(688d) Effect of Hydroxyl Groups in Lignin on Lignin-Co­­­-Poly(N-isopropyl acrylamide) Mechanical and Thermal Responsive Properties

Over the past three decades, there has been a growing interest in valorizing lignin to become a high-value material. Researchers are actively developing cost-effective and innovative techniques to access the full potential of lignin, such as incorporating lignin into synthetic polymer systems. Poly(N-isopropyl acrylamide) (PNIPAm) is a thermally responsive polymer that has been widely studied due to its lower critical temperature (LCST) ranging from 32 - 35 °C in aqueous solution. PNIPAm can solvate in water at temperatures below the LCST and precipitate out of the water above the LCST. While PNIPAm demonstrates favorable thermal responsive behavior, PNIPAm hydrogels generally exhibit poor mechanical properties, limited loading capacity, and poor biodegradability and stability. One solution to improve the mechanical properties of PNIPAm (e.g., modulus, tensile strength, and elongation at break) is to copolymerize the hydrogel with lignin. In particular, hydrogen bonding interactions between the phenolic hydroxyl groups in lignin and the amide group in PNIPAm are expected to enhance the mechanical properties of the hydrogels through the formation of physical cross-linking. It is expected that increasing the content of phenolic hydroxyl groups in lignin will increase the extent of hydrogen bonding in the composite gel. Additionally, increasing the quantity of aliphatic hydroxyl groups is expected to increase the PNIPAm grafting yield due to the increased number of grafting sites on the lignin and lower steric hindrance. While the numerous hydroxyl groups in lignin play an essential role in copolymerization and supramolecular interactions, there are limited studies focusing on the effects of hydroxyl group concentration and type (e.g., aliphatic, phenolic) on copolymerization yields and properties.

In this project, lignin-co-poly(N-isopropyl acrylamide) (L-co-PNIPAm) copolymers are synthesized to produce a sustainable smart biomaterial with enhanced mechanical properties by investigating the impact of lignin hydroxyl group type and content on PNIPAm copolymerization. Prior to copolymerization, alkaline lignin was pretreated with different modification methods to adjust the hydroxyl group type and content, including acetylation (reduce hydroxyl content), oxypropylation (increase hydroxyl content), demethylation (selectively increase aliphatic hydroxyl group content), and phenolation (promote phenolic hydroxyl group content). The modified lignins were then copolymerized with PNIPAm through an atom transfer radical polymerization (ATRP) technique. The modified copolymers were then characterized by FTIR, NMR, TGA, DSC, SEM, UV-vis, and Instron mechanical testing. This characterization suite allows for determining the chemical composition, thermal characteristics, morphology, and mechanical properties of the modified lignins and lignin copolymers. This research holds great promise for various applications, from biomedical devices and drug delivery systems to environmentally friendly materials for sustainable agriculture and water purification. The ability to fine-tune the mechanical and stimuli-responsive properties of lignin-based copolymers could contribute to developing valuable lignin-based materials for industries seeking innovative and eco-friendly polymer material solutions.