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

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• 作者：M. Flämig ; M. Becher ; M. Hofmann ; T. Körber ; B. Kresse ; A. F. Privalov ; L. Willner ; D. Kruk ; F. Fujara ; E. A. Rössler
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：August 11, 2016
• 年：2016
• 卷：120
• 期：31
• 页码：7754-7766
• 全文大小：697K
• 年卷期：0
• ISSN：1520-5207

文摘

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 C₂(t) extending over nine decades in time with a long-time power law, C₂(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 C₂(t = τ_e) ≅ S² = 0.001, where τ_e is the entanglement time and S the corresponding order parameter. Finally, we discuss the future prospects of the ²H FC NMR technique.