Fabrication and Characterization of Lignin–PVA Hydrogels with Tunable Network Structures

Author

Keturah BethelFollow

Date of Award

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Chair/Advisor

Eric M. Davis

Committee Member

Scott M. Husson

Committee Member

Mark C. Thies

Committee Member

Ken Webb

Abstract

The ability to directly tune the crosslinked network structure of hydrogels is crucial for their functional applications in various fields, such as water filtration, protein separation, and tissue engineering. By controlling the crosslink density of the hydrogel, one can directly alter the mesh size – i.e., the end-to-end distance between crosslink junctions – and, subsequently, directly alter the hydrogel performance. This work discusses the fabrication and characterization of soft composites containing the biopolymer, lignin, are discussed. Precisely, physically-crosslinked composite lignin–Poly(vinyl alcohol) (PVA) hydrogels were fabricated via the freeze-thaw (F/T) pathway, whereby solutions containing specified amounts of PVA and lignin were subjected to alternating cycles of freezing and thawing until a dense, free-standing hydrogel was formed.

In the first part of the study, lignin–PVA soft composites containing 20 wt % and 60 wt % lignin were fabricated from starting dimethyl sulfoxide (DMSO) solutions containing either 5 wt % or 10 wt % PVA. Each solution was subjected to one (1x) or three (3x) F/T cycles to obtain the final hydrogel composite. In addition, bulk, unfractionated lignin (referred to as BioChoice™ lignin) and lignin fractionated using the Aqueous Lignin Purification with Hot Agents (ALPHA) process were used in the synthesis. Various experimental characterization characterizations of these soft composites were conducted to investigate the impact of the lignin properties on the final, salient hydrogel properties. The hydrated network structure of each hydrogel was captured using scanning electron microscopy (SEM), which provided qualitative identification of varying degrees of crosslinking. By directly imaging the hydrogels, apparent changes in pore sizes and heterogeneity were identified as a function of lignin concentration and molecular weight (MW). To further probe the influence of lignin on the network structure, transport properties such as equilibrium water uptake, permeability, and diffusivity were measured. Finally, to gauge the robust nature of the hydrogels, mechanical characterization was performed using mechanical indentation and dynamic mechanical analysis (DMA). Through these characterizations, Young’s modulus and the tensile strength were determined. The results demonstrated notable changes in the network structure as the equilibrium water uptake increased with the introduction of feed lignin but then generally decreased as lignin concentration was further increased and lignin MWs varied. The number of freeze-thaw cycles also influenced the impact of lignin. In many cases, the SEM images corroborated the changes in equilibrium water uptake data as the pore sizes and structure changed accordingly.

In the second part of the study, we investigated the impact of the co(non)solvency effect on the network structure of the hydrogels. In this case, the concentrations of two good solvents —DMSO and water — in the final F/T solutions were varied from 100/0 (w/w) to 60/40 (w/w) DMSO/water. This system displayed a general increase in equilibrium water uptake as DMSO concentration decreased for the neat hydrogels. This was accompanied by a general increase in pore size and a reduction in Young’s moduli. On the other hand, the lignin–PVA hydrogels showed a drastic reduction in methylene blue (MB) permeability that continued to decrease as the DMSO concentration decreased. However, the mechanical properties remained consistent. This consistency in stiffness marks the successful decoupling of transport and mechanical properties. This research finding is significant as it highlights that the transport properties can be tailored to absorb large amounts of solvents, and the mechanical integrity of the sample is not compromised.

In the final part of this dissertation, preliminary studies to characterize the antimicrobial properties of these soft composites have been undertaken. The lignin–PVA hydrogels demonstrated growth inhibition effects against E. coli bacteria, where no formation of biofilms was detected. Future work to assess the overall applicability as a biomaterial was also addressed.

Recommended Citation

Bethel, Keturah, "Fabrication and Characterization of Lignin–PVA Hydrogels with Tunable Network Structures" (2024). All Dissertations. 3664.
https://open.clemson.edu/all_dissertations/3664