Lignin Nanoparticles: Understanding of Their Properties and Modifications for Versatile Applications

Defence is held on 5.11.2021 12:00 – 15:00 https://aalto.zoom.us/j/68644701597Kraft lignins are abundant and renewable biomass deriving from wood, which have been largely underexploited due to their structural inhomogeneity. Converting Kraft lignins into lignin nanoparticles (LNPs) triggers numerous value-added applications for lignin because LNPs have the advantages over the raw material with colloidal stability in aqueous media (pH 3 - 10), larger surface area per mass unit, and tuneable surface charge. This thesis aims to (1) fundamentally understand the solvent effects on the intrinsic properties of LNPs, since this knowledge is largely missing in the current literature; (2) broaden the application window for LNPs through various covalent and non-covalent modifications. In part one, the solvent effects on LNP size were investigated using aqueous acetone and aqueous tetrahydrofuran (THF), two common solvent systems for producing spherical LNPs. Aqueous acetone results in smaller LNPs with a narrower size distribution compared to aqueous THF. Molecular simulations (MD) results suggest that acetone has a stronger affinity towards lignin than THF, which leads to a smaller LNP size, as confirmed with 1,4-dioxane and dimethyl sulfoxide. More importantly, the LNPs resulting from aqueous acetone and aqueous THF were characterized as compact particles, which have relatively homogeneous density and very low porosity in the internal structure, relatively high hydrophilicity on the surface, and size-independent stiffness. The rigid nature of LNPs makes them "inert" (low swelling) upon the change in pH from 6 to 10. These fundamental understandings gained for LNPs are important for designing LNPs with optimum properties in applications. In part two, surface and bulk modifications were applied to LNPs for enhanced properties in applications. For instance, the addition of the cationic lignin-coated LNPs (10 wt%) into cellulose nanofibrils (CNFs) not only improved the toughness of the film by a factor of two, but also rendered the film with waterproof, antioxidant, and UV-blocking properties. Coating of LNPs with chitosan enabled the stabilization of olive oil-in-water Pickering emulsions that are promising for applications in biomedicine, cosmetics, and/or food. Moreover, a simple method was developed to incorporate bisphenol A diglycidyl ether (BADGE) into the internal structure of LNPs. The resultant hybrid LNPs could be either used for surface functionalization under strongly alkaline conditions or used as waterborne adhesives for wood. Overall, the fundamental understandings of solvent effects on LNP properties and the modifications of LNPs towards a broader application field demonstrated in this thesis pave the way for lignin varolization for materials applications