David B. Zax

Zero field NMR and NQR

Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.

Dynamical heterogeneity in nanoconfined poly(styrene) chains

Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, remarkable dynamical heterogeneities which vary on length scales as short as a fraction of a nanometer. While the surface forces apparatus provides an experimental probe of macroscopic properties of fluids in contact with atomically smooth solid surfaces, few experimental probes are available which test the microscopic origins of these heterogeneities. In this article we describe our recent efforts to apply nuclear magnetic resonance spectroscopy to nanoscopically confined poly(styrene) (PS) created by intercalation into a surface-modified fluorohectorite. A comparison between surface-sensitive cross polarization experiments with spin–echo experiments which probe the entire organic layer suggests that PS in the center of the nanopores is more mobile than the bulk at comparable temperatures, while chain segments which interact with the surface are dynamically inhibited.

STUDY OF ANISOTROPIC DIFFUSION OF ORIENTED MOLECULES BY MULTIPLE QUANTUM SPIN ECHOES

Experiments have been completed which demonstrate the greatly enhanced sensitivity of the multiple quantum version of the NMR spin echo diffusion experiment. These experiments are particularly well‐suited to studies of anisotropic diffusion where the presence of dipolar couplings may make the standard experiment technically infeasible. (AIP)

Dynamics in a poly(ethylene oxide)-based nanocomposite polymer electrolyte probed by solid state NMR

Abstract Novel nanocomposite polymer electrolytes have been synthesized by intercalation of poly(ethylene oxide) into various layered silicates. In this work we use solid state NMR to probe dynamics associated with both the cations and the polymer over a broad temperature range. Spin-lattice relaxation studies reveal the influence of the polymer backbone dynamics on the 7 Li + T 1 , and we discuss why such measurements need to be interpreted with care. Cation dynamics are separated from that of the polymer by an analysis of the temperature dependence of the 7 Li spectrum in the presence of paramagnetic impurities fixed in the silicate lattice. The intercalated polymer shows no evidence for either glass or melt transitions, and over the same temperature range exhibits thermally-activated polymer reorientation. Over the same broad temperature range, Li + cations prove to be mobile with the same activation energy. Such measurements appear to indicate that cation diffusion is much slower than polymer reorientation in these systems.

Multiple-quantum N.M.R. study of molecular structure and ordering in a liquid crystal

Proton multiple-quantum N.M.R. spectroscopy was used to investigate the anisotropic ordering in a magnetic field and molecular structure for the nematic phase of 4-cyano-4′-n-pentyl-d11-biphenyl (5CB-d11). The multiple-quantum spectra exhibit a greater resolution of line splittings than the conventional single-quantum Fourier transform N.M.R. spectrum of 5CB-d11. This greatly simplifies a spectral analysis for the proton dipole-dipole coupling constants. Splittings among the five-, six-, and seven-quantum spectra are used to obtain the biphenyl proton couplings for 5CB-d11. Two models for the biphenyl symmetry are considered in the analysis. In one model (D 4 symmetry), the two phenyl rings are assumed to be equivalent. In the other model (D 2 symmetry), the rings are inequivalent by virtue of structural or motional differences between them. Both models produce acceptable fits to the splittings assigned from the experimental spectra. However, we conclude that the spectra cannot be used to make a choice be...

Relaxation of polymers in 2 nm slit-pores: confinement induced segmental dynamics and suppression of the glass transition

Molecular Dynamics (MD) simulations are used to explore the structure and dynamics of polystyrene confined in 2 nm slit pores, between parallel, crystalline, mica-type surfaces. The systems simulated resemble experimentally studied intercalated nanocomposites, where polystyrene is inserted between layered-silicate layers. The molecular modeling perspective complements the experimental findings and provides insight into the nature of polymers in nanoscopic confinements, especially into the molecular origins of their macroscopic behavior. Namely, a comparison between simulation and NMR studies shows a coexistence of extremely faster and much slower segmental motions than the ones found in the corresponding bulk polymer at the same temperature. The origins of these dynamical inhomogeneities are traced to the confinement induced density modulations inside the 2 nm slits. Fast relaxing phenyl and backbone moieties are found in low density regions across the film, and preferentially in the center, whereas slow relaxing moieties are concentrated in denser regions in the immediate vicinity of the confining surfaces. At the same time, the temperature dependence of the segmental relaxations suggests that the glass transition is suppressed inside the confined films, an observation confirmed by scanning calorimetry.

Li+ dynamics in a polymer nanocomposite: An analysis of dynamic line shapes in nuclear magnetic resonance

Characterization of dynamics of the charge-carrying species in polymer electrolytes has proven difficult. In this work we focus on a nanocomposite polymer electrolyte created when poly(ethylene oxide) (PEO) is intercalated into a layered silicate, Li–montmorillonite. We characterize both the Li+–silicate distance and the cation dynamics by analysis of the changes in 7Li nuclear magnetic resonance (NMR) line shape observed as the temperature is changed and cation diffusion is enabled. The observed spectra are compared to spectral simulations which emphasize the role of dipolar fields, associated with the static paramagnetic Fe3+ ions randomly distributed at the Al3+ lattice sites, interacting with the mobile cations. Low temperature line shapes are asymmetric, and not simply related to line shapes of more typical NMR interactions. Simulation of 7Li NMR spectra and comparison to experimental spectra shows that the Li+ interacts most strongly with the silicate surface layer, and all our evidence indicates th...

Zero‐field NMR and NQR spectrometer

In comparison to high‐field NMR, zero‐field techniques offer advantages in terms of spectral interpretability in studies of polycrystalline or amorphous solids. This article describes a technique and apparatus for time‐domain measurements of nuclear magnetism in the absence of applied fields (Fourier transform zero‐field NMR and NQR). Magnetic field cycling and high field detection are employed to enhance sensitivity. The field cycling is accomplished with an air‐driven shuttle system which moves the sample between regions of high and low magnetic field, in combination with switchable electromagnets in the low‐field region. Sudden field steps or pulses are used to initiate coherent nuclear spin evolution in zero field and to monitor such evolution as a function of time. Experimental results are shown and analyzed. Possible variations on the basic method are described and their relative advantages are discussed.

Zero field NMR and NQR with selective pulses and indirect detection

Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo‐ and heteronuclear systems are presented as an illustration of these techniques.

Fourier transform pure nuclear quadrupole resonance by pulsed field cycling

We report the observation of Fourier transform pure NQR by pulsed field cycling. For deuterium, well resolved spectra are obtained with high sensitivity showing the low frequency nu0 lines and allowing assignments of quadrupole couplings and asymmetry parameters to inequivalent deuterons. The technique is ideally applicable to nuclei with low quadrupolar frequencies (e.g., 2D, 7Li, 11B, 27Al, 23Na, 14N) and makes possible high resolution structure determination in polycrystalline or disordered materials.