Antibacterial LDPE-based nanocomposites with salicylic and rosmarinic acid-modified layered double hydroxides

Polymer nanocomposites, prepared by dispersing layered nanofillers bearing active and natural compounds in a polymer matrix, offer a tunable way to confer antibacterial activity to traditional polymers for packaging materials. Antibacterial host-guest systems were prepared by intercalation of mono-deprotonated rosmarinic and salicylic acid (RA and SA), with recognized antibacterial and antioxidant properties, into a nitrate-intercalated MgAl layered double hydroxide (LDH) via anion exchange reaction. The structural, morphological, and thermal properties of the modified LDHs (i.e., MgAl-RA and MgAl-SA), compared with those of a fully exchanged ZnAl-SA, indicated the successful immobilization of the functional molecules. Linear low-density polyethylene (LDPE)/antibacterial-LDH nanocomposites were prepared by a two-step melt compounding procedure. X-ray diffraction analysis and scanning electron microscopy showed that MgAl-RA was better dispersed than ZnAl-SA and MgAl-SA. LDPE nanocomposites containing the highest filler content showed increased thermo-oxidation stability, with a marked effect for LDPE/MgAl-RA due to the antioxidant power of RA. Furthermore, the antibacterial activity of LDPE/MgAl-RA was high and selective toward Staphylococcus aureus. Finally, the overall migration of RA and SA from films of polymer nanocomposites immersed in ethanol solution was followed by fluorescence spectroscopy evidencing a controlled release of the active compounds.