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.

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 H 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 -terphenyl over ethylene glycol to the hydrogen-bonded liquid glycerol, indicating an increasing degree of breakdown of the SED relation.

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.

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

Using H NMR, we determine correlation times and motional mechanisms for the α and β 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. H 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.

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 H and H NMR give information about cation dynamics, while F 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.