Date of Award
5-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Biomolecular Engineering
Committee Chair/Advisor
Dr. Eric M. Davis
Committee Member
Dr. Mark C. Thies
Committee Member
Dr. Scott M. Husson
Committee Member
Dr. Nicole E. Martinez
Committee Member
Dr. Philip J. Brown
Abstract
In an attempt to reduce reliance on petroleum-based polymers, lignin – an abundant biopolymer – has gained popularity as a sustainable alternative. However, research on lignin-containing soft composites – i.e., hydrogel (bio)composites – has largely been heuristic, providing limited insight into the fundamental structure-processing-property relationships that govern performance improvements in this emerging class of ‘green’ soft composites. As such, this dissertation presents a systematic investigation of how lignin molecular weight (MW), composite composition, and the concentrations of crosslinker and accelerator impact the mechanical and transport characteristics of soft composites composed of lignin, poly(N-isopropylacrylamide) (PNIPAm), and poly(vinyl alcohol) (PVA).
In the first part of the dissertation, two series of membranes were fabricated, where the mass ratio of lignin, PNIPAm, and PVA was varied from 1:1:1 to 2:2:1 (lignin:PNIPAm:PVA). Each series of membranes was fabricated with both bulk, softwood Kraft lignin and with lignin fractionated and purified via the Aqueous Lignin Purification with Hot Agents (ALPHA) process, which generated low MW (LMW) and high MW (HMW) lignin fractions. To help isolate the impact of lignin on the network structure, a series of ‘control’ membranes was fabricated. For these control membranes, the lignin fraction was replaced with PVA. Further, within each series, the concentration of free radical accelerator, tetramethylethylenediamine (TMEDA), was varied between 5 and 10 mass %. Notably, the addition of lignin was seen to significantly decrease the permeability of methylene blue (MB), a model organic dye used to probe the impact of lignin on the transport properties, by as much as four-fold, while simultaneously increasing the elastic (Young’s) modulus by as much as 45% when compared to the PVA–PNIPAm control. For example, the MB permeability for 2:2:1 composites containing HMW lignin and a TMEDA concentration of 10 mass % decreased by more than an order of magnitude. Further, at higher lignin and PNIPAm concentrations, a clear trend of increasing Young’s modulus with increasing lignin MW was observed. Furthermore, the thermoresponsive nature of these soft composites materialized as an increase in elastic moduli, with values as large as 15 MPa observed at temperatures (~40 °C) above the volume phase transition temperature of PNIPAm.
In the second and third parts of this dissertation, the impact of lignin MW and crosslinker concentration on the release kinetics of caffeine and the soft composite network structure, respectively, was investigated. In regard to the effect of lignin in the membranes, the water uptake and diffusivity decrease for the lignin-containing membranes when compared to control gels. Water uptake also decreases across all control membranes as crosslinker concentration increases, at both room temperature and 40 °C. Notably, at temperatures above the volume phase transition temperature of PNIPAm, the diffusivity increases significantly for the control and lignin-containing membranes with 15 mass % crosslinker. The structure of the hydrogels, as measured by small-angle neutron scattering, was shown to differ for lignin-containing membranes in correlation to crosslinker content. Specifically, LMW and HMW lignin-containing gels showed an increase in the largest of two structural length scales only when crosslinker content reached 15 mass %, where 5 and 10 mass % remained similar. On the other hand, the membranes containing unfractionated lignin showed an increase in the largest length scale from 5 to 10 mass % crosslinker and then a reduction in that length scale from 10 to 15 mass % crosslinker content. This trend for bulk lignin was also reflected in measures of ultimate tensile strength, where the greatest tensile strength (~500 Pa) was demonstrated in the hydrogels with 10 mass % crosslinker.
In summary, my research is focused on bridging the gap between creating high-quality hydrogels with beneficial properties while still being conscious of the environmental impact of the materials used. My research aims to systematically investigate the interpenetrating networks that make up these hydrogels to establish structure-processing-property relationships.
Recommended Citation
Wolff, Missoury D., "Design and Characterization of Lignin-Based, Thermoresponsive Soft Composites With Improved Mechanical and Transport Properties" (2025). All Dissertations. 3952.
https://open.clemson.edu/all_dissertations/3952
Author ORCID Identifier
0000-0001-5123-2942