Salt-Tuned Mechanical Properties of Hydrogels: An O:H-O Bond Perspective

Hydrogels are versatile functional materials with applications in biomedical devices, sensors, and energy systems. However, tailoring their mechanical properties remains a critical challenge for expanding their practical utility. Here, the mechanical properties of poly(vinyl alcohol) (PVA) hydrogels prepared via the freeze-thaw method were effectively tailored through the incorporation of various chloride salts. This study reveals the molecular mechanisms by which salt ions influence the water structure, polymer chain interactions, and freezing behavior to modulate hydrogel performance. Using Raman spectroscopy, X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), we elucidate how cation-specific effects alter the crystallinity, cross-linking density, and hydrogen bond network in the hydrogels. Hydrogels containing K⁺ exhibited superior rigidity and toughness due to enhanced crystallinity and optimized water-polymer interactions, while hydrogels of Mg²⁺ and Ca²⁺ had much lower modulus and adhesion strength since the divalent ions reduced freezing points resulting in reduced polymer aggregation. These findings provide a molecular-level understanding of the salting effects on hydrogel properties, offering new insights into designing materials with tunable mechanical performance for diverse applications.