Time and temperature-dependent fracture mechanics of self-healing vitrimers

Abstract

This study investigates the formation of exchangeable bonds and recovery of load bearing in bulk vitrimers. The critical load required to fracture compact tension samples in bulk aromatic thermosetting co-polyester (ATSP) and the ability of the material to heal the crack was studied as a function of duration and temperature of healing, and number of cycles. We established a marked improvement in healing efficiency at higher temperature especially in the first healing cycle, as the thermal energy reduces the energy barriers for bond re-formation. However, the healing efficiency progressively dropped in four healing cycles, and the drop was more significant at higher temperatures. We analyzed the cure kinetics of the ATSP by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gel fraction tests. The results showed a particularly good thermal stability below 300 °C. The material loss from gel fraction tests was bound to 0.7 %. While not significant by mass, this mass will turn into volatile species and gas form, occupying a much larger volume, leading to formation of voids which serve as physical barrier to curing (material degredation). This is also evidenced in the increase in the fracture surface roughness, as improper bond exchange lead to the formation of volatile species. The formation of volatile species leads to surface asperities, reducing the contact area between the two faces of the crack, and results in a reduction in healing efficiency. The study concludes by outlining the differences between the intrinsic healing in vitrimers with high vitrification temperature and thermoplastics, highlighting the differences between the molecular mechanisms at play.