Mechanics of elastomeric molecular composites

We recently published a comprehensive article on double network elastomers in PNAS which will be of intreest ot those working in fracture and durability fo soft networks.

Millereau, P.; Ducrot, E.; Clough, J. M.; Wiseman, M. E.; Brown, H. R.; Sijbesma, R. P.; Creton, C., Mechanics of elastomeric molecular composites. Proceedings of the National Academy of Sciences 2018, 115 (37), 9110-9115.

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 (<3%) of cross-linked network, we demonstrate with mechanoluminescence experiments that the elastomer undergoes a strong localized softening due to scission of covalent bonds followed by a stable necking process, a phenomenon never observed before in elastomers. The quantification of the emitted luminescence shows that the damage in the material occurs in two steps, with a first step where random bond breakage occurs in the material accompanied by a moderate level of dissipated energy and a second step where a moderate level of more localized bond scission leads to a much larger level of dissipated energy. This combined use of mechanical macroscopic testing and molecular
bond scission data provides unprecedented insight on how tough soft materials can damage and fail.