In this study, gelatin hydrogels were soaked in ammonium ferric citrate (AFC) solutions, and the effect of AFC content on the structure and properties of the as-prepared composite hydrogels was investigated in detail. The spectral data confirmed the formation of Fe-O and Fe-N bonds between Fe³⁺ and the amide groups of gelatin, and the hydrogen bonding and hydrophobic interactions between gelatin chains were strengthened. Structural analysis suggested that the crystallinity of gelatin hydrogels was increased due to an introduction of ammonium ions and citrate ions. The tensile strength of gelatin hydrogels underwent a significant enhancement with an increase of AFC content, due to the existence of multiple crosslinks including triple helices and Fe³⁺ in hydrogels. Meanwhile, the fracture toughness was also increased by soaking treatment. Differential scanning calorimetry (DSC) results suggested that the composite hydrogels showed good freezing resistance, attributed to the strongly hydrated ammonium ions and citrate ions. These observations demonstrate that the structure and properties of gelatin composite hydrogels could be regulated by varying the AFC content.

Keywords: gelatin, composite hydrogels, freezing resistance, hydrogen bonds