Fabrication and Characterization of Lignin-Based Soft Composites

Lignin, a biopolymer and byproduct of paper making, has recently garnered attention as a green alternative to petroleum-based precursors in a wide array of applications. Due to lignin’s attractive properties, lignin-containing hydrogels have potential to replace materials traditionally leveraged in applications such as tissue engineering, membrane-based water purification, and drug delivery, to name a few. However, the lignin traditionally utilized is oftentimes unpurified, leading to lignins with high dispersity in molecular weight (MW), making it difficult to accurately characterize material properties. In this study, fractionated lignins of prescribed MW are used in the synthesis of soft composites. First, membranes with copolymers of poly(vinyl alcohol) (PVA) and lignin, of two prescribed MW and concentrations as high as 40 mass%, were fabricated using glutaraldehyde (GA) as a crosslinker. Both the mechanical and transport properties of these membranes were characterized. The Young’s modulus, water uptake, and number of monomers between crosslinks of the membranes indicated the formation of a tighter network with increasing lignin MW, as well as increasing lignin and GA concentration. Next, we attempted to fabricate composites containing a more biofriendly polymer, poly(2-hydroxyethyl methacrylate) (pHEMA). Unlike the lignin–PVA system, we were unable to synthesize robust, free-standing membranes, though we found that the properties of membranes containing pHEMA as the copolymer were more sensitive to the pH of the environment during membrane fabrication.