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

Abstract

Due to the single-particle character of the quadrupolar interaction in molecular systems, (2)H 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 (1)H; the use of (2)H is still rare. We report on the application of (2)H 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 (2)H 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 (2)H and (1)H FC NMR data shows the relevance of intermolecular relaxation in the case of (1)H 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 (2)H 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 = 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 (2)H FC NMR technique.

Segmental dynamics of polyethylene-alt-propylene studied by NMR spin echo techniques

“…24 with the addition of the data obtained in the present work by the spin echo method. Values of τ s (T ) are provided by 1 H FC NMR, shearstress rheology, 24 dielectric spectroscopy, and solid-state 2 H NMR 49 for different molecular weights of PEP, since τ s is weight independent. τ s (T ) data are interpolated by a fourparameter function 50 and this function is vertically shifted to intersect τ e , τ R , and τ d values.…”

“…Relaxation map of PEP: τ s (T) for different M determined by 1 H FC NMR (black closed symbols), shear-stress rheology (colored closed points),24 NSE (extrapolated), dielectric spectroscopy (unpublished data), and solid-state 2 H NMR (crosses) 49. τ e (T) values for PEP 200k are obtained by 1 H FC NMR,24 NSE,28 and the spin echo method presented in this work.…”

Application of proton field‐cyclingNMRrelaxometry for studying translational diffusion in simple liquids and polymer melts

“…Most of the FC studies employ 1 H NMR, although other nuclei such as 2 H, 7 Li, or 19 F are suitable, too . Polarization and detection fields higher than 1 Tesla are still difficult to put into practice, therefore, the technique faces relatively low NMR signals in FC relaxometry.…”

“…Thus, FC 1 H NMR relaxometry yields information on both, translational and rotational dynamics. The purely rotational dynamics in molecular systems may also be accessed by analysing the 2 H NMR relaxation dispersion …”

Mode Analysis of NMR Relaxation in a Polymer Melt

“…The issue of low signals inherent to low-field NMR is overcome by rapidly cycling through polarization–relaxation–acquisition intervals . Although other nuclei are possible, − the majority of FC NMR studies addresses the relaxation of the abundant proton ( 1 H), for which spin–lattice relaxation is primarily caused by fluctuations of dipole–dipole couplings, and for which Larmor frequencies on the order of tens of megahertz down to several tens of Hertz result. Recently, a low field record of 3 Hz could be achieved…”