The dielectric response of simple organic glass formers

After a short historical review concerning measurements of the dielectric response of simple glass formers, we discuss the complex dielectric permittivity <(epsilon)over cap> (nu) above and below the glass transition temperature, T-g, which we investigated in the frequency range 10(-3) Hz < nu < 10(9) Hz. (i) As a first group of glass formers (type A systems) glycerol and propylene carbonate, which do not show a beta-process, were studied, and a complete line-shape analysis of <(epsilon)over cap>(nu) is presented. The evolution of the high-frequency wing of the main relaxation (alpha-process) is the most prominent spectral change while cooling and leads to an essentially constant loss at T < T-g. This yields approximately a "1/f noise" in the spectral density, which extends at least over 7 decades in frequency (10(-2) Hz < nu < 10(5) Hz). (ii) Several glass formers exhibit a pronounced beta-relaxation in addition to the alpha-process (type B systems). Of these we investigated the dielectrically rigid systems toluene and fluoroaniline, as well as propanol and polybutadiene. The high-frequency wing observed in type A systems seems to reoccur also in type B systems. A quantitative line-shape analysis is carried out including both processes by applying the Williams-Watts (WW) approach. Compiling several properties of the beta-relaxation, that are generally observed, we give new arguments supporting Johari's idea, that the beta-process is an intrinsic property of supercooled liquids, although its relaxation strength may vary significantly among different glass formers. In some cases even no discernible beta-process is observed (type A systems). Within the WW approach, we interpret the beta-process as a local, spatially restricted reorientational process which precedes the alpha-relaxation. Above T-g, its relaxation strength strongly increases with temperature indicating that the spatial restriction imposed on the beta-process reorientation is released upon emergence of the alpha-process. (C) 1999 Elsevier Science B.V. All rights reserved.