Nuclear Spin Relaxation in Viscous Liquids: Relaxation Stretching of Single-Particle Probes

Abstract

Spin-lattice relaxation rates R 1 (ω,T), probed via highfield and field-cycling nuclear magnetic resonance (NMR), are used to test the validity of frequency−temperature superposition (FTS) for the reorientation dynamics in viscous liquids. For several liquids, FTS is found to apply so that master curves can be generated. The susceptibility spectra are highly similar to those obtained from depolarized light scattering (DLS) and reveal an excess wing. Where FTS works, two approaches are suggested to access the susceptibility: (i) a plot of deuteron R 1 (T) vs the spin−spin relaxation rate R 2 (T) and (ii) a plot of R 1 (T) vs an independently measured reference time τ ref (T). Using single-frequency scans, (i) allows one to extract the relaxation stretching as well as the NMR coupling constant. Surveying 26 data sets, we find Kohlrausch functions with exponents 0.39 < β K ≤ 0.67. Plots of the spin−spin relaxation rate R 2 rescaled by the NMR coupling constantas a function of temperature allow one to test how well site-specific NMR relaxations couple to a given reference process. Upon cooling of flexible molecule liquids, the sitespecific dynamics is found to merge, suggesting that near T g the molecules reorient essentially as a rigid entity. This presents a possible resolution for the much lower stretching parameters reported here at high temperatures that contrast with the ones that were reported to be universal in a recent DLS study close to T g . Our analysis underlines that deuteron relaxation is a uniquely powerful tool to probe single-particle reorientation.

Glassy dynamics in polyalcohols: intermolecular simplicity vs. intramolecular complexity

“…This suggests that also at low tem-peratures, intramolecular dynamics contribute to the relaxation spectrum at the high-frequency side of the α-process. However, the influence of these intramolecular modes is less strong in the supercooled regime, as it is commonly observed 61 . Thus, the PCS spectrum can be modelled using a similar sum of two GG distributions as for the TFPI spectrum, however with a reduced amplitude of the fast contribution, as shown by the orange lines in Fig.…”

“…Threitol, xylitol and sorbitol only follow the generic shape up to frequencies slightly above the peak maximum and One possible origin of the additional relaxation contribution that develops at high frequencies with increasing molecular weight is the increasing influence of intramolecular degrees of freedom, which was previously observed by different NMR techniques and computer simulations, as discussed above. Typically, dynamic decoupling of different intramolecular moieties in flexible molecules is pronounced in the liquid regime at high temperatures, and tends to be reduced in the supercooled regime, when cooperativity increases 61 . Thus, we perform high-temperature light scattering measurements to examine the possible influence of intramolecular dynamics and present TFPI data of sorbitol at high temperatures in Fig.…”

“…This is not surprising, as rotation of the outer parts (C 1 and C 6 ) of the sorbitol molecule will have a significant impact on the optical anisotropy tensor, as they make up a third of the molecule. Additionally, the dynamics of O-H groups is markedly heterogeneous 61 and decouples from the dynamics of C-H bonds 60 and, thus, can also contribute to the TFPI spectrum. Especially the broadening at ν > ν C1 could result from O-H dynamics, suggested by the very asymmetrically broadened spectrum found in 2 H-NMR for OD-labeled sorbitol 61 .…”

“…Additionally, the dynamics of O-H groups is markedly heterogeneous 61 and decouples from the dynamics of C-H bonds 60 and, thus, can also contribute to the TFPI spectrum. Especially the broadening at ν > ν C1 could result from O-H dynamics, suggested by the very asymmetrically broadened spectrum found in 2 H-NMR for OD-labeled sorbitol 61 .…”

“…60 ). Detailed analysis of the differently labeled compounds revealed that not only their characteristic relaxation times, but also their respective spectral shape of the α-process differs significantly in NMR 61 . These findings are especially relevant for interpreting BDS data, as the total molecular dipole results from the vector sum of all dipole moments of single C-O-H units in the molecule.…”