  Many new soft but tough rubbery materials have been recently discovered and
new applications such as flexible prosthetics, stretchable electrodes or soft
robotics continuously emerge. Yet, a credible multi-scale quantitative picture
of damage and fracture of these materials has still not emerged, due to our
fundamental inability to disentangle the irreversible scission of chemical
bonds along the fracture path from dissipation by internal molecular friction.
Here, by coupling new fluorogenic mechanochemistry with quantitative confocal
microscopy mapping, we uncover how many and where covalent bonds are broken as
an elastomer fractures. Our measurements reveal that bond scission near the
crack plane can be delocalized over up to hundreds of micrometers and increase
by a factor of 100 depending on temperature and stretch rate, pointing to an
intricated coupling between strain rate dependent viscous dissipation and
strain dependent irreversible network scission. These findings paint an
entirely novel picture of fracture in soft materials, where energy dissipated
by covalent bond scission accounts for a much larger fraction of the total
fracture energy than previously believed. Our results pioneer the sensitive,
quantitative and spatially-resolved detection of bond scission to assess
material damage in a variety of soft materials and their applications.
