(655f) Characterizing Network Structure in Novel Hydrogel Composites Containing Fractionated, Purified Lignins for Aqueous Separations

The fabrication of lignin-containing hydrogels has garnered attention for use in a variety of aqueous-based separations as lignin is a sustainable, naturally abundant biopolymer with a high concentration of hydroxyl groups, which can be utilized as crosslinking sites during hydrogel fabrication. However, one standing issue is our lack of understanding as to how the addition of lignin alters the network structure of these composite hydrogels given that the majority of lignin-based hydrogel studies utilize highly disperse, heterogeneous lignins, referred to as crude bulk lignins (CBLs). Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composites were synthesized utilizing ultraclean lignins (UCLs) of prescribed molecular weights (MWs) and low dispersity using two different crosslinking agents (CLAs) – methylenebisacrylamide (MBA) and glutaraldehyde. Prior to hydrogel formation, lignin hydroxyl groups were functionalized with vinyl groups, which was confirmed via ¹H and ³¹P nuclear magnetic resonance spectroscopy, allowing the lignins to chemically crosslink with PVA chains. The permeability of various pollutants (e.g., methylene blue) through the hydrated composites was measured via ultraviolet-visible spectroscopy, where penetrant permeability was found to depend on the MW of both the lignins and PVA, the source of the lignin prior to fractionation, as well as the concentration of CLAs utilized during membrane fabrication. In addition, poroelastic relaxation indentation (PRI) was used to characterize both the mechanical and transport properties of the composites. Namely, water diffusivity, hydrated mesh size, and Young’s modulus of the composites were obtained from PRI measurements. Results from this work indicate that transport of pollutants through the composite hydrogels is governed by a combination of the network structure and the strength of interaction between the pollutant and lignin.