Confocal Microscopy Visualizes Particle–Crack Interactions in Epoxy Composites with Optical Force Probe-Cross-Linked Rubber Particles

Abstract

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.