Primary and Secondary Relaxations in Neat and Binary Glass Formers Studied by Means of Dielectric Spectroscopy

The subject of this thesis is the investigation of neat and binary molecular glass formers studied with the help of dielectric spectroscopy (DS). One of two main interests followed by this thesis is to refine the generic picture of the dynamic susceptibility, including the temperature evolution of primary and secondary relaxation phenomena. The other objective is to gain insight into the microscopic origin of the beta-process (a secondary relaxation observed for T<Tg) in neat and binary systems, as well as the primary relaxation phenomena in binary systems. The results of all investigations are collected in six partially interrelated publications. Three classes of glass forming systems are analyzed: type-A systems showing no discernible beta-process, but in most cases the so-called excess wing (EW) appearing as a power law epsilon‘‘(nu)~nu^(-gamma) in the susceptibility at frequencies beyond the alpha-peak position (nu>>numax); type-B systems which have a beta-process clearly resolved as a susceptibility peak; and binary systems which are represented by mixtures of the type-A system polystyrene (PS, Mw=2 kg/mol) and the type-B system tripropyl phosphate (TPP) in this thesis. The analysis of type-A systems as presented in Papers 1 & 2 is based on the application of a specifically introduced three-parameter fit function, which interpolates continuously between Kohlrausch and Cole-Davidson spectral shapes. With the help of this function small temperature-affected spectral changes of the alpha-peak can be assigned fully to a variation of its low-frequency behavior. In the framework of this ansatz the method of spectral analysis, which has become usual to be applied for type-A systems by our group, can be advanced. The investigation of the type-A glass former quinaldine (2-methyl quinoline) leads to the discovery of a family of substances which have metastable dielectrically active crystalline polymorphs. Dielectric spectra of all observed phases are presented in Paper 3. The phase transitions, occurring in the deeply supercooled state, show temperature-dependent transformation kinetics, which are tracked and quantified by performing long-term DS experiments. As a consequence of the temperature dependence of the transformation kinetics, the metastable phases (including the supercooled liquid) can be kinetically stabilized by lowering temperature. In addition to DS experiments X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements are performed, which help to characterize the quinaldine phases and to track their phase transitions. Further, a family of neat, chemically related type-B glass formers, i.e. symmetric phosphoric esters, are investigated in Paper 4 in order to find a possible correlation between their molecular structures and the corresponding molecular dynamics, in particular concerning the beta-processes. In two aspects a universal behavior of their beta-processes is observed. Firstly, temperature-independent distributions of activation energies g(E), which control the spectral evolution of the beta-processes, are found for all systems for temperatures well below the glass transition temperature (T<0.9 Tg). Secondly, the relaxation strength Delta epsilon beta(T) is nearly constant for T<0.9Tg. When Tg is approached (T>0.9 Tg), a distinct increase of the mean activation energy as well as a strong increase of the relaxation strength is observed. Both mentioned properties are interpreted to be sensitive to the softening of the glassy sample near Tg, pointing, in turn, towards the cooperative nature of the beta-processes. Since the behavior of Delta epsilon beta(T) is known for structural as well as for orientational glasses made up by rigid molecules, and since no strong correlation between g(E) and the number of internal degrees of freedom of the phosphoric esters is found, all types of beta-processes appear to be mainly determined by intermolecular interactions and are not of merely local nature. Rather they may be considered as a generic feature of the glass transition. The system TPP/PS, a so-called asymmetric binary glass former with a Tg contrast of Delta Tg=201 K, is investigated in the form of a joint study by DS and 31P/2H NMR, supported by depolarized light scattering (DLS) as well as DSC experiments. The results are collected in Papers 5 & 6. Tripropyl phosphate (TPP), which is also subject of the just summarized study on phosphoric esters, acts now as low-Tg component with a well-resolved beta-process. The high-Tg component polystyrene (PS) has a relatively low molecular mass and shows no indication of a beta-relaxation. The beta-process found in neat TPP is also observed in the mixtures (Paper 5), and for high TPP concentrations (cTPP) its properties are nearly concentration-independent. Selective NMR experiments prove the cooperative nature of the beta-process: the beta-relaxation, which is induced by the TPP molecules, is imposed on the PS monomers in the mixture. NMR experiments show that, when cTPP is reduced continuously, there exist increasing fractions of TPP molecules as well as PS monomers not being involved in the beta-process. Due to the high Tg contrast of the components in TPP/PS, two primary relaxations (alpha1 and alpha2) and, correspondingly, two calorimetric glass transition temperatures (Tg1 and Tg2) are resolved (Paper 6). Selective NMR experiments allow for a general assignment of the alpha1-relaxation to matrix dynamics and of the alpha2-relaxation to additive dynamics. On the other hand, it is shown that a fraction of TPP molecules is involved in the matrix dynamics, i.e.,the molecules relax on the long time scale of the PS monomers. The analysis of the DS spectra shows that this fraction is temperature-dependent and vanishes above a temperature Tc. As a further result, the alpha2-process is found to change its appearance with varying TPP concentration: at high cTPP its temperature-dependent peak shape resembles the alpha-peak of a liquid in confinement, while its time constants follow a non-Arrhenian temperature dependence. While, on the one hand, NMR experiments prove that the alpha2-process is also isotropic at low cTPP, it develops, on the other hand, into a thermally activated process with a temperature-independent distribution of activation energies g(E) when cTPP is decreased. In addition, a further reduction of cTPP leads to a decrease of the mean activation energy of the alpha2-process. As a consequence, the glass transition temperature related to the alpha2-relaxation (Tg2) exhibits a maximum at intermediate TPP concentrations.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Kahlau, Robert

Contributors dc:contributor

• Rößler, Ernst

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/54/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:54