(122i) Investigating the Mechanical and Transport Properties of Thermally- and Chemically-Crosslinked Poly(vinyl alcohol)–Lignin Soft Composites

The synthesis of lignin-containing hydrogels has gained attention for use in a variety of aqueous-based separations as lignin is an abundant biopolymer with a high concentration of hydroxyl groups which can be utilized as crosslinking sites during hydrogel fabrication. However, to date, little is understood regarding how the addition of lignins alters the network structure of these composite hydrogels, which is of great importance as the vast majority of lignin-based hydrogel investigations utilize highly disperse, heterogeneous lignins, referred to as crude bulk lignins (CBLs). Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composite hydrogels were synthesized using both thermal and chemical crosslinking, as well as fabricated under both neutral and acidic conditions. Composite membranes were synthesized utilizing both CBLs and ultraclean lignins (UCLs) of well-defined molecular weights (MWs) and low dispersity, where the lignin concentration of the hydrogels was varied from 25 mass% to 60 mass%. The UCLs (M_w 940 g mol^-1 and 3800 g mol^-1) were acquired through a fractionation process of the CBLs.

The mechanical properties of the hydrated composites were characterized via tensile strength testing, mechanical indentation (to acquire Young’s modulus), and dynamic mechanical analysis (to acquire a storage and loss modulus). Significant changes in the Young’s moduli of the membranes were observed between hydrogels fabricated in neutral and acidic conditions, where a higher crosslinking density was observed for membranes fabricated in acidic conditions. Additionally, the modulus values were seen to increase with increasing lignin and crosslinker (glutaraldehyde) content. The network structure of the soft composites was characterized via small-angle neutron scattering (SANS) and swelling measurements (to acquire molecular weight between crosslinks and water uptake). The data obtained from SANS measurements was modeled using a modified Lorentzian power law model to obtain a correlation length for the composite hydrogels. For membranes containing lignin, two correlation lengths were observed in the scattering data. The permeabilities of various pollutants (e.g., methylene blue, rhodamine B, bovine serum albumin) through the hydrated composites were measured via ultraviolet-visible spectroscopy, where penetrant permeability was found to depend on the MW of both the lignins and PVA, as well as the concentration of crosslinking agent utilized during membrane fabrication. Results from this work help establish structure-process-property relationships for this emerging class of green materials.