Tough polyacrylic acid hydrogels with stable swelling and active functionalities enabled by quaternized cellulose nanofibrils and iron ions for absorbent pad interlayers

Hydrogels are highly sought-after absorbent materials for absorbent pads; however, it is still challenging to achieve a satisfactory balance between mechanical performance, water absorption capacity, and active functionalities. In this work, we presented double-network hydrogels synthesized through acrylic acid (AA) polymerization in the presence of quaternized cellulose nanofibrils (QCNF) and Fe³⁺. Spectroscopic and microscopic analyses revealed that the combined QCNF and Fe³⁺ facilitated the formation of double-network hydrogels with combined chemical and physical crosslinking. The synergistic effect of QCNF and Fe³⁺ resulted in impressive mechanical properties, including tensile strength of 1.98 MPa, fracture elongation of 838.8 %, toughness of 7.47 MJ m⁻³, and elastic modulus of 0.35 MPa. In comparison to the single-network PAA hydrogel, the PAA/QCNF/Fe³⁺ (PQFe) hydrogels showed higher and relatively stable swelling ratios under varying pH levels and saline conditions. The PQFe hydrogels exhibited notable antioxidant activity, as evidenced by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, and demonstrated effective antibacterial activity against both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). These hydrogels show promising potential as an absorbent interlayer in absorbent pads for active food packaging.