(601e) Elucidating the Impact of Lignin Molecular Weight and Composition on the Network Structure and Transport Properties of Lignin-Based Hydrogels for Sustainable Technologies

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 materials. However, the heterogenous nature of lignin poses a barrier to our understanding of how the introduction of lignin alters the network structure and ultimately the mechanical and transport properties of these soft composites. To address this issue, we have fabricated a series of lignin–poly(vinyl alcohol) (PVA) composite hydrogels using both crude bulk lignin (CBL; raw, unfractionated lignin) and ultraclean lignin (UCL; fractionated lignin with prescribed molecular weights). These soft composites were synthesized via the “Freeze-Thaw” method, whereby physical crosslinks between PVA chains are created. Specifically, the lignin concentration and molecular weight were systematically varied, ranging in lignin concentrations of 20 wt % to 60 wt % and lignin molecular weights of approximately 1250 g/mol to approximately 160,000 g/mol. To investigate the transport properties of these membranes, both the water uptake and permeabilities of a model organic pollutant, methylene blue (MB), were examined. Furthermore, to comprehensively probe the mechanical properties of the hydrogels, the Young’s modulus, as well as the loss and storage moduli were characterized via mechanical indentation and dynamic mechanical analysis, respectively. Moreover, lignin leaching from these soft composites was analyzed, where, after 14 days in 40 °C water, minimal lignin leaching was observed for all hydrogels fabricated. Finally, the antimicrobial properties of the lignin-based soft composites were explored by culturing various microbes (E.coli, C. auris, and S. aureus strains) on agar, sandwiching the streaked surface with the lignin hydrogel, and counting the number of bacterial colonies after 12 hours. Preliminary data suggest that the growth of bacterial colonies is hindered as the lignin concentration was increased. Results indicate that the lignin concentration, lignin molecular weight, and number of Freeze-Thaw cycles directly influence the water uptake, as well as the transport and mechanical properties of the soft composites. Using scanning electron microscopy, it is shown that each lignin fraction produces a unique network structure, with the low molecular weight lignin fraction resulting in the most well defined and reproducible network structure. Notably, this molecular weight also demonstrated the lowest MB permeability and highest hydrated Young’s Modulus. Ultimately, the results of this investigation indicated that the network structure of these lignin hydrogel systems can be readily modulated through varying lignin concentration, molecular weight, and number of “Freeze-Thaw” cycles, underscoring the importance of the properties of the starting lignin. These lignin hydrogels have potential to be used in a variety of technologies given their low cost, tunability, and nontoxic nature.