Quantitative analysis of the debonding structure of soft adhesives

We experimentally investigate the growth dynamics of cavities nucleating during the first stages of debonding of three different model adhesives. The material properties of these adhesives range from a more liquid-like material to a soft viscoelastic solid and are carefully characterized by small strain oscillatory shear rheology as well as large strain uniaxial extension. The debonding experiments are performed on a probe tack set-up. Using high contrast images of the debonding process and precise image analysis tools we quantify the total projected area of the cavities, the average cavity shape and growth rate and link these observations to the material properties. These measurements are then used to access corrected effective stress and strain curves that can be directly compared to the results from the uniaxial extension

Harnessing entropy to enhance toughness in reversibly crosslinked polymer networks

Reversible crosslinking is a design paradigm for polymeric materials, wherein they are microscopically reinforced with chemical species that form transient crosslinks between the polymer chains. Besides the potential for self-healing, recent experimental work suggests that freely diffusing reversible crosslinks in polymer networks, such as gels, can enhance the toughness of the material without substantial change in elasticity. This presents the opportunity for making highly elastic materials that can be strained to a large extent before rupturing. Here, we employ Gaussian chain theory, molecular simulation, and polymer self-consistent field theory for networks to construct an equilibrium picture for how reversible crosslinks can toughen a polymer network without affecting its linear elasticity. Maximisation of polymer entropy drives the reversible crosslinks to bind preferentially near the permanent crosslinks in the network, leading to local molecular reinforcement without significant alteration of the network topology. In equilibrium conditions, permanent crosslinks share effectively the load with neighbouring reversible crosslinks, forming multi-functional crosslink points. The network is thereby globally toughened, while the linear elasticity is left largely unaltered. Practical guidelines are proposed to optimise this design in experiment, along with a discussion of key kinetic and timescale considerations

Adhesion and non-linear rheology of adhesives with supramolecular crosslinking points

Soft supramolecular materials are promising for the design of innovative and highly tunable adhesives. These materials are composed of polymer chains functionalized by strongly interacting moieties, sometimes called "stickers". In order to systematically investigate the effect of the presence of associative groups on the debonding properties of a supramolecular adhesive, a series of supramolecular model systems has been characterized by probe-tack tests. These model materials, composed of linear and low dispersity poly(butylacrylate) chains functionalized in the middle by a single tri-urea sticker, are able to self-associate by six hydrogen bonds and range in molecular weight (M n) between 5 and 85 kg/mol. The linear rheology and the nanostructure of the same materials (called "PnBA3U") was the object of a previous study 1,2. At room temperature, the association of polymers via hydrogen bonds induces the formation of rod-like aggregates structured into bundles for M n \textless{} 40kg/mol and the behavior of a soft elastic material was observed (G'\textgreater{}\textgreater{}G "and G'~$\omega$ 0). For higher M n , the filaments were randomly oriented and polymers displayed a crossover towards viscous behavior although terminal relaxation was not reached in the experimental frequency window. All these materials show however similar adhesive properties characterized by a cohesive mode of failure and low debonding energies (W adh \textless{}40J/m 2 for a debonding speed of 100$\mu$m/s). The debonding mechanisms observed during the adhesion tests have been investigated in detail with an Image tools analysis developed by our group 3. The measure of the projected area covered by cavities growing in the adhesive layer during debonding can be used to estimate the true stress in the walls of the cavities and thus, to characterize the in-situ large strain deformation of the thin layer during the adhesion test itself. This analysis revealed in particular that the PnBA3U materials with M n \textless{} 40 kg/mol soften very markedly at large deformation like yield stress fluids, explaining the low adhesion energies measured for these viscoelastic gels.

Le jeu "soignant" en pédiatrie face aux douleurs iatrogènes: travail de Bachelor

Thème : Les enfants suivis en soins palliatifs pour des maladies chroniques font face à une multitude de soins, dont ils témoignent que les plus douloureux sont ceux employant une aiguille. L’objectif de cette revue de littérature est d’évaluer les bénéfices de l’utilisation de la distraction par le jeu chez l’enfant dans la prise en charge des douleurs iatrogènes induites par l’utilisation de matériel effractif nécessitant une aiguille. Contenus : La recherche documentaire s’est appuyée sur les bases de données PubMed et CINAHL, ainsi que sur le site de Pediadol. Les mots-clés utilisés ont été : enfant, aiguille, douleur, distraction et jeu : sept articles ont été identifiés et retenus. Ces derniers incluent des enfants de 0 à 19 ans ayant reçu un soin utilisant une aiguille (prise de sang, vaccination, …), et pour lesquels une autoévaluation de la douleur a été mesurée. Discussion : L’utilisation d’une distraction est efficace pour diminuer la douleur ressentie par l’enfant lors de soins effractifs utilisant une aiguille. La distraction par le jeu n’a pas été évaluée de manière spécifique, il faisait partie des distractions proposées. Pour une prise en charge efficace de la douleur chez l’enfant, les infirmiers-infirmières doivent considérer le stade de développement de l’enfant afin d’utiliser un outil d’évaluation de la douleur fiable et proposer une distraction adaptée. Il est primordial d’impliquer activement les enfants et leurs parents dans le processus de soins, et d’évaluer leur anxiété avant le soin afin de trouver ensemble la stratégie optimisant la qualité de vie

Pattern Formation During Deformation of a Confined Viscoelastic Layer: From a Viscous Liquid to a Soft Elastic Solid

We study pattern formation during tensile deformation of confined viscoelastic layers. The use of a model system (PDMS with different degrees of crosslinking) allows us to go continuously from a viscous liquid to an elastic solid. We observe two distinct regimes of fingering instabilities: a regime called "elastic" with interfacial crack propagation where the fingering wavelength only scales with the film thickness, and a bulk regime called "viscoelastic" where the fingering instability shows a Saffman-Taylor-like behavior. We find good quantitative agreement with theory in both cases and present a reduced parameter describing the transition between the two regimes and allowing to predict the observed patterns over the whole range of viscoelastic properties.Comment: 4 pages, 7 figures, typos corrected, figure 5 replace

A new sighting study for the fixed concentration procedure to allow for gender differences

The fixed concentration procedure (FCP) has been proposed as an alternative to the median lethal concentration (LC50) test (organisation for economic co-operation and development (OECD) test guideline [TG] 403) for the assessment of acute inhalation toxicity. The FCP tests animals of a single gender (usually females) at a number of fixed concentration levels in a sequential fashion. It begins with a sighting study that precedes the main FCP study and is used to determine the main study starting concentration. In this paper, we propose a modification to the sighting study and suggest that it should be conducted using both male and female animals, rather than just animals of a single gender. Statistical analysis demonstrates that, when females are more sensitive, the new procedure is likely to give the same classification as the original FCP, whereas, if males are more sensitive, the new procedure is much less likely to lead to incorrect classification into a less toxic category. If there is no difference in the LC50 for females and males, the new procedure is slightly more likely to classify into a more stringent class than the original FCP. Overall, these results show that the revised sighting study ensures gender differences in sensitivity do not significantly impact on the performance of the FCP, supporting its use as an alternative test method for assessing acute inhalation toxicity

pH/Temperature control of interpolymer complexation between poly(acrylic acid) and weak polybases in aqueous solutions

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