Atomistic Simulations on Mechanical Properties of Lignin

Mechanical properties of lignin, an aromatic heteropolymer constituting 20%-30% of plant biomass, are important to the fabrication and processing of lignin-based sustainable polymeric materials. Atomistic simulations are performed to provide microscopic insight into the mechanics of lignin. Representative samples of miscanthus, spruce, and birch lignin are studied. At temperature below the glass transition temperature, the stress-strain curve for lignin under uniaxial compression exhibits initial elastic response, yielding, and post-yield plastic response with increasing strain. The decomposition of the overall stress shows that the energetic component contributes to the elastic response and yielding, but remains low in the post-yield plastic regime until strain hardening sets in. The dissipative component dominates the post-yield regime before strain hardening. In addition to the three real lignin samples, minimalist model systems of monodisperse linear polymers consisting of only guaiacyl units and β-O-4 linkages are simulated. While the elastic response, yielding, and the plastic flow under compression do not depend on the molecular weight of the model lignin_, the strain hardening under compression is enhanced as lignin molecular weight increases.