Rubber‐Glass‐Crystal Triple Phase Transition in Polyacrylamide Hydrogels and Environment‐Adaptive Shape Memory Effect

ABSTRACT Phase transition dramatically affects the stretchability, conductivity, and other key properties of hydrogels, thereby limiting their application in various fields. Precise control of the phase transition is of profound significance to advance their applications. By combining in situ and ex situ experiments and molecular simulations, this study reveals that the polymer networks in polyacrylamide hydrogels markedly suppress water crystallization, while the water in turn hinders the glass transition of polymer networks. We demonstrate that, driven by changes in hydrogen‐bonding interaction mode and free volume, a hydrogel can switch among the rubbery, glassy, and crystalline states under the coupled temperature and moisture stimuli. Leveraging the Rubber‐Glass‐Crystal transitions, we propose an environment‐adaptive shape memory strategy for hydrogels. Notably, this shape memory behavior is programmed by temperature and moisture, rather than by manual mechanical programming as in conventional shape memory polymers, and the shape recovery occurs extremely rapidly, in contrast to existing water‐triggered systems that depend on slow solvent diffusion. In addition, a constitutive theory is developed to capture the glass‐rubber‐crystal transitions and to guide the design of soft machines. This work deepens the fundamental understanding of hydrogel phase transitions and paves the way for their application in intelligent actuators and sensors.