Breakdown of time-temperature superposition: From experiment to the coupling model and beyond

We studied chain and segmental relaxation in polyisoprene using broadband dielectric and light scattering spectroscopy. Our results demonstrate different temperature dependence of these processes (i.e. failure of the time-temperature superposition) in the entire temperature range studied. Segmental relaxation varies with temperature faster than chain relaxation at low temperatures while reverse is observed at high T. The transition between two regimes seems to occur around the dynamic crossover temperature. It appears that non-entangled and slightly entangled polymers exhibit the same behavior. The results are compared to predictions of the coupling model. We emphasize that current models cannot describe the data consistently.