Mechanische Relaxation in komplexen Fluiden

Abstract

By measurments of the frequency dependent dielectric loss modulus on organic glass formers at different temperatures, the primary relaxation due to viscous flow has been determined. It shows up in a maximum in the loss spectrum that shifts to higher temperatures with increasing frequency. This primary peak in the relaxation spectrum is often mathematically described with the empirical Havriliak-Negami function. The characteristic relaxation time obeys a Vogel-Fulcher-Tammann law.If there are additional internal degrees of freedom in the constituants of the glass, the excitation of these eigenmodes produces more loss-peaks. Examples for these systems are polymer glasses. In contrast to this, there are glassy materials where secundary relaxation processes don´t show a clear peak or a shoulder. Instead there are deviations from the theoretically expected potential law on the high frequency side or the low temperature side of the primary loss peak. The microscopic origin for these deviations, that are often named as "excess wing" are not clear even until today. Nevertheless a deeper understanding of the glass transition is only possible, if the "wing" phenomenon can be clarified. Many theories about the glass transition postulate the existence of dynamic heterogeneities and the correlated movement of groups of atoms or molecules. The "wing" could indeed be the superposition of the relaxation of these different "clusters".In this work, dynamic properties of thin films of the the metallic glass Zr65Al7.5Cu27.5 and of the polymer poly(methylmethacrylate) are investigated. The method is mechanical torsional spectroscopy with the double-paddle oscillatro (DPO). The experimental setup to work with the DPO at high temperatures is as well described as the mathematical procedure to obtain the complex shear modulus of thin films on the DPO-substrate.The developed aparatus is a powerful and effective tool. It is shown, that the damping of the bare oscillator is dominated by the thermoelastic effect up to 400 °C. The quality factor is that high, that even small effects in thin films are resolvable.The shown data of Zr65Al7.5Cu27.5 prove with no doubt, that the "excess wing" do exist in metallic glasses and therfore could be a universal property of glass forming materials. Assuming dynamic heterogeneities and cooperative processes the "wing" can be regarded as a superposition of different relaxation processes.The difference in mechanical and dielektric experiments, the latter ones are performed in cooperation with the university of Augsburg, is probably due to the different coupling mechanism of the external driving force to the glassy system.In this work, the experimental setup is described and the experimental results on both classes of materials are shown. They are interpreted by using the different theories about the glass transition and about the origin of the "excess-wing". This "wing" and the phenomenon of the glass transition are central aspects of this publication.