(4e) Characterizing the Mechanical and Transport Properties of Crosslinked Poly(vinyl alcohol)–Lignin Soft Composites for Membrane-Based Separations

Lignin-based hydrogels have recently garnered attention for use in a variety of aqueous separations as lignin is a sustainable, naturally abundant biopolymer with a high concentration of hydroxyl groups, which are potential sites for chemical functionalization and direct crosslinking. However, to date, the use of these materials in separations technologies has been hindered by our limited understanding of how the addition of lignin, both as a crosslinker and passive filler, affects the network (pore) structure of the crosslinked composite hydrogel which is of great importance as most lignin-based hydrogel investigations utilize highly disperse, undefined lignins. Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composites were synthesized utilizing ultraclean lignins (UCLs) of prescribed molecular weights (MWs) and low dispersity using two different crosslinking agents (CLAs) – methylenebisacrylamide (MBA) and glutaraldehyde (GA). The UCLs were acquired through a fractionation process of initially high disperse lignins.

Prior to crosslinking with MBA, lignin hydroxyl groups were functionalized with vinyl groups, allowing the lignins to chemically crosslink with themselves, leaving the PVA chains to thermally crosslink with themselves, creating an interpenetrating network (IPN) as shown in Fig. 1a. Successful functionalization of the lignins was confirmed via ³¹P and ¹H nuclear magnetic resonance spectroscopy as seen in Fig. 1b. The permeability of methylene blue (MB) through the hydrated composites was measured via ultraviolet-visible spectroscopy (UV-vis), where MB permeability was found to depend on the concentration of lignin in each composite, along with the dispersity of the lignins prior to fabrication. As seen in Fig. 1c, hydrogel composites containing UCLs of narrow dispersity demonstrated a smaller spread in the permeation results as compared to membranes containing highly disperse lignins, underscoring the importance of utilizing well-defined, fractionated UCLs of prescribed MWs in these hydrogel composites in acquiring repeatable measurements.

In another synthesis route, crosslinking with GA occurs at hydroxyl groups along PVA and lignin chains via condensation reaction, as shown in Fig. 2a, under both acidic and neutral conditions. The network structure of the soft composites was characterized via small-angle neutron scattering (SANS) and swelling measurements (to acquire monomers 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 mechanical properties of the hydrogel 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. As seen in Fig. 2b, modulus values increased with increasing lignin and crosslinker (glutaraldehyde) content, as well as changes in lignin MW. The permeability of various pollutants (e.g., methylene blue, bovine serum albumin) through the hydrated composites was measured via UV-vis, where penetrant permeability was dependent on the MW of both the lignins and PVA, concentration of lignin, and concentration of crosslinking agents utilized during membrane fabrication.

Results from this work underscore the importance of utilizing well-defined, low disperse lignins in the fabrication of composite hydrogels, as they allow for a direct, systematic approach to elucidating the structure-processing-property relationships in this emerging class of green materials.

Research Interests

My research interests involve polymeric materials, particular hydrogels, for membrane-based separations. Recently, there is a push for utilizing ‘greener’ alternatives to petroleum-based products directly used in fabricating hydrogels. The focus of my research project is incorporating lignin, a ‘greener’ alternative, into the hydrogel network structure to lessen the usage of fossil fuels. As such, I have gained expertise in polymer synthesis, mechanical characterization techniques, transport analysis, and neutron scattering analysis of soft materials. I strive to expand my hydrogel knowledge into more bio-based applications, particularly drug delivery or tissue engineering. I have primarily studied chemically crosslinked hydrogels and aspire to expand that knowledge into physically crosslinked materials that are suitable for swelling and releasing of drugs or proteins. Additionally, I am interested in strengthening my understanding of biopolymers suitable in hydrogel applications outside of membrane-separations.