Molecular Dynamics Study of Thermophysical and Mechanical Properties in Hydrated Lignin with Compositions Close to Softwood

The hydrogen bonding network analysis of softwood lignin is relevant to designing novel technologies to overcome the recalcitrance of plant biomass in the industrial deconstruction and manufacturing of lignin-based carbon fibers. In this work, we examine, by atomistic simulations, the hydrogen bonding network in guaiacyl-rich lignin and guaiacyl-type lignin over a wide range of temperatures. We determine the formation of stable water-bridged dimeric complexes by the interaction of phenolic and aliphatic hydroxyl groups and pi-pi stacking between phenol rings, causing a slow dynamic of lignin with temperature. The interaction strength between water oxygen and hydrogen of hydroxyl groups in these lignin complexes is established. The formation of the ice crystal structure at low temperatures in the complexes explains the anti-plasticizing action of water, increasing the Young's modulus (E) of lignin systems at high hydration. Glass-transition temperature and E were successfully predicted by CHARMM force field for lignin at different hydrations.