Interactions, microstructures, and functional characterization of starch-based films reinforced with oxalic acid-modified honeysuckle flower powder.

Starch films often exhibit poor filler-matrix compatibility when reinforced with natural plant powders, which limits their functional performance. In this study, honeysuckle flower powder was modified via stepwise oxalic acid treatment to enhance its interfacial adhesion with corn starch. The treatment progressively removed pectin and polyphenols, thereby enriching lignocellulosic fibers (cellulose, hemicellulose, lignin) and enhancing hydrogen bonding with starch. Among the modified fillers, the one treated with two-stage acid exhibited the best balance between polyphenol retention and fiber exposure, yielding composite films with a modified microstructure, the highest tensile strength, and a tightly bound water proportion of 97.35%. Molecular docking and electrostatic potential analyses further elucidated the atomic-level binding mechanisms between cellulose, starch, and polyphenols, which supported the experimentally observed interfacial reinforcement. This work provides a rationale for designing high-performance starch-based films through controlled acid modification of natural fillers.