Quantitative evaluation of fracture, healing and re-healing of a reversibly cross-linked polymer

The repeated fracture–healing characteristics of 2MEP4F polymer, a cross-linked polymer based on Diels–Alder cycloaddition, are systematically and quantitatively evaluated using a sample geometry that allows for controlled incremental crack growth so that the cracked sample remains in one piece after the test, improving our ability to realign the fracture surfaces prior to healing. The specimens have been pre-cracked to a repeatable length to enable accurate comparison between virgin and healed fracture loads, and hence fracture toughness. Moreover, multiple data points are extracted from a single sample. In this paper, we report the results of our repeated fracturing and healing cycles. We have shown that healing at temperatures between 85 and 95 °C, after repeated fracture–healing cycles, results in full fracture toughness recovery and no dimensional changes due to creep. We have then calculated the fracture toughness after each fracturing and healing cycle, using a previously developed and tested model, arriving at consistent results for repeatedly healed 2MEP4F polymer. These results show a fracture toughness of about 0.71 MPa m1/2 for this material at room temperature. We have also examined healing for shorter periods, as little as 30 min, using the same temperature–pressure cycle, arriving at essentially the same final results, i.e., full toughness recovery. That the healing process is largely independent of time is in contrast to the diffusion-controlled healing observed in the welding of thermoplastic polymers. Rather, it shows that the healing in the 2MEP4F polymer results from the repair of the broken Diels–Alder (cross-linking) bonds that seems to restore a molecular structure similar to that of the original virgin polymer with the same (or even slightly improved) overall macroscopic fracture resistance.