Soft materials are irreplaceable in engineering applications where large reversible deformations are needed, and in life sciences to replace a variety of living tissues. While mechanical strength may not be essential for all applications, excessive brittleness is a strong limitation. Yet predicting if a soft material will be tough or brittle from its molecular composition or structure relies on empirical concepts for the lack of proper tools to detect the damage occurring to the material before it breaks. Using model materials containing a variable population of internal sacrificial bonds, breaking before the material fails macroscopically, we have developed quantitative methods to detect chemical bond scission associated with macroscopic fracture of tough soft materials. We have used a combination of scattering and optical techniques such as small angle X-ray scattering, diffusive wave spectroscopy and fluorescent molecular probes to map, stress, strain, bond breakage and structure in a region ~100 µm in size ahead of the propagating crack. We have thus gained an unprecedented molecular understanding of where and when bonds break near the fracture plane as the material fails, establishing some key concepts between the architecture of soft polymer networks and their fracture energy, reistance to cavitation or to crack propagation under cyclic low amplitude loading. These results lead to a new molecular and multi-scale vision of macroscopic fracture of soft materials, that will be invaluable to design and develop better and more finely tuned soft but tough and sometimes self-healing materials to replace living tissues (in bio engineering) and make lightweight tough and flexible parts for energy efficient transport.
