Elucidating the impact of lignin molecular weight and composition on the network structure and transport properties of lignin-based hydrogel composites*

Given its biodegradability, antimicrobial properties, chemical activity, and economic appeal, lignin has proven to be a favorable choice for use in the fabrication of sustainable soft composites. However, the heterogenous nature of lignin obstructs our ability to accurately understand  how the presence of lignin alters the network structure and ultimately the mechanical and transport properties of these soft composites. To address this gap in our knowledge, a collection of physically crosslinked lignin–poly(vinyl alcohol) (PVA; MW ~133 kDa) soft composites were synthesized using the freeze-thaw method. Specifically, soft composites containing 60 mass % of either BioChoice lignin (raw/feed lignin; MW ~ 20 kDa) or fractionated lignin of various MWs (ranging from ~6 kDa to ~160 kDa) were fabricated. The synthesis of these hydrogels with varying lignin MWs provides a basis of comparison, allowing the advantages of the fractionated lignin to be highlighted. To elucidate how the mesh size of the lignin-based hydrogels can be modulated by these changes, the transport properties have been thoroughly characterized. Firstly, the influence on the mesh size was gauged through performing drug loading studies utilizing bovine serum albumin as the model drug. Secondly, the permeability of methylene blue within the resulting network structure was examined. Lastly, the diffusion coefficient of water has been extracted for these systems by applying poroelastic relaxation indentation. The results from this investigation indicated that the network structure and transport properties were influenced with varying molecular weight and dispersity of lignin.