Lineare und nichtlineare rheologische Untersuchungen von Relaxationsprozessen in glasbildenden Flüssigkeiten
Creators: name: Bierwirth, Sebastian Peter
Publisher: name: Technische Universität Dortmund
Year: 2019
Subjects: subject: Rheologie
subject: Relaxationsprozesse
subject: Gläser
subject: Nichtlinear
subject: 530
subject: Rheologie
subject: Polymere
subject: Polymere Flüssigkeit
subject: Relaxation
subject: Glasbildner

description: Analyzing and characterizing mechanical properties of polymers is a vast field of research. In this thesis, linear and nonlinear rheological investigations have been used to study the relaxation behavior of various glass-forming liquids. In simple (non-associating and non-polymeric) liquids, the microscopic flow behavior is associated to the mass transport which is defined by the dynamics of the molecules and therefore the structural relaxation. This is usually discussed in the literature in terms of shear modulus or shear compliance, however, in this work it has been additionally analyzed in terms of fluidity. This quantity seems to exhibit an universal frequency dependence which, interestingly, matches the one previously signaled for the case of conductivity probed via dielectric spectroscopy in disordered ionic conductors. Based on this equivalence, the phenomenological and theoretical concepts previously developed for the characterization of charge transport have been adopted to the mass transport that is intrinsic to the molecular flow. In complex liquids, the macroscopic flow properties are driven by supra structural motions such as the chain dynamics in polymers or the so-called Debye process in hydrogen bonded liquids. They give rise to additional low-frequency relaxation modes in the rheological spectrum. A broad correlation between the rheological properties of the supramolecular and the structural relaxation process has been found. Mixing various pure substances featuring supramolecular modes, changes in their supramolecular properties can be induced in their linear mechanical response. It was found that the investigated mixtures also follow the above-mentioned correlation of the pure substances. Furthermore, for the first time evidence for two individual structural relaxations in mixed systems of low molecular weight glass forming liquids has been found by means of linear rheology. For glass formers such as alcohols, for which network formation is caused by rather weak and transient hydrogen bonds, supramolecular relaxation processes are often hard to identify in linear measurements. To render their characterization possible the measurements have been extended to the nonlinear regime. A Fourier-analysis technique which allows the detection of the slowest relaxation modes in polymer science has been adopted and succesfully applied for low molecular weight glass-forming liquids. In this way one is able to reveal that the nonlinear parameters which characterize the slowest relaxation mode show a universal frequency dependence. The timescale of the energy transfer to the liquid from the nonlinear excitation corresponds well to the characteristic time scale of the structural relaxation process. Additionally a new correlation between the linear and nonlinear rheological parameters of both simple and complex liquids has been found.