Unconventional Stiffening Behavior of Amphiphilic Triblock Copolymers in Aqueous Environments for Thermally Activated Artificial Muscles

Biological systems achieve multifunctionality through dynamic stiffness modulation, yet replicating this adaptability in synthetic actuators remains challenging. Paradigm-shifting thermally triggered shape-memory polymers (SMPs) are reported that defies conventional thermal softening behavior. By designing amphiphilic triblock copolymers with various block chain lengths, water-induced microphase separation is exploited to achieve anomalous enhancement in stiffness-specially, a remarkable 170-fold modulus increase in aqueous environments at 25 °C, along with a sustained 1.8-fold amplification even at 80 °C. This counterintuitive thermal stiffening behavior of SMPs facilitates the design of soft actuators capable of simultaneous shape recovery and stiffening during actuation, resulting in soft actuators with high actuation multifunctionality. The system demonstrates exceptional actuation stress (650 kPa), rapid response (<4 s), and work capacity (35.2 kJ m^- ³). This thermally triggered SMPs with tunable stiffness may expand the application range of soft actuators in extreme environments, such as object manipulation in hot springs and deep-sea hydrothermal vents.