Harnessing Chain Mobility via Protonation for Tough and Isotropic Hydrogel

This study reports a scalable processing strategy to create ultra-tough, mechanically isotropic poly(vinyl alcohol) (PVA) hydrogels by sequentially modulating polymer chain mobility and physical crosslinking through (i) acidification, (ii) freeze–thaw gelation, and (iii) salting-out. Acidification protonates PVA hydroxyl groups, suppressing premature interchain hydrogen bonding and aggregation to homogenize the precursor network. A freeze–thaw step introduces minimal physical crosslinks to retain macroscopic gel integrity, followed by salting-out (e.g., sodium citrate) that deprotonates hydroxyls and drives controlled crystallite formation and dense hydrogen-bonded domains acting as strong, reversible physical crosslinks. By tuning acid concentration/species and PVA concentration, the authors achieved isotropic hydrogels with exceptional mechanical performance, including tensile strength up to 29.5 MPa, stretchability ≈2683–2684%, and record-high toughness of 424 MJ m−3 among isotropic hydrogels. Mechanistic validation combined porosity/microstructure (SEM, mercury intrusion porosimetry, optical transparency/UV–vis), crystallinity and crystal spacing (DSC, SAXS), crystalline-region mapping (FLIM), and reactive MD/DFT-supported analysis of protonation effects on hydrogen bonding and chain conformation. The hydrogel also demonstrated strong energy dissipation and fatigue resistance (fatigue threshold ~4.46 kJ m−2), and the approach was shown to be generalizable (e.g., gelatin) and compatible with additive manufacturing via DLP printing of a methacrylated PVA derivative (PVAMA) followed by the same toughening sequence.