Amidst declining fossil-based resources and environmental challenges, the focus on biobased materials has intensified. Carboxymethylation is one way to introduce reactive functionality to enhance the reactivity of lignin for a specified application. This research investigates the carboxymethylation of four lignin sources: eucalyptus kraft lignin, spruce kraft lignin, birch cyclic extracted organosolv lignin, and spruce cyclic extracted organosolv lignin. Our aim is to elucidate the role of the lignin structure in its reactivity. Using the advanced analytical techniques NMR spectroscopy, Fourier transform infrared spectroscopy, density functional theory, and size-exclusion chromatography, we provide a comprehensive characterization of the modified lignin. The findings offer valuable insights into how the chemical and physical properties of molecular lignin affect the selectivity and efficiency of the carboxymethylation reaction. These fundamental findings hold great potential for guiding considerations on the selection of lignin sources for specific applications based on their molecular properties.

The increased interest in the utilization of lignin in biobased applications is evident from the rise in lignin valorization studies. The present study explores the responsiveness of lignin toward oxidative valorization using acetic acid and hydrogen peroxide. The pristine lignins and their oxidized equivalents were analyzed comprehensively using NMR and SEC. The study revealed ring opening of phenolic rings yielding muconic acid- and ester-end groups and side-chain oxidations of the benzylic hydroxyls. Syringyl units were more responsive to these reactions than guaiacyl units. The high selectivity of the reaction yielded oligomeric oxidation products with a narrower dispersity than pristine lignins. Mild alkaline hydrolysis of methyl esters enhanced the carboxylic acid content of oxidized lignin, presenting the potential to adjust the carboxylic acid content of lignin. While oxidation reactions in lignin valorization are well documented, this study showed the feasibility of employing optimized oxidation conditions to engineer tailored lignin-based material precursors.

Chemical modification of lignin is widely used to improve its reactivity and solubility in bio-based thermosetting systems. However, the relationship between apparent solubility and true network compatibility remains poorly understood. Here, a hierarchical, multi-scale approach was used to study how lignin structure evolves from molecular modification to nanoscale organization and, ultimately, thermoset performance in fully bio-based epoxy networks derived from epoxidized linseed oil (ELO). Carboxymethylated and oxidative carboxylated lignins were examined as case studies to elucidate how different modification pathways link molecular structure, nanoscale organization, and thermoset performance. Both modification routes increased carboxylic acid content and enabled ester bond formation with ELO, as confirmed by FTIR. Despite these similarities, wide-angle X-ray scattering (WAXS) revealed distinct differences in nanoscale organization. Carboxymethylation caused a moderate change in T-shaped π–π stacking interactions, while preserving a substantial fraction of sandwich π–π stacking, resulting in partial solubility. In contrast, oxidative carboxylation produced a pronounced expansion of T-shaped distances, reflecting a more open and weakly interconnected aromatic network, leading to complete solubility but weaker network cohesion after curing. DSC and FTIR confirmed curing across all systems, although variations in thermal transitions reflected differences in resin-lignin interactions. The resulting materials formed brittle thermosets with heterogeneous network structures. NMR analysis further indicated the occurrence of side reactions, including epoxide ring opening by lignin hydroxyl groups. Leaching tests further indicated that increased carboxylic acid content enhanced solvent accessibility, particularly in oxidized lignin systems. Incorporation of triacetin as a bio-based plasticizer yielded free-standing thermosets suitable for thermomechanical characterization, confirming network formation and structure–property relationships consistent with the multi-scale structural analysis. Overall, this work demonstrates that increasing lignin functionality and solubility alone is insufficient to achieve high-performance bio-based epoxy thermosets, highlighting the critical role of nanoscale packing and short-range order in determining macroscopic properties.