Detection of Molecular Fracture in Elastomers with Mechanophores

Fracture of soft materials involves large deformations before eventual macroscopic failure occurs and the damage to the material is much less localized at the crack tip than in stiff and brittle materials such as inorganic glasses. Although fracture of simple elastic networks made of flexible polymers connected by covalent bonds, obeys relatively well the classical Lake and Thomas theory, fracture of complex soft materials is much harder to predict from knowledge of the molecular structure and architecture. Organic chemists have recently developed new molecules that can be incorporated in networks and give an optical signal (fluorescence, luminescence or change in absorption) in response to the application of a force. These optical signals can then be used to obtain quantitative information about bond scission and energy dissipation during a macroscopic fracture event. Previous work¹ has shown that it is possible to toughen and stiffen soft materials by incorporating a minority percolating filler into a stretchable matrix. Using model interpenetrated networks we demonstrate here, by incorporating mechanophore molecules in the filler or in the matrix network, how such materials break in two stages, by first softening the stiff filler network, losing mechanical percolation, and then breaking the stretchable matrix. Such mechanisms active at the crack tip are responsible for the much higher toughness of these stiffened materials and should be widely generalizable to elastomers and hydrogels reinforced by particles forming a percolating network.

¹ Ducrot, E., et al., Science, 2014. 344(6180): p. 186-189.