T. Blochowicz

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.

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 ...

Free-volume microstructure of glycerol and its supercooled liquid-state dynamics

A positron annihilation lifetime spectroscopic (PALS) study was carried out on a typical representative of simple hydrogen-bonded glass formers - glycerol - over a wide temperature region from 100 K up to room temperature. Several crossover temperatures, Tbi, were identified in the temperature dependences of the ortho-positronium (o-Ps) lifetime and the relative o-Ps intensity. The onset temperature of free-volume change, 137 K, was compared with the value of 135±3 K obtained from extrapolating thermodynamic data and with the temperatures from relaxation data. In order to determine the influence of the free-volume evolution on the relaxation dynamics above the glass transition temperature Tg, the free-volume fraction from the PALS data was related to the behaviour of the dielectric loss of glycerol. A new relationship between the temperature parameters of the onset of free-volume change and of the high-frequency wing of the dielectric loss is presented. Taking the PALS free-volume data as input for a free-volume model (Doolittle ansatz), it was found that the relaxation times τα can be reproduced above the bend temperature Tb2 = 241 K; the latter lies in the vicinity of crossover temperatures reported from mode coupling theory analyses of dynamic data. In the lower-temperature region Tb2>T>Tb1Tg, a contribution from thermally activated mobility has to be included to reproduce the dielectric relaxation time data.

Dielectric spectroscopy on the plastically crystalline phase of cyanocyclohexane

Dielectric measurements (10−2–109 Hz) on plastically crystalline cyanocyclohexane reveal a glass transition where the rotational dynamics freezes at a vitrification temperature Tg=134 K. A full line-shape analysis of the dielectric loss spectra, which include a pronounced secondary relaxation contribution (β process) in addition to the main relaxation, is performed. We show that the rotational dynamics in cyanocyclohexane is very similar to that of supercooled liquids. In particular the secondary process of cyanocyclohexane exhibits the features characteristic of the Johari–Goldstein β relaxation known from structural glass formers.

Fast and slow relaxation processes in glasses

We present dielectric relaxation (DS) and light scattering (LS) data of several glass formers. Relaxational features are compiled which are not yet properly taken into account by current models. (i) We distinguish two types of glass formers. Type A systems do not show a slow -process whereas type B systems do. A full line-shape analysis of is presented . In type A systems the evolution of the high-frequency wing of the -process is the most prominent spectral change while cooling and leads to an essentially constant loss at . The analysis of of type B systems is carried out within the Williams-Watts approach and we focus on the temperature dependence of the -relaxation strength. (ii) Concerning fast relaxations below as revealed by LS we identify relaxation with a low-frequency power-law behaviour. No indication of a crossover to a white noise spectrum as previously reported and discussed within MCT is found. Analysing this relaxation we recourse to the model of thermally activated transitions in asymmetric double well potentials. We show that the model works well in some cases and the distribution of barrier heights may be extracted, but in other systems pronounced deviations occur.

Generalization of the Cole-Davidson and Kohlrausch functions to describe the primary response of glass-forming systems.

We introduce a three-parameter step-response function which is based on a generalization of the Cole-Davidson (CD) and Kohlrausch (K) functions, and which provides a highly flexible susceptibility description for viscous liquids. A second parameter α characterizing the overall width, in addition to a parameter β determining the high-frequency behavior of the susceptibility, allows for a continuous change of the spectral shape from the CD-type to the K-type. We prove that the function fulfills mathematical conditions required for a step-response function. When applying the function to interpolate dielectric spectra of neat (pure) glass formers, it is possible to keep the high-frequency parameter β temperature-independent while varying the parameter α to account for the change of the overall width. This analysis might suggest that the failure of frequency-temperature superposition in glass formers is reflected by a broadening in the low-frequency region instead of in the high-frequency one.

Evolution of the dynamic susceptibility of simple glass formers in the strongly supercooled regime

We discuss dielectric and light scattering susceptibility spectra of simple glass formers at temperatures above as well as below the critical temperature of the mode coupling theory (MCT). Close to Tg the systems are characterized by the presence of a pronounced excess wing (type A glass formers). The data are analysed within a phenomenological approach, on the one hand, and within MCT, on the other. Among other work we present a complete interpolation of the dielectric data for glycerol (Lunkenheimer et al2000 Contemp. Phys. 41 15). The crossover temperature Tx, defined by the emergence of the excess wing upon cooling, is extracted from the phenomenological analysis and found to agree well with the critical temperature Tc, extracted from the MCT analysis at high temperatures. Below Tx the evolution of the susceptibility is characterized by a universal appearance of the excess wing. No difference is observed for the non-fragile system with respect to fragile glass formers provided that the wing parameters are studied as a function of the correlation time τα. Finally, a generalized scaling for the susceptibility minimum is proposed which is a phenomenological extension of that of MCT but now also includes the data below Tc.

Spectral response from modulus time domain data of disordered materials

The electric response of the glass-forming glycerol and crystalline ionic conductor Li0.18La0.61TiO3 is probed by modulus time domain measurements. A capacity correction algorithm is proposed to overcome the low capacitance limit of the technique. This method allows to Fourier-transform time domain data yielding undisturbed permittivity spectra. The algorithm is checked first in glycerol, where the dielectric data recorded in frequency and time domain show an overlap of several decades. It is also applied to match the dielectric data of the crystalline ionic conductor Li0.18La0.61TiO3 from modulus time domain with overlapping frequency domain data, forming data sets covering 11 decades in frequency. The extension of the electrical characterization to low frequencies allows the detection of an Arrhenius behavior for the dc ionic conductivity at low temperatures, in disagreement with previous analysis in terms of Vogel-Fulcher-Tammann laws.