Achieving high lignin content and mechanical enhancement in biobased thermosets via a high-molecular-weight soybean oil skeleton

Abstract

Developing sustainable thermosets from renewable resources that can rival their petroleum derived counterparts has attracted tremendous interest during recent years. Despite its prevalence as a reinforcement for thermosets, the utilization of lignin is constrained by its brittleness and structural heterogeneity. High lignin loadings often result in marked material embrittlement. Consequently, enhancing lignin content while preserving or improving toughness and strength presents a key dilemma. Herein, we propose a strategy to overcome this limitation by constructing a flexible, ultra-high molecular weight skeleton derived from plant oils. Specifically, we leverage soybean oil polymers with weight-average molecular weight greater than 1000 kDa to fabricate high-performance thermosets incorporating substantial lignin content (up to 50%). The homopolymer, synthesized by ring-opening metathesis polymerization (ROMP) and cross-linked at side chains with lignin, exhibits synergistically enhanced properties from rigid lignin, cross-linking, and backbone entanglement. To optimize network formation, copolymers with tunable side-chain density were engineered via ROMP, thereby achieving an effective increase in cross-linking density. The mechanical performance of the resulting thermosets, reinforced by rigid cross-links and chain entanglement, could be precisely tuned by adjusting the side-chain density and cross-linking density. This approach yielded a high content-biomass thermoset containing 50% lignin with high strength and good toughness, demonstrating a viable strategy for constructing mechanically tunable materials from abundant biomass.