Vapor-Phase Green Cross-linking of Lignocellulosic Hydrogels for Robust and Water-Stable Biobased Films

Fabrication of mechanically robust and water-stable lignocellulosic materials at high solid contents remains challenging due to excessive viscosity, inhomogeneous cross-linking, and the inefficient use of chemical cross-linkers in conventional liquid-phase processes. Herein, we report a solvent-minimized, vapor-phase green cross-linking strategy that enables uniform physicochemical cross-linking of lignocellulosic hydrogels through diffusion-controlled exposure to epichlorohydrin and ammonia vapors. By avoiding direct liquid mixing, this vapor-phase process effectively overcomes the limitations imposed by highly viscous dispersions and allows physical consolidation and covalent network formation to proceed simultaneously throughout the hydrogel matrix. As a result, homogeneous double-cross-linked lignocellulosic hydrogels can be fabricated at solid contents up to 2.0 wt %, exhibiting an enhanced storage modulus of 16.5 × 10 5 Pa. The vapor-phase cross-linked films exhibit a tensile strength of 115.6 MPa, representing a 35% increase compared with their liquid-phase cross-linked counterparts, together with a 63% decrease in swelling ratio. In addition to the improved mechanical performance, the vapor-phase approach enhances cross-linking efficiency while reducing chemical consumption and processing complexity. Overall, this strategy provides an energy-efficient and scalable route for the sustainable production of lignocellulosic films. Our results highlight how rational vapor-phase process design can help overcome long-standing sustainability challenges in biomass-based materials and support the development of high-performance, environmentally responsible, biobased films.