Elucidating the Network Structure, Mechanical and Transport Properties of Poly(vinyl alcohol)–Lignin Soft Composites Containing Fractionated, Purified Lignins*

Lignin-containing hydrogels have 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 fabrication. However, to date, little is understood regarding how the addition of lignins alters the network structure of composite hydrogels, which is of great importance as most lignin-based hydrogel investigations utilize highly disperse, heterogeneous lignins. Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composite hydrogels were synthesized utilizing ultraclean lignins of prescribed molecular weights (MWs) and low dispersity acquired via fractionation of highly disperse lignins. Mechanical properties of the hydrated composites were characterized via tensile strength, Young's modulus, and dynamic mechanical analysis. The network structure of the composites was characterized via small-angle neutron scattering and swelling measurements (molecular weight between crosslinks and water uptake). The permeabilities of various pollutants (e.g., hexavalent chromium, methylene blue, bovine serum albumin) through the hydrated composites were measured via ultraviolet-visible spectroscopy, where penetrant permeability was dependent on lignin MW.