Ionic hydrogels with transitional layers enable isochoric and wide-span stiffness modulation

Responsive ionic hydrogels with switchable stiffness exhibit broad potential for practical applications. However, the presence of hydration layers presents significant challenges in optimizing the performance of these materials. In this manuscript, we report a strategy for converting the hydration layer of ionic hydrogels into a transitional layer. This enables isochoric and reversible switching between soft ionic hydrogels and rigid polymers by adjusting phase separation, accompanied by a gigantic stiffness change from approximately 0.36 MPa to 694.90 MPa. By analyzing these mechanical responses during continuous phase separation, we identified a three-stage evolution pattern: transitioning from a soft state to a tough state, and ultimately to a robust state. Meanwhile, research has demonstrated that the presence of a transition layer not only ensures uniform stress distribution and defect-free structures in ionic hydrogels at low stiffness levels but also significantly enhances the dimensional stability, mechanical integrity, and fatigue resistance of the polymer network during stiffness-switching. This ultimately facilitates the successful fabrication of polymer materials with a broad range of tunable stiffness and enhanced toughness. This work presents a novel strategy for integrating transitional layers into ionic hydrogels, underscoring their potential for applications requiring isochoric and a broad range of adjustable stiffnesses.