Enhancement of mechanical and optical properties of modified chitosan films via grafting of multiple hydrogen bonding motifs

This study explores enhancing chitosan film (Cs) properties via supramolecular H-bonding cross-linking. The first unit, (1-(7-oxo-7,8-dihydro-1,8-naphtiridin-2-yl)urea (ODIN), was selected for its ability to self-dimerize via a sextuple H-bonding motif, while the second unit, phenyl urea (PU), was chosen as non-dimerizing unit that is still capable of forming extensive H-bonds. FTIR analysis confirmed the successful incorporation of ODIN and PU into the chitosan skeleton. The mechanical and barrier properties, and UV protection performances of the two modified polymers were determined and compared with those of pristine chitosan. These modifications resulted in enhanced surface hydrophobicity, and a substantial decrease in water vapor permeability. Mechanical testing revealed substantial reinforcement, with tensile strength increasing by up to 200 % for ODIN-modified films and up to 100 % for PU-modified ones. The UV-blocking performance of Cs-ODIN films was notably enhanced, providing complete protection across UVA, UVB, and UVC wavelengths, achieving a UPF of 161.95. The UV protective effect was further validated using the red cabbage test, which proved that Cs-ODIN films markedly reduced color degradation under UV exposure. Overall, this study shows that while PU mainly enhances barrier properties and mechanical performance through additional hydrogen-bonding interactions, ODIN not only provides similar improvements but also offers further benefits, including (i) enhanced UV-shielding and (ii) superior toughness and thermal stability driven by supramolecular dimerization.