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Dielectric Properties of Molecular Glass Formers; from the Liquid State to the Tunneling Regime
(2008)
- The main purpose of this Thesis is to extend the dielectric investigations of molecular glass forming systems down to cryogenic temperatures (2 K), but also to complement previous work above the glass transition temperature Tg. The measurements were performed on systems composed of simple, mostly rigid molecules. Having at hand a large collection of data, previously compiled in Bayreuth group, this work starts with analyzing the characteristic relaxation features in molecular systems above Tg. Here, secondary relaxation processes emerge on the high frequency side of the main (alpha) relaxation peak, namely the excess wing (EW) and the beta-process. The EW manifests itself in the dielectric spectra as a power-law, while beta-process as a second relaxation peak. A new approach is introduced to disentangle the different spectral contributions (alpha-process, EW and beta-process). At variance with previous interpretations, the spectral shape of the alpha-process is assumed to be temperature invariant, obeying frequency temperature superposition (FTS) in the full temperature range above Tg. Its corresponding stretching parameter is taken from the high-temperature spectra, where the analysis is not hampered by the appearance of secondary processes. As a result of this constraint, the EW exponent turns out to be not only temperature, but also system independent. Thus, the overall spectral evolution for systems with no beta-peak above Tg (previously called type A glass formers) is simply described by a small variation of the relative weight of the EW with respect to the alpha-peak. This weight grows upon cooling, in contrast to the behavior of a beta-process. These now called “type A characteristics” are always spoiled by a more or less pronounced manifestation of a beta-process. Based on their different temperature changes, the EW and the beta-process contributions are disentangled close to Tg, and aging experiments carried out in this work are interpreted within the new scenario. In the glass, the interplay of both EW and beta-process determines the relaxation pattern. The beta-process appears as the only feature spoiling the universality in the evolution of the dynamics in molecular glass formers, since its relaxation strength does not correlate with the molecular dipole moment. Based on the above scenario, a consistent comparison between the orientational correlation functions of rank l = 1 (probed by dielectric spectroscopy) and l = 2 (probed by field cycling NMR and light scattering) is carried out. As demonstrated for glycerol, the NMR and light scattering spectra above Tg are scaled according to FTS over 15 decades in frequency. Significant differences in the spectral shape of the susceptibilities of different ranks are recognized at the low, as well as at the high frequency side of the scaled relaxation peak. In contrast, the time constants provided by the three techniques turn out to be essentially the same. Regarding the systematic differences observed at high frequencies, they are explained by assuming that the fast dynamics (EW) proceeds via small angles. Below Tg, NMR and dielectric spectroscopy reflect the same dynamics for glycerol, i.e. an experimental temperature dependence of the susceptibility is revealed. The evolution of the secondary processes (EW and beta-process) is monitored for temperatures well below Tg by applying a high-precision bridge. The bridge was employed to investigate extremely low losses (tan(delta) < 10-5), and, for the first time, the frequency dependence (within three decades) of the permittivity down to cryogenic temperatures was accessed for molecular glasses. Two additional relaxation regimes are identified: below 10 K clear indications are found that the tunneling regime is reached. Here the dielectric loss saturates to a plateau when plotted as a function of temperature and the corresponding weak frequency dependence appears as universal, at variance with the standard tunneling model predicting no frequency dependence here. Scaling out the molecular dipole moment collapses the plateau heights to an approximately system independent value, indicating a common density of tunneling centers in molecular glasses. At higher temperatures (10 K > T > 50 K) indications for thermally activated dynamics in asymmetric double well potentials are found for these systems. Here, for some systems, the dielectric loss displays a peak when plotted as a function of temperature but not as a function of frequency. This is in accordance with the Gilroy-Phillips model, previously used to interpolate the data for inorganic glasses in this regime. The distribution of the activation barriers g(V) is directly accessed by scaling the spectra in accordance with this model. However, g(V) extracted for molecular glasses appears as a stretched exponential.
