Fabrication and Characterization of Lignin-Based, Thermo-Responsive Soft Composite Materials*

Lignin has gained significant attention due to its biocompatibility and antimicrobial properties, which make it an ideal candidate for biomedical applications. Thermo-responsive polymers, such as poly(N-isopropylacrylamide) (PNIPAm), in hydrogels can prove advantageous for bioseparations as PNIPAm exhibits a volume phase transition at a temperature below that of the human body. In this study, we fabricated two series of soft composites containing lignin, PNIPAm, and poly(vinyl alcohol) (PVA) with mass ratios of 1:1:1 and 2:2:1 (lignin:PNIPAm:PVA). Three types of lignin were used – raw lignin and two fractionated cuts of lignin with prescribed molecular weights (MWs) – and the concentration of free radical accelerator was varied between 5 and 10 wt%. The Young's modulus of each membrane was measured using mechanical indentation at room temperature and 40 °C. Young's moduli were higher for membranes at 40 °C, highlighting a volume phase transition. The permeation of methylene blue, a model pollutant, was measured by ultraviolet-visible spectroscopy and decreased with the addition of lignin, with a reduction of approximately 45:1 for membranes containing high MW lignin and 10 wt% accelerator in the 2:2:1 series. Higher water uptake was observed for the 2:2:1 membrane series.