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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.

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; 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.

Nanolayered lamellae are common structures in nanoscience and nanotechnology, but most are nearly symmetric in layer thickness. Here, we report on the structure and mechanics of highly asymmetric and thermodynamically stable soft--hard lamellar structures self-assembled from optimally designed PS1-(PI-b-PS2)3 miktoarm star block copolymers. The remarkable mechanical properties of these strong and ductile PS (polystyrene)-based nanomaterials can be tuned over a broad range by varying the hard layer thickness while maintaining the soft layer thickness constant at 13 nm. Upon deformation, thin PS lamellae (\textless{}100 nm) exhibited kinks and predamaged/damaged grains, as well as cavitation in the soft layers. In contrast, deformation of thick lamellae (\textgreater{}100 nm) manifests cavitation in both soft and hard nanolayers. In situ tensile-SAXS experiments revealed the evolution of cavities during deformation and confirmed that the damage in such systems reflects both plastic deformation by shear and residual cavities. The aspects of the mechanics should point to universal deformation behavior in broader classes of asymmetric hard--soft lamellar materials, whose properties are just being revealed for versatile applications.

International audience; A new thermodynamically stable, aperiodic “bricks-and-mortar” (B&M) cellular mesophase structure is reported in PS1-b-(PI-b-PS2)3 miktoarm copolymer and PS homopolymer blends [PS1, long polystyrene; PI, poly(isoprene); PS2, short polystyrene], where PS comprises discrete hard “bricks” and PI the continuous soft “mortar”. The mesophase is unique in its extreme domain volume fractions, its lack of positional order, and quasi-long-range orientational order. On the basis of this unusual mesophase structure, a series of PS-based thermoplastic elastomers are realized, combining rigidity from an exceptionally high content of discrete glassy PS domains (up to 82 wt %) and high extensibility with recoverable elasticity from a low content of continuous rubbery PI (down to 18 wt %). The new elastomers show sharp yielding behavior while maintaining good elasticity at large strains. Tensile-SAXS experiments reveal that voiding plays an important role for the mechanical behavior and voids can open/close reversibly with/without loading. Plastic deformation only results in a slight loss of recoverable elasticity.

International audience; From 1998 to 2003, Alan N. Gent and some co-workers published a series of papers devoted to the first measurements of fatigue crack growth properties in rubber in mode III loading conditions (Rubber Chemistry and Technology, vol. 71, pp. 76–83; vol. 71, pp. 84–94; vol. 76, pp. 1276–1289). The present paper proposes to revisit this seminal work benefiting from the capabilities of new multiaxial fatigue machines. Then, a new mode III fatigue test for elastomers is developed. A thin bonded disk of rubber with a circumferential cut is subjected to cyclic torsion to investigate the fatigue crack growth in mode III. The corresponding measurement methods for the crack growth rate and the fracture energy are proposed. As a constant small axial extension can be applied and controlled to prevent the cracking surfaces from abrasion and to ensure almost mode III loading conditions, the quality and the relevance of the experimental data are highly enhanced. As a results, the method is applied to an unfilled natural rubber and the crack growth rate is related to the fracture energy for several loading cases. The effects of axial extension, alternate cycles and positive minimum angle cycles are examined. In particular, it is shown that for fully unloaded cycles the crack growth rate laws in modes I and III coincide in the regime of high fracture energies.

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