Catalyst-Free Renewable Polyurethane Based on Depolymerized Lignin with Excellent Shape Memory Performance and Reprocessability

Using lignin as a building block for polyurethane (PU) synthesis has been challenging due to its heterogeneous, branched structure and significant steric hindrance. To overcome these limitations, this study employed lignin-derived oligomers obtained via microwave-assisted depolymerization, known as bio-oil (BO), as a sustainable alternative to petroleum-based polyol for the catalyst-free fabrication of high-performance PU sheets. The effects of BO content (10–60 wt %) on the thermal and mechanical properties, shape memory performance, antioxidant capacity, and reprocessability of PU sheets were systematically investigated. The fabricated PU sheets exhibited an ultimate tensile strength of 69.1 MPa, compared to 41.8 MPa for the conventional petroleum-based PU. Notably, despite substituting the polyol with BO, the PU sheets maintained a strong mechanical performance. Additionally, BO incorporation significantly enhanced thermal stability and antioxidant properties significantly. The PU sheets also exhibited an outstanding thermally triggered shape-memory effect with shape fixity and recovery ratios of 100% and 96.1%, respectively, over five cycles and the ability to close scratches and cuts. Finally, the material was successfully dissolved in DMF under mild conditions and was reused as an adhesive, achieving a shear strength of 1.77 MPa.