(150r) The Effect of Crosslinker Concentration on Competitive Release Kinetics and Physical Characteristics of Thermo-Responsive, Lignin-Based Soft Composites

The use of lignin in soft composites has gained recent attention due to its various favorable properties, such as biocompatibility, antimicrobial and antibacterial properties, and sustainability of sourcing. In this study, we fabricated soft composites containing: (1) a thermo-responsive polymer, poly(N-isopropylacrylamide) (PNIPAm); (2) a hydrophilic polymer, poly(vinyl alcohol) (PVA); and (3) a sustainable biopolymer, lignin. Specifically, soft composites were fabricated at a lignin concentration of 40 mass %. To form the interpenetrating network of the soft composite, the concentrations of two crosslinkers, glutaraldehyde and N,N’-methylenebisacrylamide, were varied between 5 mass % and 15 mass % (relative to their respective polymers). To investigate the role of lignin purity and molecular weight (MW), two lignin fractions were produced from the Aqueous Lignin Purification with Hot Agents, or ALPHA, process, which simultaneously fractionates and cleans the raw, Kraft lignin. Specifically, the ALPHA process was used to obtain lignin with approximate MWs of 5,700 g/mol and 160,000 g/mol. After fabrication, both the mechanical properties and the drug-release kinetics of each soft composites were characterized. Specifically, the hydrated Young’s modulus of each hydrated membrane was measured using mechanical indentation at both room temperature (approximately 21 °C) and at 40 °C, which is above the volume phase transition temperature of the PNIPAm. Furthermore, the permeability of methylene blue (MB), a model organic pollutant, across the membrane was analyzed using ultraviolet-visible spectroscopy. In addition, the equilibrium water uptake for each membrane was measured at both room temperature and 40 °C. Finally, the diffusion of caffeine and diphenhydramine, both concurrently and individually, through the network was analyzed via ultraviolet-visible spectroscopy at both room temperature and 40 °C. It was observed that the addition of lignin to the soft composites altered all four properties measured – hydrated Young’s modulus, MB permeability, equilibrium water uptake, and the drug release profiles. In particular, the diffusion of caffeine and diphenhydramine into water was suppressed with the addition of lignin, where complete release of each drug was seen to be significantly slower in membranes containing lignin. Furthermore, the crosslinker concentration and molecular weight of the lignin were also seen to impact the release kinetics of these soft composites.