Structure-Property Correlation on Solvent-Fractionated Lignin to Functional Materials

The abundance of lignin in combination with its impressive properties, i.e., a macromolecule with multifunctional groups, an amphiphilic molecular structure, and a unique nanotechnological advantage of forming nanospheres, have attracted an intensified interest in engaging this natural polyphenol in functional materials. However, native lignin is not the lignin that is available for applications, and the structure of lignin may significantly change during pulping or other biorefinery processes. In this scenario, a given sample of lignin possesses significant variability concerning impurities (e.g., extractives and carbohydrates) and has heterogeneous structural features. These aspects, together with the underlying analytical challenges, have substantially constrained the valorization of lignin. Therefore, fractionation of lignin to produce fractions with decreased heterogeneity and well-defined properties is of utmost importance, leading to breakthroughs in efficiently integrating lignin in functional materials. This thesis is dedicated to using a sequential solvent fractionation approach (isopropyl alcohol, ethanol, and methanol) to establish correlations between the structural characteristics of the lignin fractions and material properties of lignin and to reveal the determining factors of lignin utilization in certain applications. Furthermore, the lignin structure-property correlation will be used to tailor the properties of lignin integrated functional materials.
The effectiveness of this strategy was validated in the fractionation of birch and spruce alkaline lignin, where lignin fractions with well-defined properties, e.g., molar mass, content of functional groups, and degree of condensation, were obtained. The deployed lignin solvent fractionation strategy revealed fundamental insights into the correlation between the molar-mass-dependent differences of lignin fractions and the chemical accessibility to synthesize a thermosetting lignin-containing phenol-formaldehyde adhesive. In the current work, up to 70% of phenols could be replaced by birch alkaline lignin fractions.
Nano-sized lignin, such as lignin nanoparticles (LNPs), is rising as a class of sustainable nanomaterials, which can function as a template to modulate surface functionalization via interfacial interactions. This thesis proposed a high-efficacy route to integrate lignin as a bioplastic in poly (butyl acrylate-comethyl methacrylate) acrylic latex formulation by fabricating polymerizationactive LNPs with surface-arranged allyl groups. The interfacial-modulating function on the LNPs regulated the core-shell emulsion polymerization of acrylate monomers and successfully produced a multi-energy dissipative latex film structure containing a lignin-dominating core. Depending on the surface chemistry metrics of LNPs, such as the abundance of polymerization-active anchors, polymeric flexibility, and surface hydrophobicity, the LNP-integrated latex film could achieve a high toughness almost three times higher than that of the neat latex film.
In addition to chemical functionalization, this thesis also upgraded lignin through a biochemical functionalization strategy. First, a lignin solvent fractionation approach was successfully applied to reveal fundamental insights on the correlation between the lignin structural characteristics and the laccaseassisted oxidation/polymerization properties. The fractionation-dependent lignin polymerization kinetics also brought new insights into in situ polymerization of lignin fractions on nanocellulose templates, where the dispersion of nanocellulose with its fiber evenly decorated by aligned LNPs was obtained. Moreover, the cellulose-lignin nanocomposite film exhibited enhanced water barrier properties when compared to the neat cellulose film, which provides a sustainable solution for the development of functional biobased packaging materials. Second, the lignin reactivity could be fine-tuned using solvent fractionation in combination with the laccase-catalyzed polymerization approach, which endowed LNPs from laccase-polymerized lignin (L-LNPs) with dispersion durability and surface functionality in highly alkaline conditions. Subsequently, the L-LNP was utilized as a highly dispersible and nano-sized polymeric template for in situ reduction of Ag+ from silver ammonia solution (pH 11), which resulted in a uniform surfaceembedded hierarchical nanostructure of lignin-silver nanosphere. The durable dispersibility and optical properties of lignin-silver nanospheres endowed the photo-crosslinkable resin of methacrylated O-acetyl-galactoglucomannan with improved printing fidelity in three-dimensional printing. In general, this thesis provides green solutions for upgrading lignin with desired properties for efficient chemical integration in functional materials.