F. Fujara

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toDt=3·10−14 m2s−1 using a1H-NMR technique applying static field gradients up to 53T m−1 In a range of more than two decades theDt values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureTg. A change of mechanism is found for translational diffusion atTc≈1.2Tg whereDt is proportional to the inverse shear viscosityη−1 atT>Tc butDt ∼ηξ with ξ=0.75 atT<Tc. Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal toη−1 down toTg. Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.

Nonexponential 2H spin-lattice relaxation as a signature of the glassy state

Abstract High-precision measurements of 2H spin-lattice relaxation on several molecular glass-forming liquids have been performed. As a general feature the following can be stated: At temperatures more than ten to twenty degrees above the calorimetric glass transition temperature Tg the 2H spin-lattice relaxation is exponential; below that temperature regime the relaxation is nonexponential. This crossover from exponential to nonexponential magnetization recovery implies that no common spin temperature caused by spin diffusion exists in a 2H glass. This contrasts 1H spin-lattice relaxation which is found to be strictly monoexponential throughout. The occurrence of nonexponential 2H relaxation serves as a new observable at the glass transition, being a signature of the crossover from ergodic to nonergodic behaviour.

Relaxation and phonons in viscous and glassy orthoterphenyl by neutron scattering

We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from α-to β-relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureTc [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a multiple scattering simulation. Throughout the article, experimental difficulties are discussed in some detail.

Localized Motion in Supercooled Glycerol as Measured by 2H-NMR Spin-Lattice Relaxation and Incoherent Neutron Scattering

Selectively deuterated glycerol has been subjected to 2H-NMR spin-lattice relaxation and quasi-elastic neutron scattering experiments. The measurements yield relaxation rates and a non-Gaussian Q-dependence of the Debye-Waller factor which are different for the two hydrogen sites. The data analysis shows that below the onset of the glass transition α-process the hydrogens perform a local motion (≈ 10-12 s) in addition to what is expected from harmonic phonons. The resulting mean-square displacements are highly temperature dependent but are significantly smaller than those found in van der Waals glasses. Amplitudes and activation energies of the carbon-bonded and oxygen-bonded hydrogens are different. A possible mechanism is discussed.

Signatures of the glass transition in a van der Waals liquid seen by neutrons and NMR

Mode coupling theory (MCT) of the glass transition predicts the existence of a dynamic anomaly of supercooled liquids at a characteristic temperature Tc well above the caloric glass transition temperature Tg. A series of neutron scattering and NMR experiments on van der Waals liquids - a typical example of which is orthoterphenyl (OTP) - have been performed to test three predictions and to point out signatures of the glass transition. This contribution is meant to review our results in a concise manner: there is strong evidence for a Tc according to MCT from Debye-Waller factors, from line shape studies of the dynamic structure factor and from a comparison of self-diffusion and viscosity. NMR relaxation below Tc shows that there is at least one secondary process which so far is not accounted for by MCT and that the supercooled liquid becomes truely nonergodic closely above Tg on a time scale of a few seconds.

Comparative study of the Debye−Waller factor anomaly at the glass transition of isotopically substituted orthoterphenyls

Abstract We report on neutron-scattering measurements of the Debye—Waller factor of three isotopically substituted glassy orthoterphenyls: fully protonated, fully deuterated and selectively deuterated at the lateral phenyl rings. From the comparison of the observed anomaly in the glass transition region (β-process) found in the three systems, we can exclude any intramolecular phenyl-ring dynamics as the dominant motional mechanism for the β-process. The data thus support mode-coupling theory proposing a center-of-mass motion as precursor process of the glass transition.

Dynamic anomalies at the glass transition of the van der Waals glass tri-α-naphthylbenzene

Abstract The Debye-Waller factor of the van der Waals glass tri-α-naphthylbenzene, measured by high resolution incoherent quasielastic neutron scattering, shows a cusp at T c =407±5 K signaling the existence of a critical instability as has been predicted by mode coupling theory to occur in supercooled glass forming liquids. At temperatures above T c structural relaxation is observed to obey the time-temperature superposition principle.

Dynamic anomalies at the glass transition of organic van der Waals liquids

Abstract The paper discusses the question of whether there is a characteristic temperature T c above the calorimetric glass transition temperature T g . Mode-coupling theory (MCT) predicts a crossover from liquid- to solid-like dynamics at T c . Neutron scattering and gradient NMR experiments have been carried out to test MCT using the molecular van der Waals liquid ortho -terphenyl as a model system. A significant anomaly of the Debye—Waller factor and a “decoupling” of self-diffusion from viscosity support the MCT predictions. A critical discussion of the relevance of such tests and of the limitations of neutron scattering is presented.