“Freeze” Polyelectrolyte Gels with Salt: The Salt-Temperature Reciprocity Programmed Viscoelasticity

Polyelectrolyte (PE) gels, characterized by their charged nature, exhibit tunable mechanical strength and remarkable temperature- and salt-responsive dynamics, yet the mechanisms underlying governing network relaxation under broad ranges of salt concentration ( C salt ) range remain unresolved. Here, we systematically investigate the linear viscoelastic (LVE) behavior of poly(acrylic acid) (PAAc) hydrogels doped with sodium halide salts across varying temperatures and salt concentrations. By applying time–temperature superposition (TTS) and time-salt superposition (TSS), we successfully extend the dynamic spectra of PAAc gels over a broad frequency range. The resulting master curves reveal that decreasing T or increasing C salt induces a transition from rubbery to glass-like dynamics, with horizontal shift factors for TTS and TSS ( a T and a S ) obeying the reciprocal relationship where a S ∼ a T –1 . This correlation establishes a quantitative salt-temperature reciprocity, wherein the salt addition mimics a “dynamic freezing” effect analogous to cooling, effectively decelerating segment relaxation without altering intrinsic network structures. The value of ln( a T ) and T follows an Arrhenius relationship with the apparent activation energy increasing from 67.3 to 224.6 kJ/mol as C salt rises from 2 to 8 M, reflecting an enhanced energy barrier and gradual network immobilization. The value of ln( a S ) also follows a linear relationship with C salt, showing a decreased dependence of ln( a S ) on C salt with increasing temperature. Complementary low-field nuclear magnetic resonance (LF-NMR) and Fourier transform infrared (FTIR) analyses reveal a unified mechanism that the decrease in T and increase in C salt similarly drive the gel toward a less hydrated and more dynamically constrained local environment. Building on these insights, we establish a unified time-temperature-salt superposition (TTSS) framework for the PAAc gels to predict their dynamics under combined thermal and ionic stimuli and demonstrate its utility in programming viscoelasticity for shape memory and shock resistance applications, highlighting the potential of PE hydrogels as adaptive materials through simple salt addition.