Changes in the dynamics of supercooled systems revealed by dielectric spectroscopy

The dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below t o above the glass transition temperature, T-g, has been investigated throug h 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)simil ar to 1.3xT(g); moreover, a crossover temperature T(B)similar to T-S is rec ognized, marking the separation between two different Vogel-Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribut ion of the overall energy between the relaxation processes and no peculiar behavior is revealed at T-S. A strong increase characterizes the strength o f 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 t he DSE model is well verified for mono- and diepoxide up to the high viscos ity regime, while a fractional DSE law with exponent similar to 0.81, accou nting for a decoupling between translational and rotational motions, replac es the DSE relation in triepoxide for temperatures below T-S. The change of the structural dynamics, the splitting between main and secondary relaxati on and the breakdown of the DSE behavior, all occur within a narrow tempera ture range around T-S; this finding argues in favor of the existence of a c hange of the dynamics in the supercooled liquid state well above the glass transition temperature. (C) 1999 American Institute of Physics. [S0021-9606 (99)50444-3].