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We investigate the fracture properties of poly(acrylamide- co-1-vinylimidazole) dual crosslink hydrogels [P(AAm- co-VIm)-M2+ gels] containing a small fraction of covalent bonds and a majority of dynamic bonds based on metal coordination bonds (Ni2+ or Zn2+). Unlike a previous study on a different dual crosslink hydrogel system having slower dynamic bonds based on poly(vinylalcohol) and borate ions (PVA-Borax gels), the presence of these faster dynamic coordination bonds has two main effects: They significantly toughen the P(AAm- co-VIm)-M2+ gels even at high stretch rates, where the dynamic bonds should in principle behave as covalent bonds at the crack tip, and they toughen the gels at very low stretch rates, where the dynamic bonds are invisible during the loading stage. We propose two additional molecular mechanisms to rationalize this behavior of P(AAm- co-VIm)-M2+ gels: we hypothesize that fast exchanging dynamic bonds remain slow compared to the characteristic time of bond scission and are, therefore, able to share the load upon covalent bond scission even at low loading rates. We also argue of the existence of longer-lived clusters of dynamic bonds that introduce a stretch rate-dependent strain hardening in uniaxial tension and stabilize and increase the size of the dissipative zone at the crack tip, thereby introducing a strain-dependent dissipative mechanism.

The molecular level transfer of stress from a stiff percolating filler to a stretchable matrix is a crucial and generic mechanism of toughening in soft materials. Yet the molecular details of how this transfer occurs have so far been experimentally unreachable. Model multiple network elastomers containing spiropyran (SP) force sensors incorporated into the stiff filler network or into the stretchable matrix network are used here to detect and investigate the mechanism of stress transfer between distinct populations of polymer strands. We find that as the filler network progressively breaks by random bond scission, there is a critical stress where cooperative bond scission occurs and the macroscopic stretch increases discontinuously by necking. Surprisingly, SP molecules reveal that even in the necked region both filler and matrix chains share the load, with roughly 90% of the SPs force-activated in the filler chains before necking still being loaded in the necked region where significant activation of the SP incorporated into the matrix chains occurs. This result, where both networks remain loaded upon necking, is qualitatively consistent with the model proposed by Brown, where holes or microcracks are formed in the stiff regions and are bridged by stretched matrix chains. Detection of merocyanine (i.e. activated SP) fluorescence by confocal microscopy shows that such microcrack formation is also active at the crack tip even for materials that do not exhibit macroscopic necking. Additionally, we demonstrate that when the ethyl acrylate monomer is replaced by hexyl methacrylate in the first network, preventing molecular connections between the two networks, the stress transmission is less efficient. This study outlines the different roles played by these multiple networks in the onset of fracture and provides molecular insights for the construction of molecular models of fracture of elastomers., The molecular level transfer of stress from a stiff percolating filler to a stretchable matrix is a crucial and generic mechanism of toughening in soft materials.

A mechanochemistry based approach is proposed to detect and map stress history during dynamic processes. Spiropyran (SP), a force sensitive molecular probe, was incorporated as a crosslinker into multiple network elastomers (MNE). When these mechanochromic MNEs are loaded, SP undergoes a well-known force-activated reaction to merocyanine (MC) changing its absorption in the visible range (visible blue color). This SP to MC transition is not reversible within the time frame of the experiment and the color change reports the concentration of activated molecules. During subsequent loading–unloading cycles the MC undergoes a fast and reversible isomerization resulting in a slight shift of absorption spectrum and results in a second color change (blue to purple color corresponding to the loading–unloading cycles). Quantification of the color changes by using chromaticity shows that the exact color observed upon unloading is characteristic not only of the current stress (reported by the shift in color due to MC isomerization), but of the maximum stress that the material has seen during the loading cycle (reported by the shift in color due to the change in MC concentration). We show that these two color changes can be separated unambiguously and we use them to map the stress history in the loading and unloading process occurring as a crack opens up and propagates, breaking the material. Color maps on fractured samples are compared with finite element simulations and the agreement is excellent., A mechanochemistry based approach is proposed to detect and map stress history during dynamic processes.

International audience; Synthesis of the polyelectrolytes. We followed a simple procedure previously reported by our group for neutral monomers. 1,2 In this method based on free radical polymerization, a chain transfer agent used as a redox co-initiator allows to control the molecular weight. For PMADAP telomer, 1 mmol (~ 17 g) of MADAP was first dissolved in MilliQ water in a 3-necked flask. The flask was then sealed and the solution was bubbled with N2 gas for 1 h before the start of the reaction. 1 mmol of KPS and 2 mmol of AET-HCl were separately dissolved in MilliQ water and bubbled with nitrogen for 1 h. Next, the initiators were injected into the 3-necked flask in a dropwise manner and the reaction was allowed to proceed to overnight. In the end, an excess of NaOH was added into the solution to neutralize the HCl and recover the PMADAP in the basic form. The reaction medium was freeze-dried without further purification. Note that the excess sodium ions will be removed during dialysis after the next step. The dry telomer (1 mmol of MADAP) was dissolved in 120 ml of NMP after 1 h of heating (60 °C) and vigorous stirring. Separately, 20 mmol of AA and 10 mmol of the coupling agent (DCCI) were dissolved in 15 ml of NMP. Note that both AA and DCCI are in large excess to the

Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to measure spatially heterogeneous mechanical behavior in soft solids, with high sensitivity to deformation and both spatial and temporal resolution. In this paper, we discuss the mathematical approach behind the quantification of the deformation rate from MSDWS data and provide guidelines for optimizing the selection of experimental parameters in measurements. After validating the method in extensional tests on an elastomer, we provide an example of the potentiality of MSDWS by measuring the spatial distribution of the deformation rate during shear debonding of adhesive tapes. We quantitatively characterize the deformation rate distribution related to shearing and peeling under loading. A highly heterogeneous deformation rate distribution is observed, and time-dependent measurements reveal an increase in deformation localization hundreds of seconds before full debonding. This behavior, previously predicted by theory and simulation, is demonstrated here experimentally for the first time.

Significance Soft materials can be toughened by creating dissipative mechanisms in stretchy matrixes. Yet using them over a wide range of temperatures requires dissipative mechanisms independent of stretch rate or temperature. We show that sacrificial covalent bonds in multiple network elastomers are most useful in toughening elastomers at high temperature and act synergistically with viscoelasticity at lower temperature. We do not attribute this toughening mechanism only to the scission of bonds during crack propagation but propose that the highly stretched network diluted in a stretchy matrix acts by simultaneously stiffening the elastomer and delaying the localization of bond scission and the propagation of a crack. Such a toughening mechanism has never been proposed for elastomers and should guide network design.

Description, Tough elastomers that resist fracture at high loads are not optimum for sustaining many cycles at low loads., Although elastomers often experience 10 to 100 million cycles before failure, there is now a limited understanding of their resistance to fatigue crack propagation. We tagged soft and tough double-network elastomers with mechanofluorescent probes and quantified damage by sacrificial bond scission after crack propagation under cyclic and monotonic loading. Damage along fracture surfaces and its spatial localization depend on the elastomer design, as well as on the applied load (i.e., cyclic or monotonic). The key result is that reversible elasticity and strain hardening at low and intermediate strains dictates fatigue resistance, whereas energy dissipation at high strains controls toughness. This information serves to engineer fatigue-resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.

International audience; Toughness of soft materials such as elastomers and gels depends on their ability to dissipate energy and to reduce stress concentration at the crack tip. The primary energy dissipation mechanism is viscoelasticity. Most analyses and models of fracture are based on linear viscoelastic theory (LVT) where strains are assumed to be small and the relaxation mechanisms are independent of stress or strain history. A well-known paradox is that the size of the dissipative zone predicted by LVT is unrealistically small. Here we use a physically based nonlinear viscoelastic model to illustrate why the linear theory breaks down. Using this nonlinear model and analogs of crack problems, we give a plausible resolution to this paradox. In our model, viscoelasticity arises from the breaking and healing of physical cross-links in the polymer network. When the deformation is small, the kinetics of bond breaking and healing are independent of the strain/stress history and the model reduces to the standard linear theory. For large deformations, localized bond breaking damages the material near the crack tip, reducing stress concentration and dissipating energy at the same time. The damage zone size is a new length scale which depends on the strain required to accelerate bond breaking kinetics. These effects are illustrated by considering two cases with stress concentrations: the evolution of spherical damage in a viscoelastic body subjected to internal pressure, and a zero degree peel test.

International audience; We have investigated the structural changes occurring in highly crosslinked and carbon-black filled natural rubber under uniaxial extension by small and wide angle X-ray scattering using synchrotron radiation. The experiments focused on strain-induced crystallization (SIC) and nanocavitation and were carried out on a model series of materials as a function of temperature and aging conditions. We find that for all materials both SIC and cavitation decrease markedly with temperature and ageing. However the presence of carbon black filler shifts the ceiling temperature where SIC is observed to at least 120°C, presumably by a nucleating effect, maintaining the high strength of the elastomers. Interestingly, although in pure elastomers the cavitation strength decreases with temperature, we find that in these filled elastomers the critical stress for the onset of cavitation increases significantly with temperature strongly suggesting that cavitation is due to the local confinement between fillers and supporting the idea of a glassy layer near the filler. Aging for 10 days at 110°C in oxygen-free conditions decreases both SIC and cavitation and reduces the strength of the elastomer at high temperature. This is attributed to the formation of sulfur side chains hindering the crystallization.

International audience; 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 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.

International audience; Extending the mechanical lifetime of NBR (i.e., poly(acrylonitrile-co-butadiene)), a large-volume synthetic rubber, requires a better understanding of its structure–property relationships. We demonstrate that industrial-grade and uncross-linked NBR can be mechanically rejuvenated and physically aged due to an inhomogeneous distribution of monomers along the polymer chains. As opposed to its nonpolar SBR counterpart (i.e., poly(styrene-co-butadiene)), NBR experiences thermodynamic driving forces for microphase separation and kinetic barriers for processing like those of block copolymers. Extruding NBR at high temperature and shear results in a weakly microphase-separated nanostructure of low relaxation time and resistance to flow, whereas physically aging NBR leads to lamellar nanodomains, a more solid-like material, and delayed stress relaxation. This effect of rejuvenation and aging on the nanostructure and rheological properties of NBR has important consequences on processing and storage conditions, such as the formation of defect-free interfaces in multilayered parts by polymer interdiffusion.

International audience; 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.

Macromolecules 54(18), 8726 - 8737 (2021). doi:10.1021/acs.macromol.1c00934, Despite their technological relevance, the resistance of soft thermoplastic polyurethanes (TPU) to crack propagation in cyclic fatigue has never been investigated in detail. In particular, a clear shortcoming in the literature for this class of materials is the lack of connection between the cyclic fatigue resistance and the large strain behavior that has a fundamental role in defining the material���s resistance to crack propagation. We demonstrate here for the first time that when the strain-induced stiffening mechanism of TPU (already observed for large deformation) is combined with the presence of the nonhomogeneous strain, as in the case of cyclic fatigue, it produces a selective reinforcement in the crack tip area, which is the key to explain the remarkable cyclic fatigue resistance of TPU. Using commercial TPU with similar modulus (���8 MPa) but different large strain behavior, we show that the described mechanism stems from the multiphase nature of TPU and it is not necessarily linked to a specific large strain property as the case of TPU, which undergoes strain-induced crystallization., Published by Soc., Washington, DC

We investigated crack propagation in cyclic fatigue in filled (50 phR of N347 carbon black), highly crosslinked, natural rubber pure shear samples and, in parallel, mapped in detail the microstructure present at the crack tip with a microfocused X-ray synchrotron beam. We acquired data by wide-angle X-ray scattering (WAXS) to characterize strain-induced crystallization (SIC) and by small-angle X-ray scattering (SAXS) to detect nano-cavitation. Crack propagation experiments were carried out for a range of maximum energy release rates during the cycles varying from 1 to 4 kJ/m2. Each material was tested at two temperatures: room temperature and 100°C and in new and aged conditions (10 days at 100°C in an inert atmosphere). We found that although significantly less SIC at the crack tip was present at the crack tip at 100°C, the resistance to crack propagation under cyclic loading was barely affected. In aged samples SIC at the crack tip was significantly lower than in new samples at room temperature, and was not detectable at all in aged samples at 100°C. The crack propagation rate, however, only increased with aging for the more crosslinked sample at 100°C and never increased catastrophically. Finally, a few percent of nano-cavities were detected at the crack tip and the comparison of X-ray transmission and digital image correlation suggested the presence of a significant fraction of large cavities at the very crack tip. In conclusion while strain-induced crystallization in NR may be affecting fatigue resistance we could not establish a quantitative correlation between the volume crystallized at the crack tip and the crack propagation rate in cyclic fatigue.

International audience; Stimuli-responsive injectable hydrogels based on weak supramolecular interactions may represent safer alternatives to 4 chemically reactive adhesive hydrogels for biomedical applications where weak to moderate adhesion is required. We investigated the 5 linear and nonlinear rheological properties as well as the adhesive properties of two thermoresponsive graft copolymers with inverse 6 topologies, poly(N-isopropyl acrylamide)-g-poly(N,N-dimethylacrylamide) (PNIPAM-g-PDMA) and PDMA-g-PNIPAM. Except for 7 their topologies, these copolymers are analogous in terms of chemistry, architecture (graft), and monomer composition (50−50 wt 8 %). Over a wide range of concentrations, they both form injectable homogeneous solutions at room temperature and turn into soft 9 and sticky viscoelastic hydrogels close to body temperature. We find that the linear viscoelastic properties of these two hydrogels are 10 not discernible far above the thermal transition temperature. However, the PNIPAM-g-PDMA hydrogel having long 11 thermoresponsive backbones shows a strain-hardening behavior in large strains both in probe tack tests and in shear. The inverse 12 topology, PDMA-g-PNIPAM, showed no hardening and simply softened until failure. This distinction was observed regardless of the 13 polymer concentration (in the entangled regime). We attribute the hardening to a continuous, load-bearing nanostructure from 14 strong hydrophobic PNIPAM associations, while the softening is due to the easy pullout of short PNIPAM grafts from separate 15 hydrophobic clusters bridged by PDMA backbones. The findings of this work highlight the importance of macromolecular design in 16 determining the nanostructure and thereby the mechanical performance of soft hydrogels for specific applications.

Introducing additional physical and reversible crosslinks to a chemically crosslinked hydrogel is an interesting and viable alternative to increase the toughness of a hydrogel. Yet while in general the physical crosslink points provide dissipative mechanisms, there are still many details that are unknown in particular on the role that physical crosslinks play on the large strain behavior. We explore the mechanical properties in small and large strain of two dual crosslink gels made from a random copolymer of poly(acrylamide-co-vinylimidazole) with a range of elastic moduli in the tens of kPa. The interaction between vinylimidazole groups and metal ions (Zn2+ and Ni2+) results in physical crosslink points and in a markedly stretch-rate-dependent mechanical behavior. While a main relaxation process is clearly visible in linear rheology and controls the small and intermediate strain properties, we find that the strain hardening behavior at stretches of λ > 4 and the stretch at break λb are controlled by an additional longer-lived physical crosslinking mechanism that could be due to a clustering of physical crosslinks.

Embedded spiropyran mechanophores reveal and quantitatively map stresses in soft materials by changing their color., Directly quantifying a spatially varying stress in soft materials is currently a great challenge. We propose a method to do that by detecting a change in visible light absorption. We incorporate a spiropyran (SP) force–activated mechanophore cross-linker in multiple-network elastomers. The random nature of the network structure of the polymer causes a progressive activation of the SP force probe with load, detectable by the change in color of the material. We first calibrate precisely the chromatic change in uniaxial tension. We then demonstrate that the nominal stress around a loaded crack can be detected for each pixel and that the measured values match quantitatively finite element simulations. This direct method to quantify stresses in soft materials with an internal force probe is an innovative tool that holds great potential to compare quantitatively stresses in different materials with simple optical observations.

International audience; Multi-network elastomers are both stiff and tough by virtue of containing a pre-stretched stiff network that can rupture and dissipate energy under load. However, the rupture of this sacrificial network in all described covalent multi-network elastomers is irreversible. Herein, we describe the first example of multi-network elastomers with a reformable sacrificial network containing mechanochemically sensitive anthracene-dimer cross-links. These cross-links also make our elastomers mechanochromic, with coloration that is both persistent and reversible, because the fluorogenic moiety (anthracene dimer) is regenerated upon irradiation of the material. In proof-of-concept experiments we demonstrate the utility of incorporating anthracene dimers in the backbone of the sacrificial network for monitoring mechanochemical remodeling of multi-network elastomers under cycling mechanical load. Stretching or compressing these elastomers makes them fluorescent and irradiating them eliminates the fluorescence by regenerating anthracene dimers. Reformable mechanochromic cross-links, exemplified by anthracene dimers, hold potential for enabling detailed studies of the molecular origin of the unique mechanical properties of multi-network elastomers.

Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rate G(0) above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transition G(tran) far above G(0). In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, and G(tran) corresponds to the rupture of the hard phase network.

International audience; The performances of Pressure Sensitive Adhesives (PSA) are generally evaluated using different loading geometries such as tack, peel and shear tests. It is difficult to link the behaviors of PSAs in these different geometries, and to predict the result of one test from another, because the confinement of a soft and dissipative material prevents the use of standard fracture mechanics, which separates the interface debonding behavior from the dissipation associated with the bulk deformation. We present here an original experimental investigation based on the modeling strategy proposed by Creton and Ciccotti[1]. Using instrumented versions of both peel and tack measurements, we compared the adherence performances of a series of model PSAs based on styrene-isoprene block copolymers, while identifying the mesoscale mechanisms at play during debonding. This analysis method allows us to model the contribution of the large strain rheology of the PSAs in the total work of debonding. We clearly show that both the adherence performances and local mechanisms can be closely related between peel and tack when considering both similar confinement and a similar strain rate of the fibrils that are spontaneously formed during debonding. While the overall adherence properties change by a factor of 3 between the different samples, the peel tests only present a minor +20% bias in adherence, which can be attributed to the combination of a 10% increase in the average stress and a 10% increase in the maximum strain of the fibrils. This improvement in the understanding of the PSA performances opens the way to a more sound mechanical design of PSA based joints.

International audience; We report a novel bioinspired underwater adhesive based on the injectable aqueous solution of a graft copolymer with a thermoresponsive backbone, which turns into a sticky hydrogel just below body temperature. With this topology, the collapse of the backbones upon the thermal transition leads to the formation of a percolating network of strong hydrophobic domains. Similar to Pressure-Sensitive Adhesives (PSAs), the hydrogel goes through fibrillation and extensive energy dissipation in large deformations, giving it an edge over conventional chemical hydrogels which are typically elastic and inherently non-sticky. This capability comes from the hydrophobic nano-scaffold which resists large deformations to minimize its contact with water. Since hydrophobic interactions are not weakened in water, the behavior of the hydrogel is maintained in aqueous medium. Chemistry-insensitive adhesion of this hydrogel offers a major advantage over current injectable adhesives which rely on in-situ chemical crosslinking reactions with tissues.

International audience; Styrene-block-(ethylene-co-butylene)-block-styrene (SEBS) copolymers are biocompatible elastomers with outstanding stability to UV radiation. This work addresses the potentialities of this class of elastomers for the development of transdermal patches. The influence of SEBS molecular weight, plasticizer and tackifier type on rheological pattern, debonding mechanisms, adhesive properties (i.e., tack, shear and peel adhesion) as well as on the in vitro biopharmaceutical performances (i.e., drug release and skin permeability) was investigated using ibuprofen as model drug. The relationships between the linear and non-linear rheological properties and the main adhesive and biopharmaceutical properties of the prepared patches have been demonstrated. The higher the viscous component of the matrix, the lower its cohesiveness and the faster the drug release rate. The in vitro skin permeability of ibuprofen was not limited by the polymeric matrix, even if compared to the commercial reference product. In conclusion, SEBS copolymers are suitable materials to design drug in-adhesive patches. In particular, SEBS-low molecular weight is the polymer worthy of consideration because of its favorable viscoelastic behavior.

We investigate experimentally the adherence energy Γ of model polyacrylate pressure-sensitive adhesives (PSAs) with combined large strain rheological measurements in uniaxial extension and an instrumented peel test. We develop a nonlinear model for such a peel test which captures the dependence of Γ(V) with peeling rate V, revealing the key role played by the extensional rheology. Our model explains in particular why traditional linear viscoelastic approaches correctly predict the slope of Γ(V) curves for sufficiently elastic PSAs characterized by a simple rate-independent debonding criterion. However, for more viscoelastic adhesives, we identified a more complex rate-dependent debonding criterion yielding a significant modification of the Γ(V) curves, an effect that has been largely overlooked so far. This investigation opens the way toward the understanding of fibrils debonding, which is the main missing block to predict the adherence of PSAs.

A classic paradigm of soft and extensible polymer materials is the difficulty of combining reversible elasticity with high fracture toughness, in particular for moduli above 1 MPa. Our recent discovery of multiple network acrylic elastomers opened a pathway to obtain precisely such a combination. We show here that they can be seen as true molecular composites with a well–cross-linked network acting as a percolating filler embedded in an extensible matrix, so that the stress–strain curves of a family of molecular composite materials made with different volume fractions of the same cross-linked network can be renormalized into a master curve. For low volume fractions (

Correction for ‘Mechanochemical tools for polymer materials’ by Yinjun Chen et al., Chem. Soc. Rev., 2021, 50, 4100–4140, DOI: 10.1039/D0CS00940G.

We studied the rheological and mechanical properties of a series of poly(vinyl alcohol) dual crosslink hydrogels, permanently crosslinked by glutaraldehyde and transiently crosslinked by borate ions, at various crosslinking densities. The contribution of the chemical and physical crosslinks to the dynamic moduli was first determined with linear oscillatory tests, and the additivity of the two contributions was validated. The single relaxation time of the system was interpreted as the physical bond breaking time which can be slowed down by reassociation of the open physical bonds before full relaxation. By comparing the Rouse relaxation time of the strand between two adjacent crosslinks with the healing time of the physical bonds, an increase in the relaxation time with decreasing density of physical crosslinks was explained. In all cases, the extensibility and stress at break of the gels were found to increase with the addition of physical crosslinks, but the extensibility increased the most for the small...

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.

International audience; Many marine organisms have developed adhesives that are able to bond under water, overcoming the challenges associated with wet adhesion. A key element in the processing of several natural underwater glues is complex coacervation, a liquid–liquid phase separation driven by complexation of oppositely charged macromolecules. Inspired by these examples, the development of a fully synthetic complex coacervate‐based adhesive is reported with an in situ setting mechanism, which can be triggered by a change in temperature and/or a change in ionic strength. The adhesive consists of a matrix of oppositely charged polyelectrolytes that are modified with thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) grafts. The adhesive, which initially starts out as a fluid complex coacervate with limited adhesion at room temperature and high ionic strength, transitions into a viscoelastic solid upon an increase in temperature and/or a decrease in the salt concentration of the environment. Consequently, the thermoresponsive chains self‐associate into hydrophobic domains and/or the polyelectrolyte matrix contracts, without inducing any macroscopic shrinking. The presence of PNIPAM favors energy dissipation by softening the material and by allowing crack blunting. The high work of adhesion, the gelation kinetics, and the easy tunability of the system make it a potential candidate for soft tissue adhesion in physiological environments.

International audience; Most commercially available soft tissue glues offer poor performance in the human body. Inspired by sea water organisms, we have developed an injectable adhesive whose setting mechanism is activated by a variation in environmental factors (e.g temperature and/or ionic strength). The material is prepared by mimicking a mechanism observed in many natural glues: complex coacervation, a liquid-liquid phase separation between oppositely charged polyelectrolytes. Complex coacervates are characterized by a high-water content, which inevitably weakens the glue. Here, we target the increase of the adhesive properties by systematically tuning the water content, a key parameter affecting the material performance. Among the several strategies here explored, the most effective one is the mechanical removal of water using an extruder, resulting in a work of adhesion increase of one order of magnitude compared to the original formulation.

We present a detailed comparison of the microscopic dynamics and the macroscopic mechanical behavior of novel butyl rubber ionomers with tunable dynamics of sparse sticky imidazole-based sidegroups that form clusters of about 20 units separated by essentially unperturbed chains. This material platform shows promise for application as self-healing elastomers. Size and thermal stability of the ionic clusters were probed by small-angle X-ray scattering, and the chain and sticker dynamics were studied by a combination of broadband dielectric spectroscopy (BDS) and advanced NMR methods. The results are correlated with the rheological behavior characterized by dynamic-mechanical analysis (DMA). While the NMR-detected chain relaxation and DMA results agree quantitatively and confirm relevant aspects of the sticky-reptation picture on a microscopic level, we stress and explain that apparent master curves are of limited use for such a comparison. The cluster-related relaxation time detected by BDS is much shorter than the elastic chain relaxation time, although the weak conductivity does follow the latter. The systematic trends across the sample series suggest that all relaxations are dominated by a cluster-related activation barrier, but also that the BDS-based cluster relaxation does not seem to be directly associated with the effective sticker lifetime. Nonlinear stress-strain experiments demonstrate a reduction of sticker lifetime on stretching and that the stored stress and the elastic recovery depend on the deformation rate. �� 2019 American Chemical Society.

International audience; experiencing solvent exchange from good to poor, a gel usually exhibits a monotonous deswelling with time if the good and poor solvents are miscible. [7-11] During this process, the good solvent easily escapes from gels driven by osmotic pressure of solvent-solvent mixing, while the poor solvent hardly enters the gel due to the demixing of polymer and poor solvent (Figure 1a). Here, we report an abnormal swelling of hydrophobic gels in water to form "hydrophobic hydrogels." We discovered that when hydrophobic gels swollen with omniphilic organic solvents are immersed in water or aqueous solutions, instead of shrinkage, the hydrophobic gels substantially swell to reach a water content as high as 99.6 wt% at most. The significant swelling of hydrophobic gels in water leads to the formation of a novel class of "hydro-phobic hydrogels" that have a unique fruit-like structure and show unique functions such as selective water absorption. To our knowledge, such abnormal swelling phenomenon of hydrophobic gels in water has never been reported before. This abnormal swelling is attributed to the rapid formation of a semipermeable skin layer on the surface of the gel by the phase separation of the hydrophobic polymer in water, which brings about a strongly asymmetrical diffusion for the organic solvent and water. The impermeable organic solvent is effectively trapped inside the gel, and thus generates a high osmotic pressure which drives the permeable Normally, a polymer network swells in a good solvent to form a gel but the gel shrinks in a poor solvent. Here, an abnormal phenomenon is reported: some hydrophobic gels significantly swell in water, reaching water content as high as 99.6 wt%. Such abnormal swelling behaviors in the nonsolvent water are observed universally for various hydrophobic organogels containing omniphilic organic solvents that have a higher affinity to water than to the hydrophobic polymers. The formation of a semipermeable skin layer due to rapid phase separation, and the asymmetric diffusion of water molecules into the gel driven by the high osmotic pressure of the organic solvent-water mixing, are found to be the reasons. As a result, the hydrophobic hydrogels have a fruit-like structure, consisting of hydrophobic skin and water-trapped micropores, to display various unique properties, such as significantly enhanced strength, surface hydrophobicity, and antidrying, despite their extremely high water content. Furthermore, the hydrophobic hydrogels exhibit selective water absorption from concentrated saline solutions and rapid water release at a small pressure like squeezing juices from fruits. These novel functions of hydrophobic hydrogels will find promising applications, e.g., as materials that can automatically take the fresh water from seawater. Polymer gels are defined as crosslinked polymer networks that are swollen in solvents. Since the last three decades, gels, especially water-based hydrogels, have embraced increasing interest for the numerous engineering and biological applications, including bioengineering, [1,2] adhesion, [3,4] soft robotics, [5] and 3D printing. [6] Ordinarily, a polymer network swells in a good solvent having a high affinity to polymer, forming a gel. When

International audience; Recently a class of multinetwork elastomers (MNEs) was developed by swelling a filler polymer network with monomers that are subsequently polymerized to form matrix networks. Such MNEs were reported to possess remarkable stiffness and fracture toughness while maintaining the ability to sustain large deformation as found in simple elastomers. The enhancement in toughness is attained by prestretching the chains of the filler network through the introduction of one or more matrix network(s), thereby promoting energy dissipation through chain scission in the filler network. In this work, a model to capture the mechanical response of MNEs is developed, and validated with experimental data. Prestrech of the polymer chains is incorporated into the model by basing the strain energy density function on the combined effect of swelling and subsequent deformation of the completed MNE. The 2 filler network is modeled as a polydisperse network of breakable polymer chains with nonlinear chain elasticity, while the matrix networks are modeled using the generalized neo-Hookean model. Although the filler network occupies only a small fraction of the material volume, the model shows that it contributes to the majority of the stress. Finally, the hysteresis during cyclic loading is shown to correlate with the accumulation of damage in the filler network during each cycle.

The multilevel fatigue resistance mechanism of tough and self-healing hydrogels with a hierarchical structure was revealed., We investigate the fatigue resistance of chemically cross-linked polyampholyte hydrogels with a hierarchical structure due to phase separation and find that the details of the structure, as characterized by SAXS, control the mechanisms of crack propagation. When gels exhibit a strong phase contrast and a low cross-linking level, the stress singularity around the crack tip is gradually eliminated with increasing fatigue cycles and this suppresses crack growth, beneficial for high fatigue resistance. On the contrary, the stress concentration persists in weakly phase-separated gels, resulting in low fatigue resistance. A material parameter, λtran, is identified, correlated to the onset of non-affine deformation of the mesophase structure in a hydrogel without crack, which governs the slow-to-fast transition in fatigue crack growth. The detailed role played by the mesoscale structure on fatigue resistance provides design principles for developing self-healing, tough, and fatigue-resistant soft materials.

International audience; We investigate the nanostructure, the rheology and the adhesion of soft supramolecular materials elaborated by blending monofunctional and multifunctional poly(isobutene) (PIB) chains. Monofunctional PIB chains (PIBUT) are linear and unentangled polymer chains (Mn ≈3kg/mol) functionalized in the middle by a bis-urea interacting moiety, able to self-associate by four hydrogen bonds. Covalent coupling of monofunctional PIB allows us to synthesize longer chains bearing two or three interacting moieties. These chains are then added to monofunctional PIB to prepare blends containing up to 10% of multifunctional PIB (M-PIBUT). The influence of M-PIBUT on the supramolecular nanostructure, which results from the self-assembly of stickers, is studied by Atomic Force Microscopy and Small Angle X-ray Scattering at room temperature. Multifunctional and monofunctional chains are shown to interact with each other to form bundles of rod-like aggregates. The consequences of these interactions on the rheology of the blends were studied by shear tests in the linear and non linear regimes, below and above the order-disorder transition temperature. A pronounced strengthening effect of M-PIBUT is observed at room temperature: the supramolecular blends become more elastic and are more resistant to creep with increasing concentration of M-PIBUT. The effects of M-PIBUT on the nanostructure and the rheology suggest that M-PIBUT, which can link with more than one supramolecular aggregate, plays the role of a physical cross-linker. The impact of these supramolecular cross-linkers on the adhesion of the blends is studied by probe-tack tests and discussed by analyzing the in-situ deformation through the debonding images.

International audience; A new approach for the elaboration of low molecular weights pressure-sensitive-adhesives based on supramolecular chemistry is explored. The synthesis of model systems coupled with probe-tack tests and rheological experiments highlights the influence of the transient network formed by supramolecular bonds on the adhesion energy. The first step of our approach consists in synthesizing poly(butylacrylate-co-glycidyl methacrylate) copolymers from a difunctional initiator, able to self-associate by four hydrogen bonds between urea groups. Linear copolymers with a low dispersity (M n =10kg/mol, Ip

International audience; For decades, poly(vinylbutyral) (PVB) has been the polymer of choice to improve the impact resistance of laminated glass. PVB presents large rupture strain, large tensile strength and excellent dissipative properties. Here we investigate the relation between macromolecular structure and mechanical properties of plasticised PVB by combining large strain tensile experiments with calorimetry, X-ray and birefringence measurements. We find that the mechanical response is dominated by creep and that this plastic-like flow can effectively be described by a strain dependent viscosity. The commonalities with other toughened polymeric materials are outlined, and especially the role played by phase separated domains with weaker physical bonds. These results could help optimize polymer design for toughness and impact applications.