Polyurethanes synthesized from diisocyanate trimers and polyethylene glycol undergo crosslinking via thermally reversible Diels–Alder reactions

A series of self-healing polyurethane (PU) polymers were synthesized via the Diels–Alder reaction between the furan-functionalized PU prepolymer and bismaleimide (BMI). The PU prepolymer is a reaction product of polyethylene glycol (PEG) and a pentamethylene diisocyanate trimer (PDI-t). Furfuryl amine was attached to the terminal isocyanic groups of PU, resulting in furfuryl amine polyurethane (PU-FA). PU-FA was cross-linked via dynamic bonds between the furan groups and BMI, generating reversible bonds with the material. This study explored reversible polymer networks under various thermal conditions, including different reaction parameters, prepolymer ratios, and healing temperatures, to evaluate the healing capabilities of these polymers. The results demonstrated that PU-DA exhibits thermal reversibility, as evidenced by the appearance of an endothermic peak in the DSC analysis, indicating bond cleavage via the retro-Diels–Alder (r-DA) reaction. This endothermic peak re-emerged after cooling at 70 °C for 24 h, further confirming the reversibility of the reaction. Additionally, crack healing in the PU-DA film was facilitated by the synergistic effects of the thermally reversible Diels–Alder reaction and the thermal mobility of molecular chains. The self-healing performance evaluation revealed that PU-DA samples with an NCO/OH ratio of 1.5 exhibited a reduction in tensile strength after healing compared to the original PU-DA specimens before heat treatment. Conversely, samples with an NCO/OH ratio of 3 demonstrated a healing efficiency exceeding 100%, attributed to the enhanced cross-link density, which contributed to a more robust network structure.