Additive‐Free Ionic Polyurethanes with Ultrasensitive Thermo‐Switchable Conductivity and Melting Stability

Thermally switchable ion‐conductive polymers (TSIPs) show considerable application potential for flexible electronics and information encryption. However, their practical implementation is frequently affected by several inherent limitations, including ion leaching, interfacial instability, and inadequate temperature sensitivity in conductivity switching. To address these issues, a series of ionic polyurethanes featuring covalently grafted ionic groups, rather than small‐molecule additives, is reported here. This rational design not only provides ion transport pathways, but also enables precise temperature‐switchable conductivity by varying the length of crystalline soft segments. The resulting TSIPs exhibit an ultra‐sensitive thermo‐electrical response with a record‐high temperature coefficient of conductivity of up to 30.2%°C − 1 . Modeling of electrode polarization reveals that ion transport at elevated temperatures operates through a dual mechanism: a “swing‐like” segmental motion coupled with dynamic reconstruction of ionic clusters. The dynamic reorganization of ionic clusters promotes efficient charge mobility and provides physical crosslinking networks that ensure mechanical integrity in the melt state. Furthermore, they function as intrinsic fluorophores, producing stable photoluminescence. With the integration of intrinsic fluorescent properties and reversible opaque‐to‐transparent and insulating‐to‐conducting transitions, the polyurethanes developed in this work establish a versatile platform for multimodal anti‐counterfeiting applications.