Carboxyalkylated lignins as functional bio-macromolecules for tailoring biodegradable PVA films

Poly(vinyl alcohol) (PVA) is an attractive biodegradable polymer with great potential for sustainable packaging, although its high-water sensitivity and moderate mechanical and thermal performance limit its application. In this work, kraft lignin was molecularly engineered through two carboxyalkylation pathways, carboxyethylation (CEL) and carboxybutylation (CBL), to elucidate how grafted chain length and anionic charge density regulate PVA-lignin interactions and the resulting structure-property relationships of composite films. By varying the acid: lignin ratio (1:1 and 2:1) under identical synthesis conditions, we obtained lignins with controlled substitution degree, charge density, and molecular weight. Carboxyethylation proved markedly more effective, yielding the highest charge density (-4.68 meq g^-1) and degree of substitution (0.66), which translated into superior compatibility and dispersion within PVA. Films containing CEL2 (4%) exhibited exceptional multifunctional performance, including a pronounced increase in tensile strength (up to 194 MPa), a substantial rise in water contact angle (from 30° to 66°), significantly enhanced UV shielding, improved thermal stability and flame retardancy, and higher biodegradability, while maintaining low thermal conductivity (0.044-0.051 W/m·K). These improvements arose from strengthened intermolecular hydrogen bonding and a more cohesive polymer-lignin network, facilitated by the higher anionic charge and shorter grafted chains of CEL. Overall, this study establishes charge density and side-chain structure as key molecular determinants of PVA-lignin compatibility, and identifies carboxyethylation as a simple, scalable, and green strategy for producing high-performance biodegradable films suited for eco-friendly packaging, UV-protective materials, and transparent functional PVA coatings.