Mechanism of crosslinking temperature impacting the structure of calcium ion crosslinked sodium alginate/konjac glucomannan films.

Crosslinking temperature has a significant effect on the progress of the crosslinking reaction. This study systematically investigated the effect of crosslinking temperature (15-55 °C) on the structure of calcium ion crosslinked sodium alginate (SA)/konjac glucomannan (KGM) films as well as the packaging-related properties. The results indicated that crosslinking temperature not only influenced the ionic crosslinking between Ca2+ and SA, but also affected the intermolecular interactions between SA and KGM, leading to significantly different film properties. Specifically, at a relatively low crosslinking temperature (i.e., 15 °C), some tiny cracks appeared due to the excessively low crosslinking rate. However, at relatively high crosslinking temperatures (i.e., ≥35 °C), the rapid diffusion and chelation rate of Ca2+, together with fast ethanol evaporation, triggered inhomogeneous crosslinking. This phenomenon brought obvious aggregated particles on the film surface and cracks inside the cross-section. Meanwhile, elevated temperatures partially disrupted the hydrogen bonds in the SA/KGM system. Finally, the crosslinking temperature of 25 °C was considered to contribute to the highest degree of uniform crosslinking and the strongest intermolecular interactions, which conferred the films with the smoothest and densest structure. Although its water contact angle (72.5°) was not the highest, the other properties were optimum, including tensile strength of 82.65 MPa, swelling index of 123.59%, soluble solid loss rate of 8.08%, and water vapor permeability of 1.42 × 10-12 g·cm/(cm2·s·Pa). The findings highlighted the importance of an appropriate crosslinking temperature in determining the structure and physical properties of the ion crosslinked polysaccharide films.