(32h) Fabrication and Characterization of Lignin-Based, Thermo-Responsive Soft Composite Materials

The use of lignin in the fabrication of soft composites has garnered significant attention as lignin possesses several attractive qualities such as biocompatibility, antimicrobial properties, as well as the fact that it can be obtained from renewable feedstock. The antimicrobial and antibacterial properties of lignin make it an ideal candidate for use in biomedical applications that require precise engineering, such as stimuli-responsive polymers. The introduction of thermo-responsive polymers – e.g., poly(N-isopropylacrylamide) (PNIPAm) – into composite hydrogels can prove advantageous for bioseparations and bind-and-release of biomacromolecules, as it exhibits a lower critical solution temperature near the temperature in the human body. However, hydrogels fabricated entirely from PNIPAm (i.e., pristine PNIPAm) have shown delayed swelling dynamics and weaker mechanical properties than soft composites containing PNIPAm as one of the copolymers. In this study, we fabricated two series of soft composites of interpenetrating networks containing lignin, PNIPAm, and poly(vinyl alcohol) (PVA). Specifically, the two series of membranes were fabricated at two different concentration ratios (lignin:PNIPAm:PVA) of 1:1:1 and 2:2:1. In each soft composite series, the concentration of the crosslinker, glutaraldehyde, was held constant, while the concentration of accelerator, tetramethylethylenediamine (TMEDA), was varied between 5 wt% and 10 wt%. Furthermore, three types of lignin were used in the fabrication of the soft composites – unfractionated Kraft lignins (referred to as crude bulk lignins, CBLs) and two molecular weight fractions of approximately 1250 g/mol and approximately 4200 g/mol, referred to as ultra-clean lignins (UCLs).

Following fabrication, the Young’s modulus of each membrane was measured using mechanical indentation at both room temperature (~21 °C) and 40 °C. For both series, the Young’s moduli were higher for membranes tested at 40 °C, where the highest observed increase was for the 1:1:1 membrane series. In addition, the Young’s moduli of soft composites fabricated with 10 wt% TMEDA were greater than those fabricated with 5 wt%, indicating a tighter network structure in these hydrated composites. In addition, the transport properties of the membranes were characterized by permeation and water uptake experiments. The permeation of methylene blue (MB), a model pollutant, was measured by ultraviolet-visible spectroscopy. The permeability of MB was seen to decrease for both series of membranes when compared to control membranes fabricated with no lignin. Furthermore, the MB permeability was lowest for soft composite containing the high molecular weight cut of lignin (4200 g/mol). Finally, higher water uptake was observed for the 2:2:1 membrane series. Results from this study highlight how lignin concentration and molecular weight can be leveraged to systematically tune the mechanical and transport properties of lignin-based, thermo-responsive soft composites.