Comparative study of sodium alginate films loaded with limonene and poly(limonene): interactions, properties, and functional activities

Abstract

D-Limonene (LIM) is a terpene widely used as an additive in biopolymer-based active films, but its low thermal stability limits its applications. This study proposes using its thermally stable oligomer derivative, poly(limonene) (PLM), as an active component in sodium alginate (SA) films. PLM was synthesized via photopolymerization of LIM under mild conditions, yielding non-volatile oligomers with enhanced thermal stability. Scanning electron microscopy revealed a homogeneous surface distribution of the active oligomer in the films. Confirmatory analyses using Fourier-transform infrared spectroscopy and X-ray diffraction indicated reduced crystallinity after PLM incorporation. The addition of the active compounds did not significantly affect moisture content or water vapor permeability. While pure LIM exhibited inhibitory antimicrobial effects, none of the film solutions showed antimicrobial activity, likely due to low additive concentrations. Nonetheless, films with PLM demonstrated improved thermal stability and mechanical properties, with tensile strength increasing from 36 ± 3 MPa to 46 ± 5 MPa and elongation at break from 3.7 ± 0.8% to 14 ± 5% compared to the control films. Additionally, they presented a noticeable reduction in UV transmittance, indicating an improved UV light barrier. Films containing PLM also showed higher antioxidant activity than both the control and LIM films, with DPPH and ABTS scavenging effects of 7.8 ± 0.7% and 77 ± 16%, respectively. Overall, incorporating PLM is a promising strategy for utilizing citrus industry by-products, adding value to this natural compound while producing functional, biodegradable packaging materials with enhanced barrier and antioxidant features. Importantly, this work highlights the unexplored potential of PLM in SA films and provides new insights into how molecular structure affects film functionality. These advances contribute to the design of active packaging systems with improved stability and multifunctional performance.