Does the “excess wing” in the relaxation spectrum of glass formers have a fine structure?

The relaxation spectrum of glassy materials as determined by various spectroscopic techniques, including dielectric, light scattering, NMR as well as mechanical contains several universal features. In particular at higher frequencies than the α-relaxation peak one finds a broad wedge-like feature commonly referred to as the excess wing. Depending on the material, the excess wing may or may not be followed (when going to still higher frequencies) by another peak known as the β-relaxation peak. It is still a subject of debate whether the excess wing and the β-peak are manifestations of the same underlying mechanism. This is because the materials that only possess the excess wing under standard thermal history begin to exhibit a peak or at least a shoulder when aged for a sufficiently long time. We report on dynamic mechanical experiments on several epoxy systems quenched and aged. In the analysis we show that the relaxation spectrum needed to describe the sub-Tg data consists of multiple relatively narrow peaks (of which the β-peak is but one representative) rather than the continuous “wing”. The relaxation processes giving rise to these peaks have Arrhenian temperature dependence with the activation energies systematically ordered.