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

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. The binary ionogel composed of an amphiphilic polymer network and a hydrophobic ionic liquid demonstrates an abnormal moisture-induced stiffness boosting, which is in complete contrast to water-induced softening of common polymer materials. Comprehensive understanding of this moisture-induced stiffening of the ionogel is achieved from multiscale perspectives, including molecular interactions, microphase separations, and macro mechanical performances. image