C. Tschirwitz

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).

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 ...

A 2H NMR and dielelectric spectroscopy study of the slow β-process in organic glass formers

Abstract We study the slow β-process of several organic compounds by recording dielectric and 2 H NMR spectra below the glass transition temperature T g . For the neat systems toluene, polybutadiene, cis -decalin and ethanol as well as for the binary glasses chlorobenzene/ cis -decalin and toluene/2-picoline, the β-process manifests itself in very similar changes of the 2 H NMR spectrum when varying the spatial resolution by increasing the interpulse delay t p in the solid-echo pulse sequence. These findings indicate that for all considered glasses, in particular, for both components of the binary mixtures, molecular dynamics involved in the β-process are highly comparable. A line-shape analysis reveals that the secondary relaxation is mainly caused by a highly restricted, step-wise reorientation of essentially all molecules.For the neat glasses 2-picoline, glycerol and polystyrene (PS) which do not show a β-peak in dielectric spectroscopy, there is no evidence for molecular reorientation in the 2 H NMR spectra at T T g .

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.

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.

Evolution of the dynamic susceptibility of paradigmatic glass formers below the critical temperature Tc as revealed by light scattering

Abstract We have studied the molecular glass formers toluene and picoline in the liquid and glassy state by quasi-elastic light scattering applying tandem Fabry–Perot interferometry. Also, we perform an analysis of the susceptibility spectra of the inorganic glasses calcium potassium nitrate (CKN) and silica at T T g . At high temperatures the evolution of the susceptibility minimum in toluene and picoline is described by mode coupling theory (MCT), and the critical temperature T c can be determined consistently. Below T c the asymptotic laws of MCT fail, and the excess wing of the α process is rediscovered in a similar way as in the dielectric spectra. The wing contribution controls the evolution of the susceptibility minimum and is attributed to a secondary relaxation process different from the slow β process. The fast dynamics spectra exhibit a trend to white noise at low frequencies ( T T c ). We conclude, regarding the fast dynamics, that MCT predictions hold also below T c . In CKN the relaxation mechanism changes below T g and the fast relaxation in both CKN and silica is reproduced by assuming thermally activated transitions in asymmetric double well potentials, while this is not possible for picoline or toluene.

Slow dynamics in supercooled liquids and plastically crystalline solids

Abstract We report on the dielectric response of supercooled organic liquids. Main and secondary relaxation process are described by a quantitative line shape analysis. The results are compared with those obtained for supercooled plastically crystalline phases. Both types of glass forming systems exhibit very similar dynamic susceptibilities which are discussed in detail.