Hydrogen Bonding Competition Mediated Phase Separation with Abnormal Moisture‐Induced Stiffness Boosting

Abstract Moisture usually deteriorates polymers’ mechanical performance owing to its plasticizing effect, causing side effects in their practical load‐bearing applications. Herein, a simple binary ionogel consisting of an amphiphilic polymer network and a hydrophobic ionic liquid (IL) is developed with remarkable stiffening effect after moisture absorption, demonstrating a complete contrast to water‐induced softening effect of most polymer materials. Such a moisture‐induced stiffening behavior is induced by phase separation after hydration of this binary ionogel. Specifically, it is revealed that hydrogen (H)‐bonding structures play a dominant role in the humidity‐responsive behavior of the ionogel, where water will preferentially interact with polymer chains through H‐bonding and break the polymer‐IL H‐bonds, thus leading to phase separation structures with modulus boosting. This work may provide a facile and effective molecular engineering route to construct mechanically adaptive polymers with water‐induced dramatic stiffening for diverse applications.