Leveraging the Co(non)solvency Effect of Dimethyl Sulfoxide and Water to Fine-tune Hydrogel Network Structures for Enhanced Transport and Mechanical Properties*

Given its biocompatibility, biodegradability, and economic appeal, lignin is a favorable choice for use in the fabrication of “green” soft bio-composites. In this study, the impact of cononsolvent concentration (in this case, dimethyl sulfoxide (DMSO) and water) on the network structure, and thus, material properties of poly(vinyl alcohol) (PVA) and lignin soft composites was investigated. Specifically, the mass ratio of the two solvents, DMSO/H₂O, was varied from 100/0 (w/w) to 60/40 (w/w). Using the “freeze-thaw” method, physically crosslinked membranes were synthesized, where the lignin concentration was varied between 0 mass % and 60 mass %. The impact of the cononsolvent concentrations was elucidated via transport, microscopy, and mechanical characterization. Transport results showed that the permeability and diffusivity of a model organic dye, methylene blue, generally decreased as the concentration of DMSO was increased. Conversely, the equilibrium water uptake was seen to decrease  as DMSO concentration increased. Mechanical characterization indicated that the hydrogels became stiffer with increasing DMSO concentrations. Lastly, direct imaging from scanning electron microscopy (SEM) showed a hierarchical network structure over a small window of cononsolvent concentrations, which was further enhanced with the addition of lignin. Results from this study underscore how mixed solvents can be used to further increase crosslinking density without need of additional additives.