five pure proteins. We have analyzed the dependence of velocity on N-WASP surface density, on the concentration of capping protein, and on external force. Movement was not slowed down by increasing the diameter of the beads (0.2 to 3 microm) nor by increasing the viscosity of the medium by 10(5)-fold. This important result shows that forces due to actin polymerization are balanced by internal forces due to transient attachment of filament ends at the surface. These forces are greater than the viscous drag. Using Alexa488-labeled Arp2/3, we show that Arp2/3 is incorporated in the actin tail like G-actin by barbed end branching of filaments at the bead surface, not by side branching, and that filaments are more densely branched upon increasing gelsolin concentration. These data support models in which the rates of filament branching and capping control velocity, and autocatalytic branching of filament ends, rather than filament nucleation, occurs at the particle surface.

In an attempt to establish connections between classical rheology and aging in paste colloidal suspensions, we report in this paper a large set of experimental results on a given system. We have studied suspensions of polyethylene oxide-protected silica particles and performed classical rheology experiments that exhibit a very nonlinear behavior. We have then evidenced aging through stress relaxation as observed in various glassy systems, and finally show other manifestations of aging through various rheological experiments. Qualitative agreement between these experimental results and the predictions of a simple model suggests that the behavior observed experimentally is governed by the competition between aging and mechanically induced rejuvenation.