Kinetically Arrested Interfacial Polyelectrolyte Complexes Enable Ionic Rectification in Ionoelastomer Heterojunctions

Abstract At interfaces between oppositely charged polyelectrolytes, ionic double layers (IDLs) emerge that are often treated as ionic counterparts of semiconductor depletion layers. Yet this analogy overlooks the molecular deformability of polymer-tethered charges. Here, we show that IDLs in soft ionoelastomer(IE) heterojunctions undergo interfacial complexation, resulting in the formation of a kinetically arrested polyelectrolyte complex (PEC) layer. Selective counterion-exchange staining with ionic fluorophores reveals a micron-scale unstained interfacial PEC region─far exceeding the depletion length predicted from thermodynamic considerations of IDLs. This exclusion arises from a kinetically frozen glassy PEC that suppresses counterion exchange with ionic fluorophores. Forward bias, elevated temperature, or salt addition reversibly drives a glass-to-rubber transition in the interfacial PEC, restoring fluorophore uptake, as confirmed by optical, mechanical, and impedance measurements. This transition produces asymmetric ionic conductivity, demonstrating that ionic current rectification in IE heterojunctions is governed by interfacial PEC formation and its kinetically arrested glassy state.