Details

Autor(en) / Beteiligte

• Flämig, M
• Becher, M
• Hofmann, M
• Körber, T
• Kresse, B
• Privalov, A. F
• Willner, L
• Kruk, D
• Fujara, F
• Rössler, E. A

Titel

Perspectives of Deuteron Field-Cycling NMR Relaxometry for Probing Molecular Dynamics in Soft Matter

Ist Teil von

• The journal of physical chemistry. B, 2016-08, Vol.120 (31), p.7754-7766

Ort / Verlag

United States: American Chemical Society

Erscheinungsjahr

2016

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Due to the single-particle character of the quadrupolar interaction in molecular systems, 2H NMR poses a unique method for probing reorientational dynamics. Spin–lattice relaxation gives access to the spectral density, and its frequency dependency can be monitored by field-cycling (FC) techniques. However, most FC NMR studies employ 1H; the use of 2H is still rare. We report on the application of 2H FC NMR for investigating the dynamics in molecular liquids and polymers. Commercial as well as home-built relaxometers are employed accessing a frequency range from 30 Hz to 6 MHz. Due to low gyromagnetic ratio, high coupling constants, and finite FC switching times, current 2H FC NMR does not reach the dispersion region in liquids (toluene and glycerol), yet good agreement with the results from conventional high-field (HF) relaxation studies is demonstrated. The pronounced difference at low frequencies between 2H and 1H FC NMR data shows the relevance of intermolecular relaxation in the case of 1H NMR. In the case of the polymers polybutadiene and poly­(ethylene-alt-propylene), very similar relaxation dispersion is observed and attributed to Rouse and entanglement dynamics. Combination with HF 2H relaxation data via applying frequency–temperature superposition allows the reconstruction of the full spectral density reflecting both polymer as well as glassy dynamics. Transformation into the time domain yields the reorientational correlation function C 2(t) extending over nine decades in time with a long-time power law, C 2(t) ∝ t –0.45±0.05, which does not conform to the prediction of the tube-reptation model, for which ∝ t –0.25 is expected. Entanglement sets in below C 2(t = τe) ≅ S 2 = 0.001, where τe is the entanglement time and S the corresponding order parameter. Finally, we discuss the future prospects of the 2H FC NMR technique.