Structural and molecular-weight-dependency in the formation of lignin nanoparticles from fractionated soft- and hardwood lignins

Abstract

The use of lignin from forests as a renewable resource is a greener alternative to the petrochemical industry and accelerates the progress towards the development of more environmentally friendly industrial processes. A better understanding of the complexity of lignin as a raw material is necessary for creating new sustainable value chains, and a deeper understanding of the forces and interactions driving the self-assembly of lignin nanoparticles (LNPs) is required to create new, more advanced lignin nanomaterials. In the current study, “a library” of LNPs made from both softwood (spruce) and hardwood (eucalyptus) lignins was prepared utilizing green solvents-fractionated kraft lignins with narrow structural and molecular weight dispersity, and the LNPs were thoroughly characterized with respect to their size, shape, and surface properties. For both spruce and eucalyptus lignin fractions, the size of the LNPs decreased with increasing M_w with a decreasing number of phenolic hydroxyls and an increasing number of aliphatic hydroxyl units in the lignin fraction. The diameter of the LNP's could be varied between 80 and 500 nm, depending on the M_w of the initial lignin and its concentration. The number of methoxy and phenolic groups in the aromatic ring, the aliphatic hydroxyls and β-O-4 bonds in side chains in lignin fractions affect the morphology and surface structure of the LNPs to a significant degree. The LNP's with shapes ranging from doughnut-like structures to filled interconnected spheres were prepared, depending on the type of lignin phenylpropanoid units (botanical origin), concentration, and other properties of the lignin fractions. The identified strong dependence of the properties of the LNPs on the inherent properties of the lignin from which they were derived reveals that it is of crucial importance to select the appropriate starting lignin materials for the controlled design and synthesis of LNPs. This reduces costs for the subsequent purification and further processing of the LNPs, and prevents environmental pollution by minimizing the usage of resources. The obtained knowledge provides a clear guideline for the design of new biomass-based materials.