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As recently published in Adv. Materials, a novel mode of gel toughening has been highlighted

By exploring original network topologies, phase-separated gels demonstrated advanced mechanical properties. Fracture systematically proceeded by crack bifurcation, unreported so far in gels.

Phase transition of covalently cross-linked gels entails a drastic volume-change that makes it difficult to reveal the role of phase transition on the mechanical toughening independently of the polymer concentration effect.
To clear up this ambiguity, we have designed covalently cross-linked gels that phase-separate at constant macroscopic volume and quite high level of hydration, i.e. typically more than 80 wt% water.
By exploring original network topologies, we have demonstrated the remarkable asset that may represent the phase-separation process for the gel design displaying responsive advanced mechanical properties : excellent fatigue resistance, perfect recovery and more interestingly remarkable fracture properties can be triggered at a given temperature, Tc.
Beyond the achieved high values of fracture energy (1000 J m-2) for such high hydration level, the gel fracture patterns have highlighted a systematic crack bifurcation, as commonly observed in natural rubbers but unreported so far in the case of gels.

Left : Crack propagation features crack bifurcation. Pictures were taken during the crack propagation and illustrate the crack propagation process in phase-separated gels at 60°C. Initial notch was made prior test with a razor blade (as denoted with the white arrow). Upon monotonic loading, fracture proceeded in the material with deviations from its original direction (black arrows). Right : Fracture energies for different temperatures. Above the critical temperature Tc, fracture proceeds by crack bifurcation. The scheme is a view of the reinforcing mechanisms at micro-scale, specifically at the crack tip where the strain amplification level is high. Dotted lines refer to damage mechanisms and define weak interfaces.

 

Guo, H., Sanson, N., Hourdet, D. and Marcellan, A. (2016), Thermoresponsive Toughening with Crack Bifurcation in Phase-Separated Hydrogels under Isochoric Conditions. Adv. Mater., 28 : 5857–5864. doi:10.1002/adma.201600514
http://onlinelibrary.wiley.com/doi/10.1002/adma.201600514/epdf