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Article Dans Une Revue Proceedings of the National Academy of Sciences of the United States of America Année : 2022

A new molecular interpretation of the toughness of multiple network elastomers at high temperature

Résumé

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. Abstract Unfilled elastomers often suffer from poor fracture resistance at high temperature where viscoelastic dissipation is low. A molecular design based on multiple interpenetrating networks composed of a brittle filler network isotropically prestretched to a value λ0 by swelling it in an extensible matrix leads to a dramatic increase of fracture energy Γc , typically attributed to sacrificial bond scission creating a dissipative damage zone ahead of the propagating crack. However, the molecular mechanisms controlling the size of the damage zone when the crack propagates are currently unknown. Here, we combine fluorogenic mechanochemistry with quantitative confocal mapping and mechanical testing to characterize both Γc and the extent of bond scission in the sacrificial network detected on the fracture surfaces for different stretch rates and temperatures. We find that increasing the prestretch λ0 of the filler network leads to a large increase in Γc mainly at temperatures well above the glass transition temperature of the elastomers, where viscoelasticity is inactive, but also at lower temperatures where both mechanisms are coupled. Yet, we show that there is no direct linear relation between the extent of filler network scission and Γc. We mainly attribute the large increase in Γc to the dilution of highly stretched strands in the entangled and unstretched matrix, which delocalizes stress upon bond scission and effectively protects the matrix network from scission and the material from crack growth. Delaying the localization of bond scission by network design is a promising strategy that will guide molecular designs able to toughen elastomers even in the absence of viscoelastic dissipation.
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Dates et versions

hal-03673377 , version 1 (20-05-2022)

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Juliette Slootman, Joshua Yeh, Pierre Millereau, Jean Comtet, Costantino Creton. A new molecular interpretation of the toughness of multiple network elastomers at high temperature. Proceedings of the National Academy of Sciences of the United States of America, 2022, 119 (13), pp.e2116127119. ⟨10.1073/pnas.2116127119⟩. ⟨hal-03673377⟩
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