Linking the Evolution of Raman-Active Vibrational Modes to the Glass Transition of a Polymer Mixed Conductor

Thin films and bulk samples of an organic mixed ionic-electronic conductor (OMIEC), i.e., a thienothiophene (TT) based conjugated polymer with oligoether side chains, are investigated using variable-frequency dynamic mechanical thermal analysis (DMTA) and variable-temperature Raman spectroscopy. Macroscopically observed thermal transitions are correlated with molecular-scale dynamics. This multianalytical approach identifies a low-temperature β-relaxation originating from side-chain motion and a higher-temperature α-relaxation linked to backbone rearrangement. Specifically, temperature-dependent Raman spectra reveal shifts in characteristic CC/C-C, C-H, and C-S-C vibrational modes, which were quantified using a sigmoid model. Furthermore, a refined assignment of key vibrational modes ensures accurate spectroscopic interpretation. These results establish temperature-dependent Raman spectroscopy as a sensitive probe for glass and subglass transitions, providing a direct link between vibrational dynamics and thermo-mechanical properties of an exemplar mixed ionic-electronic conductor.