Lineare und nichtlineare rheologische Untersuchungen von Relaxationsprozessen in glasbildenden Flüssigkeiten
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Date
2019
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Abstract
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.
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Keywords
Rheologie, Relaxationsprozesse, Gläser, Nichtlinear