Changes in the dynamics of supercooled systems revealed by dielectric spectroscopy

The dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below to above the glass transition temperature, T-g, has been investigated through the temperature behavior of relaxation times, strengths, and conductivity, determined in a wide frequency range (10(2)-2x10(10) Hz). In all systems the main and secondary relaxations define a splitting temperature T(S)similar to 1.3xT(g); moreover, a crossover temperature T(B)similar to T-S is recognized, marking the separation between two different Vogel-Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribution of the overall energy between the relaxation processes and no peculiar behavior is revealed at T-S. A strong increase characterizes the strength of the secondary relaxation on crossing the glass transition from the lower temperatures. Conductivity data have been analyzed to test the dynamics in terms of the Debye-Stokes-Einstein (DSE) diffusion law. The prediction of the DSE model is well verified for mono- and diepoxide up to the high viscosity regime, while a fractional DSE law with exponent similar to 0.81, accounting for a decoupling between translational and rotational motions, replaces the DSE relation in triepoxide for temperatures below T-S. The change of the structural dynamics, the splitting between main and secondary relaxation and the breakdown of the DSE behavior, all occur within a narrow temperature range around T-S; this finding argues in favor of the existence of a change of the dynamics in the supercooled liquid state well above the glass transition temperature. (C) 1999 American Institute of Physics.