Secondary relaxations in a series of organic phosphate glasses revealed by dielectric spectroscopy

Dielectric susceptibility spectra of six chemically similar organic phosphate glass formers are analyzed in order to elucidate the spectral evolution of a multitude of secondary (β) relaxation processes dominating the dielectric loss below the glass transition temperature T(g). By doing the spectral analysis we forgo any data fitting procedure and apply a model independent scaling approach instead. This approach assumes a constant distribution of activation energies g(E) underlying a plurality of thermally activated processes, which determine the β-relaxation. The scaling reveals temperature independent, asymmetric distributions g(E) for temperatures well below T(g). Simultaneously, the temperature dependence of the relaxation strength of the β-processes Δε(β)(T) is yielded, being nearly constant well in the glassy state. Already somewhat below T(g) the spectral scaling fails and reveals an apparent increase of the mean activation energy, leading to a weaker temperature dependence of the mean relaxation times τ(β)(T). In the same temperature regime Δε(β)(T) starts to increase drastically, i.e., the softening of the glass near T(g) is reflected directly in the loss of temperature independence of g(E) and Δε(β)(T). Comparing the different glasses made from phosphate molecules with increasing number of internal degrees of freedom we do not see systematic changes in the spectral evolution. In some cases even identical distributions g(E) are found.