Simultaneous robustness, reprocessing and self-healing of castor oil-based polyurethane vitrimers enabled by supramolecular nitrogen-coordinated dynamic covalent boronic ester

Abstract

Striving for high-performance polyurethanes from renewable resources has attracted increasing attention, due to the continuously increased environmental concerns and resource crisis related to traditional fossil-based materials. Castor oil is a renewable raw material, while the traditional castor oil-based polyurethane chains are highly flexible and permanently cross-linked, resulting in poor mechanical strength and toughness that cannot be reprocessed. To obtain robust and reprocessable castor oil-based polyurethane materials, herein we report the utilization of supramolecular nitrogen-coordinated boronic ester crosslinking. A series of castor oil-based polyurethane (COPUX) vitrimers with different amounts of dynamic covalent boronic ester crosslinking and boron-nitrogen (B-N) coordination were designed and synthesized by a solvent- and catalyst-free two-step method. The permanent crosslinking degree decreases while the strength, moduli, ductility and heat resistance significantly increase accordingly with increasing boronic ester content. The B-N coordination can further improve the mechanical properties of materials due to supramolecular interactions. The tensile behaviors are tunable over a broad range (either as elastomers or as plastics) depending on the compositions. The tensile strength of COPU4 was improved by 9 times from 1.9 to 17.1 MPa compared to COPU0. The Young's modulus for COPU3 was improved by 119 times from 2.9 to 345.1 MPa and the yield strength reaches 18.3 MPa. Boron-nitrogen coordination enhances mechanical strength without loss of material self-healing properties. COPU3 and COPU4 exhibit excellent self-healing as well as reprocessability. The investigation provides the feasibility of future shifts from traditional fossil-based thermosets to bio-based reprocessable vitrimers, offering new hints for the design and manufacture of high-value sustainable polymer materials.