Physical, mechanical and antioxidant properties of chitosan films grafted with different hydroxybenzoic acids

Five kinds of hydroxybenzoic acids including gallic acid (GLA), gentisic acid (GTA), protocatechuic acid (PA), syringic acid (SA) and vanillic acid (VA) were grafted onto chitosan (CS) through carbodiimide mediated coupling reaction. The obtained CS derivatives were further developed into films by casting. The physical, mechanical and antioxidant properties of hydroxybenzoic acid-g-CS films were compared for the first time. The grafting ratios of hydroxybenzoic acid-g-CSs varied from 34.9 to 111.0 mg/g. Structural characterization showed both amide and ester linkages were formed between CS and hydroxybenzoic acids with decreased crystallinity. The thicknesses of CS and hydroxybenzoic acid-g-CS films were similar (0.028–0.031 mm). The grafting of GLA and PA onto CS significantly decreased the whiteness but increased redness and yellowness of film. Hydroxybenzoic acid-g-CS films exhibited better UV light and moisture barrier properties as compared with CS film. Most hydroxybenzoic acid-g-CS films showed higher tensile strength and elongation at break than CS film. The surface morphology of CS film became rough after grafting with hydroxybenzoic acid. Antioxidant activity assays showed hydroxybenzoic acid-g-CS films had higher DPPH scavenging activity than CS film. As compared to CS and other hydroxybenzoic acid-g-CS films, GLA-g-CS film possessed the highest UV light and moisture barrier, mechanical and antioxidant properties.