Comprehensive investigation of the interactions between natural rubber and lignin by molecular dynamics simulation

Currently, the interaction behavior of natural rubber-lignin (NR-L) remains unclear. In this study, we successfully replicated the density, glass transition temperature, and mechanical properties of NR-L using molecular dynamics coupled with GAFF. Using these models, we rigorously investigated the interaction behavior of these systems, which revealed that when the ambient temperature was lower than the thermal decomposition temperature, there was almost no impact on system properties, including density, structure, interaction forces, radial pair distribution function, free volume, radius of gyration, and lignin hydrogen bonding, as no significant enthalpy change occurred. The influence of hardwood lignin and softwood lignin on NR strength varied according to the concentration of lignin. At low concentration, the electron density cloud between highly branched hardwood lignin and NR is lower, leading to smaller interactions. However, at higher concentrations, the van der Waals term between the less polar and larger number of atoms of hardwood lignin and NR becomes stronger, resulting in the opposite trend being observed. Furthermore, NR-L is governed primarily by van der Waals forces, while lignin-lignin is governed primarily by electrostatic interactions. These detailed characterizations offered valuable insight for future research endeavors aimed at designing and synthesizing green NR-L composites at the atomic scale.