Superionic Bifunctional Polymer Electrolytes for Actuators

The modulus and ion conductivity of polymers depend on the relaxation of polymer segments and ion transport, respectively. Enhancing conductivity often sacrifices modulus due to their coupled dynamics. Block copolymers with nanostructured hard and soft domains are believed to simultaneously improve both properties. However, current systems still use polymers as the soft segment, where ion mobility remains much lower than in small-molecule liquids, limiting significant improvement in conductivity. Extending the smartness of phase separation from polymer/polymer to polymer/small-molecule systems, where rapid ion transport occurs in the small-molecule domains, may resolve this issue. Yet, attractive Coulombic forces between counterions typically result in homogeneous solutions or gels rather than structured ion channels in blends. To avoid this, we propose using hydrogen bonding to modulate electrostatic interactions, as they exhibit similar magnitude of binding energy. Here, we introduced -OH and -SO₃H at adjacent sites in each repeating unit. The resulting intra-monomer hydrogen bond effectively weakens Coulombic interactions between acidic polymers and ionic liquids, facilitating the formation of liquid ion channels. A polymer electrolyte with a conductivity of ~10⁻³ S/cm and a modulus of ~100 MPa was achieved and applied in soft actuators exhibiting millisecond response and megapascal output stress.