Brittle fracture
often compromises the durability of glassy polymers.
This can be mitigated by reinforcing the matrix with a filler, activating
a range of toughening mechanisms. Therefore, it is desirable to better
understand the mechanical response of polymer composites, but a direct
visualization of the mechanical fate of second-phase inclusions upon
material fracture was previously unavailable. Here, rubbery poly(hexyl
acrylate) particles, cross-linked with an optical force probe (OFP),
are dispersed within an epoxy matrix, the material is fractured, and
particle–crack interactions are visualized with confocal laser
scanning microscopy. The dual-fluorescence character of the OFPs allows
the differentiation between particles that remain intact and those
that are stressed beyond bond scission upon interaction with a propagating
crack. The localized activation of OFPs reveals stress gradients within
the particles and crack direction pathways, hence providing a new
layer of information over fracture events in polymer composites.