Synergistic Effects of Dynamic and Stable Networks on the Mechanical Behavior of Poly(vinyl alcohol) Hydrogels

We investigate the effects of dynamic and stable network structures on the mechanical behavior of poly(vinyl alcohol) hydrogels prepared through diverse methods, including freeze-thawing (F), salting out (S), dry annealing (A), and their combinations (F-S, F-A, F-S-A). Utilizing in situ small-angle X-ray/neutron scattering (SAXS/SANS) and rheometry, we elucidate the structural evolution and mechanical response mechanisms of these hydrogels. Salting out induces a dynamic network (generally refers to the network with highly reversible cross-link points such as hydrogen bonds), enhancing energy dissipation and self-healing, while annealing forms a stable network (generally refers to the network with low-reversibility cross-link points such as crystals), improving strength and stiffness. The synergistic effect of salting out and annealing achieves a balanced network structure, optimizing crystal formation, uniformity, and mechanical performance. This tunable network design offers a universal strategy for adaptive hydrogels in biomedical devices. Additionally, the process of fracture and recrystallization of crystals within the structure is closely related to the yielding, hardening, fracture, and other behaviors of the sample, revealing a direct correlation between microstructural evolution and macroscopic mechanical properties.