Reversing fatigue damage in self- healing vitrimer and its composites

Thermosetting polymeric composites which are ubiquitous in structural applications due to their superior strength and stiffness compared to their thermoplastic counterparts. These superior properties are principally imparted by dense and permanent crosslinking present at the molecular network level. However, as thermosets are subjected to fatigue loading, they accumulate damage in terms of ruptured crosslinks which eventually result in catastrophic failure. Nanocomposites fabricated by the addition of nanofillers create interactions with incipient cracks and prolong the fatigue life¹. But since the crosslinking rupturing is irreversible process, the cracks eventually propagate to a critical size which leads to failure. Thus, the same crosslinking which imparts strength and stiffness to thermosets is responsible for the accumulation of irreversible fatigue damage. If crosslinks in the thermosets are made reversible under application of external stimulation, thermosets can retain their superior properties while potentially possessing the ability of reversing the fatigue damage. Liebler et al proposed an epoxy-based network called vitrimer which can reform crosslinks at elevated temperature². Vitrimers have attracted wide attention in the past decade, however their fatigue performance remains widely unexplored.

We report a vitrimeric system which is prepared using epoxy resin and adipic acid with a suitable catalyst. The system has mechanical properties comparable to conventional epoxies and is defined by two characteristic temperatures namely, the glass transition temperature (T_g) and topology freezing temperature (T_v). We demonstrate that the vitrimer can be periodically healed by heating at elevated temperature. Further, we also report a carbon fiber vitrimer composite which can be similarly healed by periodic healing. Such composites can pave way for next generation of structural components which are recyclable and have much longer service lifespans.