Evolution of Glass Transitions in Homogeneous Organic Mixed Ionic-Electronic Conductors upon Ion Uptake and Electrochemical Doping

Fueling advances in bioelectronics and neuromorphic computing, the performance of organic mixed ionic-electronic conductors (OMIECs) is dictated not only by crystalline order but also fundamentally by the segmental dynamics within their amorphous domains. Here, by fabricating an electrochemical cell in situ on the chip calorimetry, we unveil the evolution of the glass transition temperature (Tg) of homogeneous OMIEC films across dry, swollen and electrochemically doped states. Dry films display distinct glass transitions for the side-chain (Tg,s) and backbone (Tg,b) domains, governed by backbone rigidity and side-chain linkages, with flexible oligoethylene glycol (oEG) side chains acting to lower Tg,b. Electrolyte swelling elevates Tg,s through ion confinement in side-chain lamellae while plasticizing the backbone, lowering its Tg,b but preserving the intrinsic rigidity crucial for electronic pathways. Critically, the response to electrochemical doping is dependent on the initial polymer crystallinity. For low-crystallinity OMIECs, anion injection enhances amorphous phase mobility, reflected in a decreasing Tg,b. Conversely, in highly crystalline polymers, doping-induced ion intercalation partially disrupts the ordered lamellae, creating a new amorphous phase with restricted mobility characterized by a higher-temperature glass transition (Tg,b2). This newly formed phase can further grow through high carrier density-induced backbone planarity, which may foster the mesoscale order for mixed conduction. This work reveals the intricate coupling between molecular architecture, doping-induced morphology, and segmental dynamics, providing fundamental insights for the structural evolution between ordered and amorphous segments in OMIEC systems.