Temperature-Dependent Fracture Toughness of Epoxy Vitrimers

Vitrimer, a class of covalent dynamic networks, is promising for polymeric applications requiring reshaping, recycling, and self-healing. The dynamic bonds in vitrimer can be reversibly broken and reformed under thermal actuation. This work investigates the effects of temperature on the fracture toughness of soft epoxy vitrimers. We measured the fracture toughness above the topology freezing temperature. Elevated temperatures reduce both fracture stretch and toughness, while they linearly increase the modulus. Given the invariant cross-linking density, the temperature dependence of fracture toughness is attributed to the sensitivity of the bond-exchange reactions. We extend the Lake-Thomas theory by incorporating kinetic bond scission to elucidate such dependence in epoxy vitrimers. Unlike conventional elastomers with a threshold of decreased fracture toughness at elevated temperatures, our theory indicates the continuous decrease of fracture toughness until approximating a zero value, aligning with the unique viscous behavior of vitrimers.