Using forest lignin as a renewable resource is a greener alternative to the petrochemical industry and accelerates progress in developing greener industrial processes. A better understanding of the complexities of lignin as a raw material is required to create new sustainable value chains, and a deeper understanding of the forces and interactions that drive 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 made using green solvent fractionated kraft lignins with narrow structure and molecular weight dispersity, and the LNPs have been thoroughly characterized in terms of their size, shape, and surface properties. For both spruce and eucalyptus lignin fractions, the size of the LNPs decreased with increasing size M.w with a decreasing number of phenolic hydroxyl groups and an increasing number of aliphatic hydroxyl units in the lignin fraction. The diameter of the LNPs can be varied between 80 and 500 nm M.w of the initial lignin and its concentration. The number of methoxy and phenol groups in the aromatic ring, the aliphatic hydroxyl groups and β-O-4 bonds in side chains in lignin fractions have a considerable influence on the morphology and surface structure of the LNPs. The LNPs, with shapes ranging from donut-like structures to filled, interconnected spheres, were made depending on the nature of the lignin phenylpropanoid units (botanical origin), the 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 shows that it is of crucial importance to select the appropriate starting lignin materials for the controlled design and synthesis of LNPs. This reduces the costs for the subsequent cleaning and further processing of the LNPs and prevents environmental pollution by minimizing the consumption of resources. The knowledge gained provides a clear guideline for the design of new biomass-based materials.

Graphic summary: Structural and molecular weight dependence in the formation of lignin nanoparticles from fractionated soft and hardwood lignins

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