Slow secondary relaxation in simple glass formers

We investigate the secondary relaxation (beta-process) in simple glass formers by dielectric spectroscopy covering the frequency range 10(-2) Hz less than or equal to v less than or equal to 10(9) Hz. Data on the dielectrically rigid molecules toluene and 3-fluor aniline are presented. Furthermore, 1-propanol and polybutadiene are studied. Performing a lineshape analysis, we find the following features: (i) The beta-process can be described by assuming a broad distribution of thermally activated Debye processes with a Gaussian distribution of activation enthalpies, E-beta, and entropies, Delta S-beta(not equal). (ii) The mean activation enthalpy [E-beta] is related to the glass transition temperature T-g through [E-beta] = 24RT(g), and (iii) [Delta S-beta(not equal)] approximate to 10R is found. The latter finding explains the small but system independent prefactor of the Arrhenius function fitted to the data (tau(o) approximate to 10(-16) s). (iv) The relaxation strength is constant for T < T-g, but varies among the systems and increases at temperatures >T-g. We conclude that the beta-relaxation is intrinsic to the glassy state (Johari). (C) 1998 Elsevier Science B.V. All rights reserved.