Ionic Elastomers with Hierarchical Structures by In-Situ Phase Separation for Ionotronics

Abstract

Ionically conductive elastomers (ICEs), bridging the gap between mechanical and electrical properties, are promising for realizing human-machine interfaces, bioelectronics, or wearable sensors. Current design strategies for these materials are mainly based on electrolytes dissolved in small molecule solvents or solvent-free systems, which usually suffer from problems associated with the solvent (leakage, evaporation, toxicity) or poor mechanical properties. Here, we demonstrate a strategy to fabricate ionic elastomers by polymerizing one monomer in a mixture of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) with salts, using PEG/PPG as solvent avoids solvent leakage, evaporation, toxicity. Then, polymerized elastic networks present distinct solubility with PEG (highly soluble) and PPG (poorly soluble), resulting in a macroscopically homogeneous network with in situ phase separation. In detail, the polymer-rich phase (PPG) with hydrogen bonds dissipates energy and strengthens the gel; and the solvent-rich phase (PEG) enables large strain. In addition, incorporating ions into PEG/PPG solvent led to an additional enhancement of both the strength and the toughness. Thus, the hierarchical structure was built with the phase separation (micro-level) and ionic interactions (nano-level), showing high fracture strength (12 MPa), fracture energy (54 kJ m^-2), and elastic modulus (60 MPa) while being highly stretchable (over 500% strain). Further, the designed ICEs were used to fabricate electrodes for electrocardiograms and pneumatic artificial muscles, showing their versatility in ionotronic devices. The proposed strategy offers fundamental and extensible materials for various ionotronics, further inspiring the design of novel ionic elastomers.