pH-responsive lignin-incorporated PVA/chitosan hydrogels

Abstract

Hydrogels are three-dimensional polymeric networks capable of retaining large amounts of water while maintaining structural integrity. Among them, poly(vinyl alcohol)/chitosan (CS) systems have attracted attention due to their complementary mechanical and physicochemical properties; however, they often suffer from limited functionality, insufficient environmental responsiveness, and suboptimal performance under dynamic conditions. The incorporation of lignin, an abundant aromatic biopolymer, has emerged as a promising strategy to enhance hydrogel performance. However, challenges such as poor dispersion and aggregation of kraft lignin (KL) can restrict its effectiveness. In this thesis, the effects of chemical modification and nanoscale fabrication on hydrogel performance were systematically investigated by incorporating kraft lignin (KL), lignin nanoparticles (KL-NP), carboxyethylated lignin (CE), and carboxyethylated lignin nanoparticles (CE-NP) into CS hydrogels at 6% and 8% loadings. KL-NP systems demonstrated improved dispersion, porosity, and antibacterial activity compared to raw KL, confirming the advantages of nanoscale structuring. Carboxyethylation further enhanced hydrophilicity and functional group accessibility, while nanoparticle formation promoted more uniform distribution within the polymer network. Hydrogels containing CE-NP exhibited the most uniform and interconnected porous structure, with high surface area (148–150 m²/g) and pore volume (0.138–0.140 cm³/g). Notably, these hydrogels achieved a swelling ratio exceeding 2000%, along with enhanced mechanical properties, including tensile strength of 0.246 MPa and elongation at break of 138.1%. In addition, CE-NP systems showed improved pH responsiveness and antibacterial activity against S. aureus and E. coli, particularly. Importantly, all lignin-containing hydrogels demonstrated complete UV-blocking across the UVC, UVB, and UVA regions, highlighting the intrinsic photoprotective capability imparted by lignin. These hydrogels also demonstrated excellent UV durability, with negligible changes in surface wettability and stable or enhanced mechanical properties after UV exposure. Lignin effectively protected the polymer networks from UV-induced degradation, while carboxyl functionalization and nanoparticle reinforcement promoted UV-induced crosslinking, resulting in increased stiffness without compromising surface hydrophilicity. Overall, the results demonstrate that the synergistic combination of carboxyethylation and nanoparticle formation significantly enhances polymer–filler interactions, network homogeneity, and functional performance. This work provides a systematic understanding of how lignin modification strategies can be used to design high-performance hydrogels with tunable structural and physicochemical properties.