The dielectric response of simple organic glass formers

被引:397
作者
Kudlik, A [1 ]
Benkhof, S [1 ]
Blochowicz, T [1 ]
Tschirwitz, C [1 ]
Rössler, E [1 ]
机构
[1] Univ Bayreuth, Inst Phys, D-95440 Bayreuth, Germany
关键词
dielectric spectroscopy; glass transition; organic liquids;
D O I
10.1016/S0022-2860(98)00871-0
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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.
引用
收藏
页码:201 / 218
页数:18
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