E. A. Rössler

Structural characterisation of silicon carbonitride ceramics derived from polymeric precursors

Abstract Polymeric precursors with tailored structures were prepared from functionalised chlorosilanes. Pyrolysis under inert atmospheres led to amorphous Si–C–N–(H) ceramics at 1000°C. Further heat treatment caused the transformation into the thermodynamically stable crystalline phase assemblage. The structural changes associated with crosslinking, pyrolysis and crystallisation were studied by characterising the solid intermediates between 300 and 1600°C applying 29 Si and 13 C solid state nuclear magnetic resonance (NMR) spectroscopy. In addition, Fourier transformed infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermoanalytical techniques and density measurements were employed. The combination of these methods and the use of polymeric precursors with tailored structures pointed to a correlation of the polymer architecture with the structure of the amorphous ceramic material.

The dynamics of strong and fragile glass formers: vibrational and relaxation contributions

Abstract There are two contributions to the low-frequency excitation spectra (Raman and neutron scattering, specific heat, etc.) of glass formers: relaxations and vibrations. It is shown from the analysis of low-frequency Raman spectra that the relative weight of vibrational over relaxational excitations is larger for less fragile (in Angells classification) glass formers. The spectra are compared with predictions of mode coupling theory (MCT) for relaxation processes. Qualitatively the predicted behaviour is observed in all analyzed systems. However, some quantitative disagreement due to significant vibrational contribution to the spectra is found for intermediate glass formers. The spectra are also analyzed using a model of vibrations coupled with relaxations. It is found that the temperature at which overdamping of the low-frequency vibrations happens is essentially the critical temperature of MCT. New details of a scenario for the liquid-glass transition are suggested.

Universal representation of viscosity in glass forming liquids

Abstract Temperature dependence of the viscosity η ( T ) of fragile and strong polymer and non-polymer glass formers was analyzed. A temperature T x was introduced which marks the boundary between a high- and low-temperature regime, where different approaches are applied to interpolate η = η ( T ). At T T x a master curve is obtained for polymer and non-polymer glass formers by scaling the quantity ( T g / T −1) with a factor F = T x /( T x − T g ) where T g is the glass transition temperature. F is related to the steepness parameter m via F ≅0.079 m . A continuous transition from the master curve of non-polymer systems to the one of polymers is found if molecular weight is systematically increased. T x is found to be close to the critical temperature T c reported from analyses of the dynamic susceptibility by mode coupling theory. It is tentatively concluded that the magnitude of the interval between T x and T g defines the degree of fragility. At T > T x deviations from the master curves occur, and the Vogel–Fulcher–Tammann equation provides a very good fit to highest temperatures. The two master curves at T T x can be interpolated by several formulae.

Slow secondary relaxation in simple glass formers

Abstract We investigate the secondary relaxation (β-process) in simple glass formers by dielectric spectroscopy covering the frequency range 10 −2 Hz ⩽ν⩽10 9 Hz . Data on the dielectrically rigid molecules toluene and 3-fluor aniline are presented. Furthermore, 1-propanol and polybutadiene are studied. Performing a lineshape analysis, we find the following features: (i) The β-process can be described by assuming a broad distribution of thermally activated Debye processes with a Gaussian distribution of activation enthalpies, Eβ, and entropies, ΔSβ≠. (ii) The mean activation enthalpy 〈Eβ〉 is related to the glass transition temperature Tg through 〈Eβ〉=24RTg, and (iii) 〈ΔSβ≠〉≈10R is found. The latter finding explains the small but system independent prefactor of the Arrhenius function fitted to the data ( τ 0 ≈10 −16 s ). (iv) The relaxation strength is constant for T Tg. We conclude that the β-relaxation is intrinsic to the glassy state (Johari).

Two glass transitions in ethanol: a comparative dielectric relaxation study of the supercooled liquid and the plastic crystal

Ethanol is known to form two different kinds of glassy state, namely a structural glass and a glassy crystal, both showing the same glass transition temperature . The molecular dynamics in the different phases (supercooled liquid/plastic crystal) and during the corresponding phase transition is studied by dielectric spectroscopy in the frequency range (30 K-250 K). Extracting the time constant, width parameter and relaxation strength of the main relaxation (the -process), very similar dielectric loss is found in both phases, including also the high-frequency wing. Comparing the temperature dependence of the time constants , the plastic crystal exhibits a less fragile behaviour. Additionally, a secondary relaxation is detected, again essentially the same in the two glassy states. We conclude that the motional mechanisms probed in the plastic crystal and in the supercooled liquid state are very similar.

Nuclear-Magnetic-Resonance Measurements Reveal the Origin of the Debye Process in Monohydroxy Alcohols

Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes, an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on translational diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so-far puzzling observations made for this class of hydrogen-bonded glass forming liquids.

Evidence of secondary relaxations in the dielectric spectra of ionic liquids

We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methyl imidazolium cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency (10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An ionic conductivity is observed above the glass transition temperature T_{g} with a relaxation in the electric modulus representation. Below T_{g}, two relaxation processes appear, with the same features as the secondary relaxations typically observed in molecular glasses. The activation energy of the secondary processes and their dependence on the anion are different. The slower process shows the characteristics of an intrinsic Johari-Goldstein relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found, as observed in molecular glasses.

Orientational and translational dynamics in room temperature ionic liquids

The authors investigate the dynamics of a series of room temperature ionic liquids, based on the same 1-butyl-3-methylimidazolium cation with different anions, by means of broadband (10(-6)-10(9) Hz) dielectric spectroscopy and depolarized light scattering in the temperature range from 400 K down to 35 K. Typical ionic conductivity is observed above the glass transition temperature Tg. Below Tg the authors detect relaxation processes that exhibit characteristics of secondary relaxations, as typically observed in molecular glasses. At high temperatures, the characteristic times of cation reorientation, deduced from the light scattering data, are approximately equal to the electric modulus relaxation times related to ionic conductivity. In the supercooled regime and close to Tg, the authors observe decoupling of conductivity from structural relaxation. Overall, room temperature ionic liquids exhibit typical glass transition dynamics, apparently unaltered by Coulomb interactions.

The spectral density in simple organic glass formers: Comparison of dielectric and spin-lattice relaxation

The spin-lattice relaxation time T1 of simple organic glass formers is analyzed by introducing a spectral density obtained from broadband dielectric susceptibility data χ″(ω). For this purpose χ″(ω) was measured for several glass formers, that do not exhibit a Johari-type secondary relaxation process, covering a frequency range between 10−2 Hz and 109 Hz at temperatures above and below the glass transition temperature Tg. We introduce an analytical function to fit the shape of the main relaxation (α-process) above Tg, in particular taking into account high-frequency contributions in χ″(ω) commonly known as high-frequency wing. Below Tg the latter feature appears as a power law susceptibility χ″(ω)∝ω−γ, with γ<0.1 and a characteristic temperature dependence χ″(T)∝exp(T/const.), yielding almost 1/ω behavior in the spectral density. On the base of this complete description of χ″(ω), a quantitative comparison of dielectric and NMR spectroscopy is possible, which is carried out in full detail for glycerol-d3 (...

Susceptibility functions for slow relaxation processes in supercooled liquids and the search for universal relaxation patterns

In order to describe the slow response of a glass former, we discuss some distributions of correlation times, e.g., a generalized gamma distribution (GG) and an extension thereof (GGE), the latter allowing us to reproduce a simple peak susceptibility such as of the Cole–Davidson type as well as a susceptibility exhibiting an additional high-frequency power law contribution (excess wing). Applying the GGE distribution to the dielectric spectra of glass formers exhibiting no β process peak (glycerol, propylene carbonate, and picoline) we are able to reproduce the salient features of the slow response (10−6–109 Hz). A line shape analysis is carried out either in the time or frequency domain and in both cases an excess wing can be identified. The latter evolves in a universal way while cooling and shows up for correlation times τα>10−8 s. It appears that its first emergence marks the breakdown of the high-temperature scenario of mode coupling theory. In order to describe a glass former exhibiting a β process ...