Ester crosslinking of sodium alginate using bio-based 1,3-propanediol: Structure-property relationships and...

Sodium alginate (NSA) is a GRAS-certified natural polysaccharide that has been widely investigated for its food packaging applications. However, its commercial translation is limited by moisture sensitivity, water solubility, and susceptibility to microbial contamination. In the present study we developed a 1,3-propanediol (PDO)-crosslinked sodium alginate film (NSAP) with an improved physicochemical profile compared to the native polymer. The crosslinking reaction time was optimized by time-resolved FTIR spectroscopy where ester bond formation was monitored over 120 h. Successful incorporation of PDO in NSA matrix was verified through XRD, DSC, TGA, and ¹H NMR analyses. Morphological investigations using SEM and AFM revealed notable alterations in surface topology due to PDO-induced crosslinking. NSA-P exhibited a significant reduction in swelling power (80.9%) and water solubility (91.89%), along with an increase in contact angle (81%), indicating enhanced hydrophobicity. Furthermore, an increase in the wetting constant, k(s^-1), from 0.00143 (NSA) to 0.0097 (NSAP) suggested a transition in the wetting mechanism from bulk diffusion to surface-controlled spreading. The water vapour transmission rate (WVTR) of NSA-P was found to be 156.91 g/m².24h at 90% humidity, which was comparable to the conventional biopolymeric films. In addition, NSA-P demonstrated a substantial improvement in tensile strength, increasing from 12.14 ± 1.80 MPa (NSA) to 23.11 ± 3.12 MPa, indicating enhanced mechanical integrity due to effective crosslinking. Finally, NSA-P showed reduced susceptibility to microbial degradation against Gram-positive and Gram-negative bacteria, as well as fungal strains, highlighting its potential utility as moisture-resistant biopolymeric film for food packaging and biomedical applications.