Abstract
<jats:p>Lignin-based vitrimers offer a promising route to integrating renewable feedstocks with reprocessable covalent adaptable networks. In this study, co-solvent-enhanced lignocellulosic fractionation (CELF) lignin was carboxylated and subsequently reacted with epoxidized soybean oil (ESO) to construct bio-based vitrimer networks. Carboxylation increased the carboxyl groups content of CELF lignin from 0.43 to 6.14 mmol g-1 by converting its aliphatic and phenolic hydroxyl contents. The formation of ester-crosslinked networks was confirmed by Fourier-transform infrared spectroscopy (FTIR), including the consumption of epoxy groups and the appearance of a dominant ester carbonyl band at 1729 cm-1. Varying the L-COOH/ESO feed ratio enabled systematic tuning of the mechanical properties with the tensile strength from 3.06 to 8.82 MPa and the Young’s modulus from 24.28 to 188.05 MPa, while decreasing the elongation at break from 66.3% to 9.1%, consistent with an increase in relative crosslink density. Stress-relaxation measurements revealed conventional monotonic relaxation at 190-200 °C but an unusual partial stress recovery at temperatures of 210 °C and above. Detailed FTIR analysis provided evidence of the disassociation of hydrogen bonding and the coordination of the Zn2+ with oxygen-containing groups that may cause and the rearrangement of segmental structures which may support the stress recovery in the stress relaxation. These findings demonstrate a composition-dependent balance between stiffness and extensibility in lignin-ESO vitrimers and reveal a potentially important role of metal-mediated noncovalent interactions in their high-temperature mechanical response.</jats:p>