Comparison of the Interaction and Structure of Lignin in Pure Systems and in Asphalt Media by Molecular Dynamics Simulations

Lignin is a class of organic aromatic polymers contributing to the rigidity and strength of plants and has been proposed as a modifier to improve asphalt performance on road pavement. However, contradicting experimental results on the lignin miscibility in asphalt were found from different studies, and lignin has been found to self-assemble in different solutions. Thus, investigating the interaction and microstructure of lignin in asphalt media in molecular detail is necessary. Molecular dynamics (MD) simulations using both the LAMMPS program with the OPLS-aa force field and the NAMD program with the CHARMM force field have been conducted on pure lignin (including lignin monomer, dimer, and polymer with 17 and 31 units) and their mixtures with model asphalt molecules at different temperatures. Consistent results were observed from both programs and force fields in terms of density, hydrogen bonds, diffusion coefficient, radius of gyration, and radial distribution function. Glass transition was observed in the pure lignin systems based on density and diffusion coefficient calculations at different temperatures. Lignin can form intramolecular hydrogen bonds and intermolecular hydrogen bonds with other lignin and 1,7-dimethylnapthalene in the asphalt mixture, which has dependence on temperature and lignin chain length. Correlating the lignin size and chain length using the power-law relationship showed that lignin polymers in pure systems are in quasi-relaxed structures at different temperatures; lignin molecules stay in quasi-relaxed structures in asphalt mixtures at high temperatures but in collapsed structures at low temperatures. Implementing lignin monomer, dimer, and polymer into the model asphalt mixture can improve its density. Although lignin in different chain lengths aggregates in asphalt, lignin can modify the packing between different components in asphalt media at different temperatures. The work suggests that temperature can significantly influence the miscibility of lignin polymer in asphalt and that lignin can function as both a modifier and a resin in asphalt.