Synergistic Strengthening and Toughening of Hydrogels: Ultralow Loading Nanosilica Coupled with Annealing to Modulate Heterogeneous Nucleation

While nanofillers are widely employed to reinforce hydrogels through heterogeneous nucleation, polymer crystallization theoretically requires only trace amounts of nucleating agents, whereas excessive nanofiller loading may disrupt polymer-chain ordering and compromise mechanical performance. However, at conventional high loadings (1–15 wt %), these adverse effects can be masked by the dominant contribution of volume-filling reinforcement. Thus, this study explores ultralow loadings (0–0.2 wt %) of nanosilica to minimize conventional composite effects and elucidate the role of nanoparticle-mediated heterogeneous nucleation in annealing-driven crystallization. Nanosilica particles act as heterogeneous nucleation sites, facilitating the formation of numerous fine and relatively uniformly distributed crystalline domains during the gelation of poly(vinyl alcohol) (PVA) hydrogels. Upon subsequent annealing, these crystalline domains undergo further growth and spatial reorganization, accompanied by an increase in overall crystallinity. Crucially, excessive nanosilica loadings may hinder polymer-chain ordering and compromise crystallinity under both freeze–thaw and annealing conditions. With a mere 0.05 wt % nanosilica, the fracture energy of the hydrogel annealed for 30 min exceeds twice that of the pristine hydrogel subjected to the same thermal treatment. Across the ultralow-loading regime (0–0.2 wt %), the annealed hydrogels achieve tensile strengths of 1.41–7.67 MPa and fracture energies of 1.08–6.32 kJ m–2, collectively outperforming previously reported isotropic PVA hydrogels reinforced through crystalline architecture manipulation. This study provides fundamental insights into nanofiller-mediated crystallization in hydrogels and establishes a nucleation-annealing pathway for precise regulation of crystalline-domain architecture and mechanical properties at ultralow nanofiller loadings.