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

by Robert Kahlau

Institution: Universität Bayreuth
Department: Mathematik, Physik und Informatik
Degree: PhD
Year: 2014
Record ID: 1099756
Full text PDF: https://epub.uni-bayreuth.de/54/


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…