Balancing Hydrogen Bonding and Crystallinity in Enzymatically Functionalized Lignin–Poly(vinyl alcohol) Composite Films

Poly(vinyl alcohol) (PVA) is a promising biodegradable polymer; however, its reliance on petroleum and incompatibility with raw lignin additives hinder sustainable PVA composite development. In this study, mechanically robust PVA/functionalized acid-soluble lignin (fLGAS) composite films were developed by means of aqueous solution casting without toxic chemical crosslinkers. Commercial kraft lignin was enzymatically modified using laccase from Coriolopsis spp., improving its compatibility with the PVA matrix. Molecular characterization confirmed the formation of an intermolecular hydrogen-bonding network between PVA and fLGAS, which significantly enhanced the thermal stability of the composites. Additionally, this network improved moisture response, achieving complete surface wetting (water contact angle decreasing from 47.04° to 0°) and swelling ratios exceeding 250% after 2 h. Nanoindentation and dynamic mechanical analysis characterizations revealed that mechanical properties enhanced with fLGAS loading up to 5 wt%, while the overall properties were governed by the balance between hydrogen bonding and crystallinity. This work provides an eco-friendly framework for industrial lignin valorization and sustainable functional film design.