Solvent-Driven Nanostructural Tuning of Lignin/Poly(N,N-dimethylacrylamide) Hydrogels

Non-covalent molecular self-assembly serves as a distinctive strategy for enhancing the mechanical performance of lignin-based composite hydrogels. Nevertheless, the self-assembly process can be significantly influenced, leading to alterations in the nanostructure of the hydrogel, because of the diverse conformational reorganizations of lignin in different solvents. In this research, a solvent exchange process was employed to generate a phase-separated structure comprising hydrophobic lignin domains and hydrophilic poly(N,N-dimethylacrylamide) (PDMA) domains through the aggregation of lignin, thereby forming tough lignin/PDMA hydrogels. By adjusting the solvent composition, the hydrogels exhibit distinct nanostructural transformations that are precisely correlated with the changes in Hansen Solubility Parameters (HSPs) of the solvent mixtures. Balanced HSPs facilitates the formation of small-scale lignin domains with high-domain density, which act as crosslinking points for the establishment of a reinforced network. Remarkably, lignin/PDMA hydrogels prepared at a boundary solvation condition unexpectedly induced the formation of large and highly condensed lignin domains, which displayed a radius of gyration (Rg) of 7.7 nm and an inter-domain distance (d-spacing) of 98.1 nm within the hydrogel network. These unique nanostructural features further contribute to its superior mechanical performance, including excellent tensile strength of 3.2 MPa, Young's modulus of 5.7 MPa, and fracture energy of 41.2 kJ m^-2, which outperforms most reported lignin hydrogels. Additionally, it offers a strong adhesion and rapid drying approach, rendering the hydrogel more suitable for applications as hydrogel coatings.