Preparation and properties of grafted polyacrylic acid modified poly(3-hydroxybutyrate-co-3-hydroxyvalerate) antioxidant film

To reduce the need to add metal chelators to lipid-based foods to prevent their oxidation, antioxidant packaging is receiving increasing attention. In this study, acrylic acid (AA) was used as the metal chelating ligand and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was used as the substrate. AA was covalently immobi-lised onto the surface of PHBV films by UV grafting to produce the non-releasing PHBV-g-PAA anti-oxidant film with metal chelating ability. The results show that the successful grafting of polyacrylic acid (PAA) onto the surface of PHBV films was demonstrated by structural tests on the composite films. Observation of the composite film morphology and structure by SEM revealed that the density of graft products gradually increased as the grafting time increased. When the grafting time was 20 minutes, the dense uniformity of the PAA grafted layer on the film surface was the best. The crystalline properties of the composite film were tested by DSC and XRD. It showed that the crystallinity decreased from 63.97% to 56.23%, which was conducive to improving the toughness of the film, and the best toughness of the film was achieved at 20 minutes of grafting. Toluidine blue (TBO) staining and flame atomic absorption spectrometry were used to determine the carboxyl density and Cu²⁺ chelation on the surface of the composite membrane. When the carboxyl density was 392.65 nmol/cm², the corresponding Cu²⁺ chelating amount was 115.09 nmol/cm². The ratio of the two was close to 4, indicating that a stable five-membered ring chelating structure could be generated, thus acting as an antioxidant. Through the mechanical properties test, it was found that the tensile strength and elongation at break of the grafted films both increased first and then decreased. Tensile strength and elongation at break increased by 27.51% and 99.02% respectively at 20 minutes of grafting. The Cu²⁺ chelating ability and mechanical properties of the prepared non-release antioxidant film are better than those of the PHBV film, which has promising applications in the food packaging field.