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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.

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

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.

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; 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.

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...

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.

We propose a simple theoretical model describing the debonding process of soft adhesives in the probe-tack test. In this model, the expansion dynamics of interfacial cavities is determined by the balance between the strain energy release rate and the rate-dependent fracture energy. As a result, we obtain analytical solutions for the cavity size, stress-strain curve, peak stress, strain at the peak stress, maximum strain, as well as the adhesion energy. Furthermore, we discuss the validity of our theoretical results by comparing them with experiments.

International audience; In this study, aqueous emulsion polymerization of n-butyl acrylate is performed in batch conditions without surfactants using a poly(acrylic acid)-trithiocarbonate macro-RAFT agent to control the polymerization and to stabilize the emulsion. According to the polymerization-induced self-assembly (PISA) approach, well-defined amphiphilic PAA-b-PBA diblock copolymers form and self-assemble during synthesis to yield highly stable core-shell particles with an extremely thin hard PAA shell. For the first time, we report here the specific properties of films obtained from these particular latexes. After drying the aqueous dispersion, tough and transparent films are obtained. Although the films are not chemically cross-linked, they do not dissolve in good solvents for PBA. Moreover, they remain transparent even after immersion in water. Rheology shows that the films are both stiff and ductile, thanks to the nanostructured but very low volume fraction (less than 3 wt%) of PAA forming a percolating network in the soft PBA. Compared with conventional core-shell-based films, this approach affords for the first time a route to a thin percolating honeycomb nanostructure with a sharp and strong interface between the two phases. The versatility of the synthetic procedure opens perspectives for a large range of functional materials.

International audience; Hybrid polyurethane/acrylic waterborne pressure sensitive adhesives were synthesized by miniemulsion polymerization. The free radical polymerization of a mixture of acrylic and methacrylic monomers, containing 2-hydroxyethyl methacrylate, was carried out in the presence of a diisocyanate terminated polyurethane prepolymer and a diol. The resulting polymer microstructure was varied by altering the PU content and the number of links between the PU and the acrylic chains. The structure of the hybrid polymer was characterized by a combination of macroscopic measurements (gel fraction, swelling ratio, and sol molecular weight distribution) and computer simulations. A detailed study of the relationship between structure and adhesive properties was carried out.

The concomitant appearance of crystallites and nanocavities under uniaxial strain is investigated by X-ray scattering in a model natural rubber system. The nanocavities appear after crystallization and only when the true stress is above a critical cavitation stress σCav. The presence of crystallites alone does not influence the calculation of the void volume fraction ϕvoid. The nanocavities formed are 20–50 nm in size with a constant aspect ratio. The presence of filler shifts the critical crystallization extension ratio λCry, λCav, and σCav to lower values. The clear correlation between σCav and the crystallinity at the onset of cavitation χC(λCav) implies that the crystallites take most of the mechanical loading thus delaying the cavitation in the amorphous phase. Under cyclic loading, nanocavitation is significant only in the first loading and in the successive loadings if the extension ratio is above its maximum historical value. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1125–1138

International audience; The formation of interpolymer complexes (IPC) between poly(acrylic acid) (PM) and poly(acrylamide) (PAM), poly(N,N-dimethylacrylamide) (PDMA), and statistical copolymers of acrylamide (AM) and N,N-dimethylacrylamide (DMA) has been studied as a function of pH, salt concentration and temperature (0 -70 degrees C). The cloud points of dilute solutions were measured by turbidimetry and phase diagrams were determined as a function of temperature and pH in pure water and as a function of pH and salt concentration at room temperature. For each temperature and salt concentration a critical pH (pH(crit)) below which IPC are observed was defined. In the case of PAA/PAM, pH(crit) continuously decreased with increasing temperature, from pH 3.5 at 0 degrees C to pH 1.9 at 60 degrees C (UCST-type). In the case of PAA/PDMA, pH(crit), increased with temperature. The LCST-type behavior of the hydrogen-bonding complex formed between PM and PDMA was attributed to the dimethyl Substitution of amide groups that puts in hydrophobic interactions at high temperature. PAA and statistical copolymers P(AM-co-DMA) showed an intermediate behavior between PM/PAM and PAA/PDMA with a continuous shift from UCST-type to LCST-type with increasing amount of DMA. This behavior can be attributed to changes in configurational entropy due to the IPC formation and (for PDMA) to the release of water molecules initially confined in hydrophobic hydration cages around DMA units. While at low salt concentration, the stability of PM/PAM and PAA/PDMA complexes only slightly increases with the screening of ionized acrylic units, there is a sharp increase of pHcrit at high salt concentration in relation with the weakening of the solvent quality. In this regime, the complex formation of PAA/PDMA is greatly enhanced compared to PM/PAM due to the interference of hydrophobic interactions. (C) 2011 Elsevier Ltd. All rights reserved.

International audience; Polymers made of even slightly different repeat units are usually immiscible and form materials with two separate phases, like oil and water. Not only do different polymers not mix, but the interfaces between them are very sharp and mechanically weak. This lack of interfacial strength poses a very serious challenge to the recycling of blends of different polymers and, notably, polyethylene (PE) and isotactic polypropylene (iPP), the two polymers most commonly found in the industrial and domestic waste that come from packaging. If these two tough polymers are simply blended together, the resulting material is brittle and cannot be used. On page 814 of this issue, Eagan et al. (1) report that adding just 1% of a suitable block copolymer—a chain of PE connected to a chain of iPP—can create molecular stitches between the two phases and make the resulting blend as tough as iPP and PE themselves.

The synthesis of hybrid star-shaped polymers was carried out by atom transfer radical polymerization of n-butyl acrylate from a well-defined multifunctional titanium-oxo-cluster initiator. Conditions were identified to prevent possible side reactions among monomer, polymer, and the titanium-oxo-cluster ligands. Polymerizations provided linear first-order kinetics and the evolution of the experimental molecular weight is also linear with the conversion. 1 H DOSY NMR and cleavage of the polymeric branches from the multifunctional initiator by hydrolysis were used to (i) prove the star-shaped structure of the polymer, and (ii) demonstrate that the shoulder observed on size exclusion chromatograms is not due to a noncontrolled polymerization but to ungrafting of polymeric branches during analysis. Rheological properties of the hybrid star-shaped poly(n-butyl acrylate) were studied in the linear regime and show that the Ti-oxo-cluster not only increases significantly the viscosity of the polymer relative to its ungrafted arm but has a rheological signature which is qualitatively different from that of stars with organic cores suggesting that the Ti cluster reduces significantly the molecular mobility of the star.

The feasibility of independent control of the gel fraction and sol molecular weight distribution in the miniemulsion polymerization of 2-ethylhexyl acrylate using symmetrical and nonsymmetrical cross-linkers and a chain transfer agent was investigated. It was found that the very reactive symmetrical cross-linker had only a limited effect on polymer architecture, whereas the nonsymmetrical cross-linker and the chain transfer agent had a profound effect allowing decoupling gel and sol molecular weight. Adhesive properties were mainly controlled by the gel content of the polymer.

The synthesis and mechanical characterization of novel, tough poly(N,N-dimethylacrylamide) (PDMA)−silica hydrogel hybrids are presented to understand the role played by strong physical interactions between silica nanoparticles and the PDMA polymer on the properties of chemically cross-linked highly swollen PDMA networks. A detailed comparison of the hybrids with unmodified PDMA gels indicates that the incorporation of silica nanoparticles in the hydrogel increases the compression strength and the fracture toughness of notched samples up to an order of magnitude while increasing its modulus by a factor of 6 with a volume fraction of particles of the order of only 7%. The hybrid gels present a strain-dependent hysteresis but no permanent damage or residual strain upon unloading even after repeated cycling, a very unique property for such tough hydrogels. The reason for this exceptional increase in toughness is attributed mainly to the combined effect of breakable silica/polymer bonds and of a wide distribut...

The wetting and adhesive properties of model pressure-sensitive adhesives synthesised by miniemulsion polymerization were investigated. Wetting experiments of aqueous solutions of three different emulsifiers showed that Silwet L-77 (nonionic ethoxylated trisiloxane) was the best wetting agent. Probing adhesive properties, two different structure modifications of the polymer were investigated: a change in the polymer microstructure by the addition of a chain transfer agent and the introduction of a hydrophobic monomer into the polymer backbone. An addition of chain transfer agent was sufficient to obtain a polymer with a significantly different microstructure and, consequently, enhanced adhesion energy but reduced shear resistance. On the other hand, stearyl acrylate (SA) was employed as the hydrophobic monomer and the synthesis was carried out in miniemulsion polymerization which enables the incorporation of hydrophobic compounds without any diffusion limitations. There were almost no differences in the polymer microstructure, whereas, some differences were observed in adhesion properties.

The mechanical properties of nanostructured polyelectrolyte hydrogels have been investigated targeting specifically the nonlinear deformation regime at large strains and the fracture properties. The hydrogels were synthesized using a thiol-ene cross-linking chemistry of polyacrylic acid precursor chains previously functionalized with double bonds. Some polymers were also grafted with C 12 side chains able to form hydrophobic clusters in water. The large strain deformation of these nanostructured gels was tested in uniaxial compression by performing loading/unloading cycles at different strain rates and up to different maximum strains. All hydrogels displayed a pronounced strain hardening before fracture and a strain dependent hysteresis but the magnitude of these effects depended on the details of the composition. The most pronounced strain hardening was observed for the unmodified hydrogels in pure water while the largest hysteresis was found for the gels containing the highest concentration of nanoclusters. These dissipative processes barely influence the mechanical behaviour at small strains where the gel remains very elastic but become important at large strains and improve the resistance to fracture of the gels.

The major objective of this article is to present recent advances in the methodology to fine tune the adhesive performance of a PSA. In addition to the so-called Dahlquist criterion requiring a low modulus, we propose two additional rheological predictors of the adhesive properties. The first one is derived from the description of the detachment of a linear elastic layer from a rigid substrate. We made an approximate extension of this analysis to the viscoelastic regime and showed that the transition from interfacial cracks to cavitation and fibrillation can be quantitatively predicted from the easily measurable ratio tan(δ)/G′(ω). If a fibrillar structure is formed, the nonlinear large strain properties become important. We showed that the ability of the fibrils to be stretched before final debonding can be predicted from the analysis of simple tensile tests. The softening, which occurs at intermediate strains, and, more importantly, the hardening which occurs at large strains, can be used to predict the...

The failure of interfaces between a series of styrene-butadiene random linear monodisperse polymer melts was investigated with a contact mechanics method. A thin (1μm) and a thick (150μm) film of the same polymer were deposited on a metallic cylindrical probe and on a glass slide, respectively. The two films were put in contact for up to 1000s in a very confined geometry and the mechanical strength of interfaces formed in these conditions was then characterized by separating the probe from the glass slide. We observed different deformation micromechanisms of the layers depending on the time of contact, the debonding velocity, and the polymer used. We found that these different behaviors could be represented in a general deformation map as a function of two reduced parameters: the ratio between the contact time and the reptation time and the Deborah number. For values of De Decrit, two regimes were obse...

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress.

Summary: We report the synthesis and characterization of a family of hydrophobically modified hydrogels designed to have an improved fracture resistance. A backbone of poly(acrylic acid) (PAA) was functionalized with double bonds and hydrophobic groups. The functionalized PAA was then crosslinked with a dithiol. The chemical structure of the gels was characterized with a combination of NMR, titration methods and rheological techniques. The crosslinked structure of the hydrogel was found to be dependent on the polymer concentration only, while the dissipative properties of the gel increased strongly at all frequencies, with the introduction of hydrophobic groups which formed reversible associations. We expect these viscoelastic hydrogels to display dramatically different properties from the unmodified hydrogels when stretched and fractured.

The interfacial thickness and mechanical strength of several model interfaces between immiscible polymer melts were investigated. The interfacial width at equilibrium was determined by neutron reflectivity, and the adhesion energy was determined with an asymmetric probe test method inspired of the tests used for soft adhesives. A very good correlation was found between the adhesion energy and the measured interfacial width in a qualitatively similar way as what was observed for interfaces between glassy polymers. For polymer pairs with χ values ranging from 0.002 to 0.05, the mechanical strength of the interface was controlled by the degree of interpenetration, or in other terms, by the interfacial free energy between the two polymers. For lower values of χ, the interfacial strength was comparable to the fracture strength of the weakest of the two bulk polymers, while for higher values of χ, the thermodynamic work of adhesion and the polymer surface mobility are probably controlling the interfacial strength.

The effect of a composition gradient along the thickness in soft adhesive films was investigated. The adhesion properties of bilayer films made from acrylic solutions, one layer being more cohesive and the other more dissipative, were studied by performing probe tack experiments. To understand the mechanisms that determine the bulk and the interfacial contributions to the debonding, tests on different surfaces were carried out. The results show that the presence of a composition gradient can enhance the adhesive properties, particularly on a low-energy surface such as polyethylene. On steel, the presence of the thin layer of a more elastic adhesive in contact with the adherent can influence significantly the debonding mechanism, transforming the fracture from cohesive to adhesive.

The effect of surface tension γ on cavity nucleation and growth in an incompressible neo-Hookean material of initial elasticity modulus E is examined. In a numerical analysis of potential energy limited to spherically symmetric configurations, a bifurcation problem is identified. If the applied uniform radial tensile dead-load is sufficiently large, a branch corresponding to cavity expansion bifurcates from the branch corresponding to cavity constriction due to surface tension. The bifurcation is interpreted in terms of a sudden growth to a finite cavity radius which bears some reminiscence of the snap-through buckling phenomenon in structural mechanics. The finding is compared to experimental results obtained in probe tack tests of soft adhesives where rapid cavity growth is also observed. Some of the experimental evidence is explained by assuming that in the adhesive there is a distribution of pre-existing cavities whose radius compares in a way to the length scale γ/E such as to make them susceptible to the influence of surface tension.

The adhesion between a homopolymer matrix and a diblock copolymer is shown to depend on the length of the non-adsorbing block that penetrates the matrix chains. The tack or short-time adhesion was measured using a probe-tack set-up consisting of a thick (∼100 μm) polystyrene, PS, adhesive layer with degree of polymerization (DP), 1923, brought into contact with a monolayer of poly(deuterated styrene- block -methyl methacrylate), dPS- b -PMMA, deposited on a glass substrate. Experiments performed at 130 °C, above the glass transition temperatures, show that the maximum debonding stress increases from 1.4 MPa for the glass to 2.3 MPa for the copolymer. Also, the adhesion energy increases dramatically as the non-adsorbing block length, N dPS , increases from below to above the entanglement DP of PS. These observations suggest a difference in the debonding mechanism between the nude glass, which undergoes fragile rupture, and the glass covered by dPS- b -PMMA, which exhibits increasing cavity formation with increasing N dPS . After normalizing by the chain areal density, the adhesion dissipation is observed to increase by a factor of 4 as N dPS increases from 100 to ca. 1000. These results suggest that entanglements between matrix chains and the non-adsorbing block impart good stress transfer and interfacial strength across the interface.

A reliable measure of the adhesion between a very deformable material and a solid surface is rather difficult, since the interface boundary conditions and the bulk deformation of the layer are closely and very nonlinearly coupled. In this article, a new methodology to assess the adhesion of a soft viscoelastic layer on a solid surface is proposed, where we have used a specific experimental geometry minimizing the bulk deformation of the layer. A flat-ended probe is first put in contact with a thin layer of soft material and removed at a constant velocity. The probe is then stopped at a preset level of tensile force and the time for complete debonding of the layer from the probe is measured. For our model system, comprised of a soft acrylic removable adhesive and a silicone-coated surface, the higher the applied force the faster the interfacial fracture occurs, leading to an experimental curve of the adhesion energy as a function of average crack velocity. We find that the methodology is relatively simple ...

Mechanical properties of miscible blends of high molecular weight poly(N-vinyl pyrrolidone) (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) of molecular weight 400 g/mol have been examined as a function of PVP–PEG composition and degree of hydration. The small-strain behavior in the linear elastic region has been evaluated with the dynamic mechanical analysis and compared with the viscoelastic behavior of PVP–PEG blends under large strains in the course of uniaxial drawing to fracture and under cyclic extension. A strong decoupling between the small-strain and the large strain properties of the blends has been observed, indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen on tensile tests under fast drawing, is attributed to the peculiar phase behavior of the blends and the molecular mechanism of PVP–PEG interaction. Nevertheless, for the PVP blend with 36% PEG, under comparatively low extension rates, the reversible contribution to the total work of deformation up to e=300% has been found to be maximum at around 70%, while the blends containing 31 and 41% PEG behave rather as an elastic–plastic solid and a viscoelastic liquid, respectively.

The phase behaviour of poly(N-vinyl pyrrolidone)–poly(ethylene glycol) (PVP–PEG) blends has been examined in the entire composition range using Temperature Modulated Differential Scanning Calorimetry (TM-DSC) and conventional DSC techniques. Despite the unlimited solubility of PVP in oligomers of ethylene glycol, the PVP–PEG system under consideration demonstrates two distinct and mutually consistent glass transition temperatures (Tg) within a certain concentration region. The dissolution of PVP in oligomeric PEG has been shown earlier (by FTIR spectroscopy) to be due to hydrogen bonding between carbonyl groups in PVP repeat units and complementary hydroxyl end-groups of PEG chains. Forming two H-bonds through both terminal OH-groups, PEG acts as a reversible crosslinker of PVP macromolecules. To characterise the hydrogen bonded complex formation between PVP (Mw=106) and PEG (Mw=400) we employed an approach described in the first two papers of this series that is based on the modified Fox equation. We evaluated the fraction of crosslinked PVP units and PEG chains participating to the complex formation, the H-bonded network density, the equilibrium constant of complex formation, etc. Based on the established molecular details of self-organisation in PVP–PEG solutions, we propose a three-stage mechanism of PVP–PEG H-bonded complex formation/breakdown with increase of PEG content. The two observed Tgs are assigned to a coexisting PVP–PEG network (formed via multiple hydrogen bonding between a PEG and PVP) and a homogeneous PVP–PEG blend (involving a single hydrogen bond formation only). Based on the strong influence of coexisting regions on each other and the absence of signs of phase separation (evidenced by Optical Wedge Microinterferometry) we conclude that the PVP–PEG blend is fully miscible on a molecular scale.

The cohesive failure of soft adhesives is studied using a yield stress fluid as a model adhesive, which allow to relate the viscoelastic properties of the fluid directly to its adhesive performance. We derive a theoretical expression for the force—distance curve as a function of the yield stress, which describes our experimental results very well. The theoretical prediction is obtained by assuming a circular air—adhesive interface; surprisingly, good agreement between theory and experiment is also obtained when strong fingering instabilities are observed in the experiment. This result is confirmed by the fact that we do not find a significant reduction in the work of adhesion when fingering instabilities are present. In addition, we discuss the morphology of the fingering instabilities.

Adhesively bonded elastic layers with thicknesses that are small relative to their lateral dimensions are used in a wide variety of applications. The mechanical response of the compliant layer when a normal stress is imposed across its thickness is determined by the effects of lateral constraints, which are characterized by the ratio of the lateral dimensions of the layer to its thickness. From this degree of confinement and from the material properties of the compliant layer, we predict three distinct deformation modes: (1) edge crack propagation, (2) internal crack propagation, and (3) cavitation. The conditions conductive for each mode are presented in the form of a deformation map developed from fracture mechanics and bulk instability criteria. We use experimental data from elastic and viscoelastic materials to illustrate the predictions of this deformation map. We also discuss the evolution of the deformation to large strains, where nonlinear effects such as fibrillation and yielding dominate the failure process.