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Herein, a series of novel, lignin-based hydrogel composites was fabricated by incorporating ultraclean lignins (UCLs), of controlled molecular weight and low dispersity, into poly(vinyl alcohol) (PVA). The UCLs were obtained from a novel liquid–liquid fractionation of high dispersity crude bulk lignins (CBLs) obtained from Kraft black liquor. A complementary series of composite hydrogels was fabricated using these CBLs. Both the CBLs and UCLs were functionalized with vinyl-containing acrylate groups allowing the lignins to chemically crosslink with themselves, forming an interpenetrated network with the thermally-crosslinked network of PVA chains. Successful functionalization of the UCLs was demonstrated by proton and phosphorous nuclear magnetic resonance. PVA–lignin hydrogels containing 20 wt% UCL saw a reduction in methylene blue (MB) permeability by approximately two orders of magnitude when compared to neat PVA. Further, for composite hydrogels containing either 50 wt% UCL or CBL, no MB was detected in the receiving reservoir over the duration of the permeation experiment. In general, an increase in Young's moduli was observed in PVA–lignin hydrogels containing CBLs, where hydrogels composed of 50 wt% CBLs exhibited ∼40% increase when compared to neat PVA. In contrast, a ∼10% reduction in Young's moduli was observed for composite hydrogels containing 20 wt% UCLs ormore »less, though these membranes exhibited the lowest MB permeabilities of all membranes investigated. However, the largest increase in membrane stiffness was observed for composite hydrogels containing 50 wt% UCLs, where a ∼70% increase in Young's modulus was observed. Finally, the concentration and functionalization of the lignins was seen to have a direct impact on the network structure of the soft composites, where in general, the molecular weight between crosslinks is seen to decrease with increasing lignin concentration.« less