Identifying the origin of intrinsic self-healing gradual decay in vitrimer carbon fiber reinforced polymer composites

The performance of materials degrades over time. One group of materials which stands out with their high maintenance costs is carbon fiber reinforced polymer (CFRP) composites. Vitrimers, an emerging class of self-healing thermosetting polymers can regain their properties upon damages via bond reformation. In this study, we evaluated self-healing in vitrimer composites subjected to multiple fracture-healing cycles. The study focused on a vitrimer with high thermal resistance (Aromatic thermosetting polyester - ATSP). To assess the self-healing, we performed Mode I fracture double cantilever beam test, followed by healing at 280 degrees C for five cycles. The results demonstrated that there was complete healing after the first two cycles, with mode I fracture toughness of similar to 300 J/m2, albeit with a gradual decline of healing after the third cycle, reaching to similar to 80% healing efficiency after the 5th cycle. From scanning electron microscopy, optical microscopy, and micro-computed tomography X-ray imaging, surface and internal morphology of the vitrimer CFRPs closely resembled the virgin samples after two cycles. However, integrity of matrix and carbon fiber structure around crack was gradually depleted after the third cycle. Infrared spectroscopy ruled out material degradation. The performance decline in vitrimer composites was associated with the irreversible local mechanical damages which accumulated during multiple fractures and the limited chain diffusion in vitrimers. Hence, bond exchanges are more likely to occur between chains on one side of the crack rather than across the crack. The study outlines the advantages and limitations of exchangeable bonds in achieving high performance composites with intrinsic and sustained self-healing ability.