Biobased Composite Films from Chitosan and Lignin: Antioxidant Activity Related to Structure and Moisture

Composite films based on chitosan and lignin biopolymers were investigated for their mechanical, barrier, surface, and antioxidant properties and linked to an extensive microscopic analysis of their external and internal structure. In particular, the fluorescence properties of lignins were exploited, using two-photon microscopy, to achieve a 3D representation of its distribution within the chitosan matrix. The lignin incorporation generated small aggregates homogeneously distributed in the film. The aggregates slightly weakened the network as reflected by the mechanical properties. Lignin as an antioxidant provided to the film a radical scavenging activity, essentially governed by a surface activity mechanism. Accordingly, the film surface showed a chemical reorganization induced by the presence of lignin as highlighted by surface hydrophobicity and X-ray photoelectron spectroscopy. On the molecular scale, solid state NMR also revealed the absence of covalent bonds between lignin and chitosan and the establishment, but to a small extent, of low energy dipole–dipole interactions. Finally, lignin is a promising compound for a good added-value due to radical scavenging in a chitosan matrix.