Lignin demethylation enabled robust and reprocessable bio-polyurethane elastomer with adaptable cross-linking network

The development of sustainable bio-polyurethanes (bio-PU) from nonfood lignin has been attracting extensive attention owing to the abundant reserves, stable aromatic structures, and rich hydroxyls of lignin. However, exploiting a high-performance lignin-based PU integrating excellent mechanical properties with reprocessability is still a challenge. Herein, we proposed a dynamic phenolic-carbamates to construct robust and reprocessable bio-PU elastomers based on the reaction of phenolic lignin (PL) with isocyanate. Benefiting from the low Mw of 1692 Da and the abundant phenolic hydroxyls of 8.3 mmol/g, the adaptable cross-linking network consisting of dynamic hydrogen bonds and phenolic-carbamates were formed by PL. The synthesized phenolic lignin-based PU (PLPU) displayed tunable mechanical properties via adjusting PL substitution amount. The tensile strength, modulus and toughness of PLPU reached 58.8 MPa, 1350.3 MPa, and 57 MJ/m3 when the substitution amount of PL was 70%, respectively. Importantly, the reversible cross-linking structure endowed PLPU with excellent multireprocessability. After 5 reprocessing cycles, the retention ratio of strength, breaking elongation, modulus and toughness was 87.6%, 58.9%, 218.1% and 63.3%, respectively, which were 1.6, 1.1, 3.5, 2.2 times higher than those of commercial lignin-based polyurethane (CLPU). Therefore, this work not only presents a robust and multi-reprocessable lignin-based PU elastomer with high substitution amount, but also provides a promising sustainable alternative to replace petroleum-based plastic.