Dynamics of the lignin glass transition

Abstract

The dynamics of lignin, a complex and heterogeneous major plant cell-wall macromolecule, is of both fundamental and practical importance. Lignin is typically heated to temperatures above its glass transition to facilitate its industrial processing. We performed molecular dynamics simulations to investigate the segmental (α) relaxation of lignin, the dynamical process that gives rise to the glass transition. It is found that lignin dynamics involves mainly internal motions below T_g, while segmental inter-molecular motions are activated above T_g. The segments whose mobility is enhanced above T_g consist of 3–5 lignin monomeric units. The temperature dependence of the lignin segmental relaxation time changes from Arrhenius below T_g to Vogel–Fulcher–Tamman above T_g. This change in temperature dependence is determined by the underlying energy landscape being restricted below T_g but exhibiting multiple minima above T_g. The Q-dependence of the relaxation time is found to obey a power-law up to Q_max, indicative of sub-diffusive motion of lignin above T_g. Temperature and hydration affect the segmental relaxation similarly. Increasing hydration or temperature leads to: (1) the α process starting earlier, i.e. the beta process becomes shortened, (2) Q_max decreasing, i.e. the lengthscale above which subdiffusion is observed increases, and (3) the number of monomers constituting a segment increasing, i.e. the motions that lead to the glass transition become more collective. The above findings provide molecular-level understanding of the technologically important segmental motions of lignin and demonstrate that, despite the heterogeneous and complex structure of lignin, its segmental dynamics can be described by concepts developed for chemically homogeneous polymers.