Semi-Interpenetrating Polyurethane Network with Fatigue Elimination and Upcycled Mechanical Performance

Recyclable elastomers based on adaptable covalent networks fabricated via different types of dynamic bonds have been designed and prepared to resolve the urgent problems generated by waste elastomers. However, due to inevitable side reactions during thermal recycling (e.g., oxidation, permanent cross-linking), mechanical recovery efficiency after thermal recycling is normally <90% for most recyclable elastomers, making it difficult to achieve comparable performance. Herein, we report a novel semi-interpenetrating network design that achieves mechanical reinforcement after thermal recycling via network topology isomerization and formation of new cross-linking points. The designed semi-interpenetrating network includes an aromatic disulfide bond-containing polyurethane network and long linear polyurethane chains with side-chain vinyl groups. Triggered by heat during reprocessing, the disulfide bonds inside the cross-linked network break and the generated phenyl sulfur radicals undergo thiol–ene reactions with the side-chain vinyl groups in the linear polyurethane chains, resulting in increased cross-linking density. Electron paramagnetic resonance testing, in situ Fourier transform infrared spectrometry, high-temperature stress relaxation testing, and cross-linking density measurements were utilized to monitor the process. The tensile strength and extensibility recycling efficiencies reached 186 and 131% of original values after reprocessing twice, realizing mechanical reinforcement after thermal recycling. Interestingly, based on a similar mechanism, mechanical fatigue after repeated stretch and release cycles was efficiently eliminated upon thermal treatment. Such a strategy achieving polymer design with reinforced physical properties after recycling will benefit practical applications of sustainable elastomers and other thermosetting materials.