Costantino Creton

Toughening elastomers with sacrificial bonds and watching them break

Toughening Up Elastomers Elastomers are soft polymer materials widely used in industry and daily life. Inspired by recent work on double-network hydrogels, Ducrot et al. (p. 186; see the Perspective by Gong) designed interpenetrated network elastomers that contained isotropically prestretched chains as the first network. Double- and triple-network structures yielded elastomers with very high strength and toughness in comparison with the corresponding single networks. Network elastomers based on hydrogel structures show increased toughness through the incorporation of sacrificial bonds. [Also see Perspective by Gong] Elastomers are widely used because of their large-strain reversible deformability. Most unfilled elastomers suffer from a poor mechanical strength, which limits their use. Using sacrificial bonds, we show how brittle, unfilled elastomers can be strongly reinforced in stiffness and toughness (up to 4 megapascals and 9 kilojoules per square meter) by introducing a variable proportion of isotropically prestretched chains that can break and dissipate energy before the material fails. Chemoluminescent cross-linking molecules, which emit light as they break, map in real time where and when many of these internal bonds break ahead of a propagating crack. The simple methodology that we use to introduce sacrificial bonds, combined with the mapping of where bonds break, has the potential to stimulate the development of new classes of unfilled tough elastomers and better molecular models of the fracture of soft materials.

Direct Observation of Cavitation and Fibrillation in a Probe Tack Experiment on Model Acrylic Pressure-Sensitive-Adhesives

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

Deformation and failure modes of adhesively bonded elastic layers

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 fai...

How does tack depend on time of contact and contact pressure

The tack of polymer melts on rigid substrates under conditions of short contact times and low pressures is examined. The substrate is modeled as a random rough surface with a distribution of asperities heights. The true contact area between the adhesive and the substrate is calculated for a given total load and elastic modulus of the substrate. The dependence of tack on contact time is accounted for by introducing the relaxation of the adhesive through a time-dependent elastic modulus. For relatively high pressures the tack is predicted to scale with 1/E so that for short contact times, t c , the tack is predicted to scale with (t c /τ e ) 1/2 , where τ e is the entanglement time. For lower pressures this simple scaling low is no longer valid and we predict a complex variation of tack with contact time and molecular parameters.

Adhesion and Fracture of Interfaces Between Immiscible Polymers: from the Molecular to the Continuum Scal

In order to obtain a measurable fracture toughness, a joint between two immiscible polymer glasses must be able to transfer mechanical stress across the interface. This stress transfer capability is very weak for narrow interfaces and a significant reinforcement can be achieved, either by the use of connecting chains (block copolymers), or by a broadening of the interface (random copolymers). In both cases, the stress is transferred by entanglements between polymer chains. The molecular criteria for efficient stress transfer, by connecting chains and by broad interfaces, are reviewed here with a special emphasis on the role of the molecular architecture (diblock, triblock or random copolymers) and molecular weight of the chains present at the interface. Recent theoretical developments in the relationship between macroscopic fracture toughness and interfacial stress transfer are also discussed, and the essential role of bulk plastic deformation properties of the polymers on either side of the interface are specifically addressed.

Micromechanics of flat‐probe adhesion tests of soft viscoelastic polymer films

Because of recent experiments (H. Lakrout et al., J Adhes 1999, 69, 307) conducted with an instrumented probe-tack device, a better description of the debonding mechanisms of soft viscoelastic adhesives from a hard interface can be proposed. Because the deformed volume is about the magnitude of the sample size and the deformation is highly inhomogeneous, adhesion cannot be studied independently of the geometry of the test. One of the simplest geometries is that of the debonding of a cylindrical flat-ended probe from a thin film. In this case, a uniform displacement field is applied over the entire top surface of the film, and the stress field at the probe-film interface is strongly dependent on the degree of confinement of the film characterized by the ratio between the probe radius a and the film thickness h. For a very confined film, the nucleation of interfacial cavities is predicted to occur over most of the surface of the probe. These cavities will grow in the plane of the film and can be assimilated to multiple penny-shaped interfacial cracks. We present a semiquantitative model of how the lateral growth of these cavities is controlled by the competition between a critical energy-release rate at the interface G c and the elastic modulus of the film E. Finally, either these cavities coalesce and the film is debonded from the surface or the walls between these cavities become elongated in the tensile direction and form a fibrillar structure. The conditions for the formation of this fibrillar structure are qualitatively discussed.

Rate-dependent frictional adhesion in natural and synthetic gecko setae

Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30 000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s−1 to 158 mm s−1. To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick–slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick–slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity (V*) and then decrease at velocities greater than V*. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick–slip events in the population of flexible fibrils, confirming our model predictions.

Fracture and adhesion of soft materials: a review

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

Micromechanisms of tack of soft adhesives based on styrenic block copolymers

The tackiness of model soft adhesive layers based on styrene-isoprene-styrene block copolymers and a tackifying resin were investigated with a flat-ended cylindrical steel probe. The contact between the probe and the adhesive was maintained for 1 s at a nominal pressure of 1 MPa before being detached at a constant velocity. The effect of resin content, probe velocity during debonding and temperature were systematically investigated. Failure was initiated by two main mechanisms: an interfacial cavitation at low debonding rates, giving relatively low adhesion energies, and a bulk cavitation process at higher debonding rates, which gave much higher adhesion energies. In both cases failure occurred at the end by interfacial detachment of fibrils. The characteristic probe velocity where the transition between these two mechanisms took place was controlled primarily by the linear viscoelastic properties of the adhesives. However, the important quantitative parameters obtained from a tack test, i.e., the maximum debonding stress and the adhesion energy, could not be predicted by the linear viscoelastic properties of these adhesives alone.

Relation of glass transition temperature to the hydrogen bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers: 3. Analysis of two glass transition temperatures featured for PVP solutions in liquid poly(ethylene glycol)

Abstract 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.