Strain Stiffening Governs the Thermal Memory Window in Swelling-Preset Networks

Constrained liquid–liquid phase separation in polymer networks is fundamental; however, how nonlinear elasticity modulates the critical threshold remains elusive, particularly in noncovalent lower critical solution temperature hydrogels. To decouple thermodynamics from nonlinear elasticity without altering molecular compatibility, we developed physically crosslinked poly(vinyl butyral) hydrogels. By fixing the degree of acetalization and varying the molecular weight, we utilized chain length to topologically tune the crosslinking density and extensibility. Analysis indicates that thermal memory primarily stems from metastable supersaturation stabilized by elastic constraints. Although the low swelling temperature sets the thermodynamic driving force, the molecular weight determines the strain-stiffening intensity. Crucially, strain stiffening amplifies the elastic penalty of phase separation, establishing a quantitative correlation between critical supersaturation and network elasticity. Consequently, the thermoplastic window is directly driven by the strain-stiffening-modified modulus. Finally, we demonstrated that this hydrogel can serve as an electricity-free time–temperature indicator for cold chains, thereby establishing a mechanothermodynamic framework for the design of topologically smart hydrogels.