Stretchable Self-Healing Polymeric Dielectrics Cross-Linked Through Metal–Ligand Coordination

TLDR

A self‑healing dielectric elastomer is achieved by incorporating metal‑ligand coordination cross‑linking sites into nonpolar PDMS polymers. The polymer uses 2,2′‑bipyridine‑5,5′‑dicarboxylic amide ligands cross‑linked with Fe²⁺ or Zn²⁺ salts, where Zn‑bipyridine coordination provides rapid ambient self‑healing. Transistors using FeCl₂‑ or ZnCl₂‑cross‑linked PDMS dielectrics exhibited higher dielectric constants, hysteresis‑free transfer, low leakage after 1000 strain cycles, and mechanical robustness, whereas dielectrics with large anions showed hysteresis, underscoring the role of small anions for stable, stretchable electronics.

Abstract

A self-healing dielectric elastomer is achieved by the incorporation of metal-ligand coordination as cross-linking sites in nonpolar polydimethylsiloxane (PDMS) polymers. The ligand is 2,2'-bipyridine-5,5'-dicarboxylic amide, while the metal salts investigated here are Fe(2+) and Zn(2+) with various counteranions. The kinetically labile coordination between Zn(2+) and bipyridine endows the polymer fast self-healing ability at ambient condition. When integrated into organic field-effect transistors (OFETs) as gate dielectrics, transistors with FeCl2 and ZnCl2 salts cross-linked PDMS exhibited increased dielectric constants compared to PDMS and demonstrated hysteresis-free transfer characteristics, owing to the low ion conductivity in PDMS and the strong columbic interaction between metal cations and the small Cl(-) anions which can prevent mobile anions drifting under gate bias. Fully stretchable transistors with FeCl2-PDMS dielectrics were fabricated and exhibited ideal transfer characteristics. The gate leakage current remained low even after 1000 cycles at 100% strain. The mechanical robustness and stable electrical performance proved its suitability for applications in stretchable electronics. On the other hand, transistors with gate dielectrics containing large-sized anions (BF4(-), ClO4(-), CF3SO3(-)) displayed prominent hysteresis due to mobile anions drifting under gate bias voltage. This work provides insights on future design of self-healing stretchable dielectric materials based on metal-ligand cross-linked polymers.