A green approach to toughening poly(vinyl alcohol) soft composites using renewably processed and fractionated lignin

This study investigates the fabrication and mechanical characterization of poly(vinyl alcohol) (PVA)-lignin biocomposite hydrogels prepared using unfractionated lignin and its low- and high-molecular-weight (LMW and HMW) fractions. Hydrogels were fabricated from 5 wt % and 10 wt % PVA-DMSO precursor solutions containing 20 wt % or 60 wt % lignin (relative to total solute) and physically crosslinked through one or three freeze-thaw (F/T) cycles. The effects of lignin molecular weight, lignin concentration, PVA concentration, and F/T processing on the hydrated mechanical properties were evaluated through measurements of Young's modulus and ultimate tensile strength (UTS). Increasing the number of F/T cycles generally increased hydrogel stiffness, with the greatest effect observed for hydrogels prepared from 5 wt % PVA solutions. Hydrogels fabricated from 10 wt % PVA solutions exhibited approximately twofold higher Young's moduli and fourfold higher UTS values than those prepared from 5 wt % solutions. LMW lignin produced some of the highest hydrated Young's moduli measured in this study, reaching values of approximately 2 MPa and 4 MPa in 5 wt % and 10 wt % PVA hydrogels, respectively. However, these formulations also exhibited the lowest UTS values, revealing a distinct tradeoff between stiffness and tensile strength. In contrast, hydrogels containing unfractionated lignin achieved simultaneous improvements in Young's modulus and UTS, yielding some of the highest tensile strengths among all formulations. These findings demonstrate that lignin molecular weight distribution, lignin concentration, and processing conditions can be strategically tuned to tailor hydrogel mechanical performance, establishing fundamental structure-processing-property relationships for PVA-lignin biocomposites while advancing the development of sustainable lignin-based materials for biomedical and engineering applications.