Glass spectrum, excess wing phenomenon, and master curves in molecular glass formers: A multi-method approach.

The relaxation spectra of glass formers solely displaying an α-peak and excess wing contribution collected by various methods are reanalyzed to pin down their different spectral evolution. We show that master curve construction encompassing both α-peak and emerging excess wing works for depolarized light scattering (DLS) and nuclear magnetic resonance (NMR) relaxometry. It reveals the self-part of the slow dynamics' spectrum. Master curves are to be understood as a result of a more extensive scaling covering all temperatures instead of strict frequency-temperature superposition. DLS and NMR display identical relaxation spectra; yet, comparing different systems, we do not find a generic structural relaxation at variance with recent claims. Dielectric spectroscopy (DS) spectra show particularities, which render master curve construction obsolete. The DS α-peak is enhanced or suppressed with respect to that of DLS or NMR, yet, not correlated to the polarity of the liquid. Attempting to single out the excess wing from the overall spectrum discloses a stronger exponential temperature dependence of its amplitude compared to that below Tg and a link between its exponent and that of the fast dynamics' spectrum. Yet, such a decomposition of α-peak and excess wing appears to be unphysical. Among many different glasses, the amplitude of the excess wing power-law spectrum is found to be identical at Tg, interpreted as a relaxation analog to the Lindemann criterion.

A Relation between the Formation of a Hydrogen-Bond Network and a Time-Scale Separation of Translation and Rotation in Molecular Liquids.

We study the relation between the translational and rotational motions of liquids, which is anticipated in the framework of the Stokes-Einstein-Debye (SED) treatment. For this purpose, we exploit the fact that 1H field-cycling nuclear magnetic resonance relaxometry and molecular dynamics simulations provide access to both modes of motion. The experimental and computational findings are fully consistent and show that the time-scale separation between translation and rotation increases from the van der Waals liquid o-terphenyl over ethylene glycol to the hydrogen-bonded liquid glycerol, indicating an increasing degree of breakdown of the SED relation. Furthermore, the simulation results for two ethylene glycol models with different molecular conformations indicate that the translation is more retarded than the rotation when the density of intermolecular hydrogen bonds increases. We conclude that an increasing connectivity of a hydrogen-bond network leads to an increasing time-scale separation and, thus, to a stronger SED violation.

NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers.

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of 1H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole-Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

On the molecular mechanisms of α and ß relaxations in ionic liquids.

Using 2H NMR, we determine correlation times and motional mechanisms for the α and ß relaxations of glass-forming imidazolium-based ionic liquids, explicitly, for the associated cation reorientation dynamics. It is shown that the α relaxation is faster, its nonexponentiality is stronger, and the fragility is higher for bis(trifluoromethylsulfonyl)imide anions than that for tetrafluoroborate anions. 2H NMR stimulated-echo studies reveal that the overall reorientation dynamics involved in the α relaxation is isotropic and composed of jumps about small angles, where the mean jump angles are smaller for larger cations. Moreover, we demonstrate that, in addition to a cation-specific ß relaxation, all studied ionic liquids exhibit the genuine Johari-Goldstein ß relaxation of glass-forming liquids. Various 2H NMR results consistently indicate that the associated rotational motion is spatially highly restricted. Altogether, our findings show that, despite strong electrostatic interaction and prominent microphase separation of ionic liquids, their glassy dynamics resemble that of their nonionic counterparts, including similar microscopic mechanisms for intrinsic α and ß relaxations.

On the relation between reorientation and diffusion in glass-forming ionic liquids with micro-heterogeneous structures.

We investigate complex structure-dynamics relations in glass-forming ionic liquids comprising 1-alkyl-3-methylimidazolium cations and bis(trifluoromethylsulfonyl)imide anions. In doing so, we exploit the microheterogeneous structures emerging when the alkyl length is increased in the range n = 1-12 and use that 1H and 2H NMR give information about cation dynamics, while 19F NMR reports on anion motions. Furthermore, we combine spin-lattice relaxation analysis, including field-cycling relaxometry, with stimulated-echo experiments to follow reorientation dynamics related to structural relaxation in wide dynamic ranges and we apply static field gradients to probe translational diffusion. The resulting correlation times τ and diffusion coefficients D show Vogel-Fulcher-Tammann temperature dependence. Moreover, they indicate a moderate slowdown of both cation and anion dynamics with increasing alkyl length n. However, the relative diffusivities of the ionic species depend on the cation size, where cations are more mobile for n < 6 and anions for n > 6. Finally, we relate rotational and translational motions in the framework of the Stokes-Einstein-Debye (SED) approach. We find that the SED relation is obeyed for anion dynamics in all samples, while it breaks down for cation dynamics when n is increased. The origin of this SED breakdown is shown to differ fundamentally from that reported previously for conventional glass formers. We argue that an emergence of cation clusters causes a retardation of cation diffusion relative to cation reorientation upon cooling, i.e., the studied ionic liquids show a complex interplay of structural and dynamical properties.