Molecular simulations of lignin and its behavior in fiber walls

Lignin is a major component of wood cell walls and wood fibers, and as such, contributes to physical properties of pulp fibers and their material applications. Despite the abundance and industrial potential of lignins, their structure-function relationships remain poorly understood. The underlying reason is their heterogeneous nature, which stems from their formation through stochastic radical synthesis. Recent advances in chemical analysis have enabled a statistical description of the complex and heterogeneous molecular structure of lignins [1]. This, in turn, has made molecular modelling a useful tool for studying their fundamental properties.

In this work, we report molecular simulations on the structure and physical properties of softwood lignin. This involves its role in the moisture behavior of wood cell wall structures and thermal response in the hot-pressing process for a fiber web. We first create a molecular model for native-like softwood lignin based on a statistical description of its chemical structure [1]. Using a molecular force field specifically designed for lignins [2], we carry out molecular dynamics simulations to predict the behavior of cellulose-lignin interfaces and neat lignin at different moisture contents and temperatures. We compare the predictions at ambient temperature with X-ray scattering experiments on native and delignified spruce samples to understand the role of lignin in the moisture response of microfibril structures [3]. We compare the predictions at elevated temperatures with hot pressing experiments on fiber webs at varied temperatures, sample moisture contents and pressing times to determine the possible contribution of lignin softening and diffusion to the observed wet strengthening.

Our work is an early step toward more systematic use of molecular modelling to understand lignin structure-function relationships. We anticipate that combined chemical analysis and simulation will find increasing use in the development of lignin-based materials, and can also guide the thermal and wet processing of lignin-containing fiber materials.

[1] M. Balakshin, E. A. Capanema, X. Zhu, I. Sulaeva, A. Potthast, T. Rosenau and O. J. Rojas. Spruce milled wood lignin: linear, branched or cross-linked? Green Chem., 22:3985-4001, 2020.
[2] J. Vermaas, L. Petridis, J. Ralph, M. F. Crowley and G. T. Beckham. Systematic parameterization of lignin for the CHARMM force field. Green Chem., 21:109-122, 2019.
[3] A, Zitting, A. Paajanen, M. Altgen, L. Rautkari and P. A. Penttilä. Role of Lignin in Moisture Interactions of Cellulose Microfibril Structures in Wood. Small Struct., 5:2400167, 2024.