Enhanced Mechanical Properties of Composite Hydrogels Containing Fractionated and Purified Lignin

Herein, a series of poly(vinyl alcohol) (PVA)− lignin hydrogel composites was synthesized by crosslinking both unfractionated and fractioned lignin of varying molecular weights directly with PVA via a condensation reaction involving glutaraldehyde (GA) as the crosslinker. The fractionated lignin was obtained from a liquid−liquid extraction process of bulk lignin obtained from Kraft black liquor. The mechanical properties of the hydrated PVA−lignin composites were characterized using ultimate tensile strength (UTS) testing, dynamic mechanical analysis (DMA), and mechanical indentation. Most notably, soft composites containing 40 wt % fractionated lignin and 3 wt % GA exhibited 2 orders of magnitude increase in UTS when compared to neat PVA. The storage moduli obtained from DMA were used to calculate the molecular weight between crosslinks (M_C) of the various soft composites, where a direct correlation between enhanced mechanical properties and lower M_C for soft composites containing fractionated lignin was observed. Finally, the hydrated network structure of the hydrogels was directly imaged using scanning electron microscopy. These images showed a more disrupted network structure for soft composites containing unfractionated lignin, potentially explaining the unpredictability in mechanical properties observed for these composite hydrogels.