A recyclable polyurethane with characteristic thermal stiffening behavior via B-N coordination with reversible B-O bonds

Thermal softening is an inevitable process in the physical network. Polyurethane (PU), a typical commercial material, is constructed by physical networks, which undergoes the serious thermal decay on mechanical properties at high temperature. Herein, a physically cross-linked PU with a unique thermal stiffening behavior has been developed by incorporating B-N coordination with reversible B-O bonds. The B-N coordination can significantly improve the mechanical properties of the PU. The reversible B-O bonds (temperature dependent reversible transformation between B-OH and B-O-B) are conducive to constructing more multi-coordination macromolecular crosslinking points and more stable B-N coordination bonds at high temperature, endowing the PU with the special thermal stiffening behavior for the first time. Such thermal stiffening behavior compensates for the bond breakage and the network destruction caused by heat, significantly expands the rubbery plateau and delays the entire chain motion of the thermoplastic PU. As a result, the terminal flow occurs at a higher temperature up to 200°C. The modulus retention ratio of the materials is up to 87% even at 145°C, which is much higher than that of the existing PU elastomer with the physical network and even some covalent cross-link PU. Simultaneously, the physical network ensures the recyclability of the PU, and the thermal stiffening behavior is still obtained in recycled PU. This work provides a simple strategy to impart thermal stiffening behavior to the physically crosslinked PU, thereby significantly extending the operating temperature range of thermoplastic PU, which can potentially expand the scopes of PU in applications under harsh conditions.