High-resolution Quasielastic Neutron Scattering Research Articles

Inelastic and quasi-elastic neutron scattering. Application to soft-matter

Microscopic dynamical events control many of the physical processes at play in condensed matter: transport, magnetism, catalysis and even function of biological assemblies. Inelastic (INS) and Quasi-Elastic Neutron Scattering (QENS) are irreplaceable probes of these phenomena. These experimental techniques reveal the displacements of atoms and molecules over distances spanning from angstroms to a few tens of nanometers, on time scales ranging from a fraction of picoseconds to microseconds. In this context, the different INS and QENS machines (Time-of-Flight (ToF), Backscattering (BS) and Neutron spin-echo (NSE)) stand at a central position. After introducing an underlying basic theoretical toolbox for neutron scattering, the principles and key elements of a ToF measurement are described. While, here, we mainly focus on disk choppers spectrometers, all the INS/QENS instruments share a common ground: they directly and simultaneously probe correlation functions in both time and space, so that the scattering vector (Q) dependence of the systems characteristic time(s) can be measured. To illustrate, the potentialities of the technique in the field of soft-matter, we show a multiscale approach of the dynamics of a polymer melt. The system is probed from the molecular to the mesoscopic scale (1 ps to 0.6 μs and 0.1 to 40 nm), in bulk and under nanometric confinement. We address the different dynamical modes of a high mass entangled polymer chain: local monomer dynamics, Rouse modes up to the reptation process. This study exemplifies that, used in conjunction with hydrogen/deuterium isotopic effects, high resolution QENS can be bridged to the Zero Average Contrast (ZAC) method to probe, in a non destructive way, the dynamics of a single polymer chain in bulk but also under severe nanometric confinement. Connection and complementarity of the neutron derived analysis with Pulsed-Field Gradient and Relaxation NMR techniques are discussed.

Neutron Scattering Studies of the Interplay of Amyloid β Peptide(1-40) and An Anionic Lipid 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol.

The interaction between lipid bilayers and Amyloid β peptide (Aβ) plays a critical role in proliferation of Alzheimer’s disease (AD). AD is expected to affect one in every 85 humans by 2050, and therefore, deciphering the interplay of Aβ and lipid bilayers at the molecular level is of profound importance. In this work, we applied an array of neutron scattering methods to study the structure and dynamics of Aβ(1–40) interacting 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) bilayers. In the structural investigations of lipid bilayer’s response to Aβ binding, Small Angle Neutron Scattering and Neutron Membrane Diffraction revealed that the Aβ anchors firmly to the highly charged DMPG bilayers in the interfacial region between water and hydrocarbon chain, and it doesn’t penetrate deeply into the bilayer. This association mode is substantiated by the dynamics studies with high resolution Quasi-Elastic Neutron Scattering experiments, showing that the addition of Aβ mainly affects the slower lateral motion of lipid molecules, especially in the fluid phase, but not the faster internal motion. The results revealed that Aβ associates with the highly charged membrane in surface with limited impact on the structure, but the altered membrane dynamics could have more influence on other membrane processes.

Quasi-elastic neutron scattering studies of the slow dynamics of supercooled and glassyaspirin

Aspirin, also known as acetylsalicylic acid (ASA), is not only a wonderful drug, but also agood glass former. Therefore, it serves as an important molecular system to study thenear-arrest and arrested phenomena. In this paper, a high-resolution quasi-elastic neutronscattering (QENS) technique is used to investigate the slow dynamics of supercooled liquidand glassy aspirin from 410 down to 350 K. The measured QENS spectra can beanalyzed with a stretched exponential model. We find that (i) the stretched exponentβ(Q) is independent of thewavevector transfer Q in the measured Q range and (ii) the structural relaxation timeτ(Q) follows a power-lawdependence on Q.Consequently, the Q-independent structural relaxation timeτ0 can beextracted for each temperature to characterize the slow dynamics of aspirin. The temperature dependenceof τ0 can be fitted with the mode-coupling power law, the Vogel–Fulcher–Tammann equationand a universal equation for fragile glass forming liquids recently proposed by Tokuyamain the measured temperature range. The calculated dynamic response functionχT(Q, t) using the experimentally determined self-intermediate scattering function of the hydrogenatoms of aspirin shows direct evidence of the enhanced dynamic fluctuations as the aspirinis increasingly supercooled, in agreement with the fixed-time mean squared displacement⟨x2⟩ and the non-Gaussianparameter α2 extracted from the elastic scattering.

Heterogeneous dynamics of poly(vinyl acetate) far above Tg: A combined study by dielectric spectroscopy and quasielastic neutron scattering

We have investigated the dynamics of poly(vinly acetate) using broadband dielectric spectroscopy (DS) covering over 14 decades in frequency up to 20 GHz and high-resolution quasielastic neutron-scattering (QENS) technique. The dielectric results have been interpreted in terms of the phenomenological Kohlrausch-Williams-Watts [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Alvarez, A. Alegria, and J. Colmenero, Phys. Rev. B 47, 125 (1993)] description. Because of the wide frequency range covered by DS, it provides a precise determination of dynamics over a wide temperature range and it revealed a crossover in polymer dynamics at 387 K through different dielectric parameters, viz., characteristic times, asymmetric shape parameter, and dielectric strength. Moreover, shape parameter is found to be higher in comparison to other typical polymeric systems. The characteristic relaxation times observed by QENS displayed an anomalous dependence of momentum transfer, indicating the possible existence of heterogeneities in the system even at the high temperatures. In addition, spin-lattice relaxation times, T(1), were found to be decoupled from dielectric characteristic times. Based on these results, a model was proposed to account for heterogeneities where we consider coexistence of different regions with standard polymeric behavior but with different characteristic times, leading to a distribution of relaxation times. The model is found to account for the anomalous behavior and an inherent shape parameter is found to account for the shape of alpha relaxation. This model is also found to predict the T variation of T(1) characteristic time scales at all temperatures. The origin of the heterogeneous domains is believed to lie in the microstructure of polymer chains.

Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations.

Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.

45ScNMR and high-resolution quasielastic neutron scattering studies of localized H(D) motion inα−ScHx(Dx)

Nuclear magnetic resonance measurements of the ${}^{45}\mathrm{Sc}$ spin-lattice relaxation rates in the solid solutions $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScD}}_{0.05},$ $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScD}}_{0.22},$ and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScH}}_{0.27}$ have been performed over the temperature range 4.2--299 K and the resonance frequency range 11--86.2 MHz. For all of the samples studied, the ${}^{45}\mathrm{Sc}$ relaxation rate shows a peak near 100 K resulting from the electric quadrupole interaction modulated by localized hopping of H(D) atoms. In addition to the strong effect of isotope $(\mathrm{H}\ensuremath{\leftrightarrow}\mathrm{D})$ substitution on the amplitude of this peak, with a larger peak appearing in the deuterides, we have found that in the region of the peak, the frequency dependence of the ${}^{45}\mathrm{Sc}$ relaxation rate for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScH}}_{0.27}$ is much weaker than for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScD}}_{x}.$ These results indicate that the hopping rate distribution for H atoms in scandium is shifted to much higher frequencies from that for D atoms. The results of our high-resolution quasielastic neutron-scattering measurements on $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ScH}}_{0.27}$ in the temperature range 12--302 K are consistent with this conclusion.

Quasi-elastic neutron-scattering studies of intercalated molecules in charge-deficient layer silicates. Part 2.—High-resolution measurements of the diffusion of water in montmorillonite and vermiculite

The relative influence of water–surface and water–cation interactions in hydrates of the charge-deficient expanding-lattice layer silicates montmorillonite and vermiculite have been studied using high-resolution quasi-elastic neutron-scattering (QENS) measurements made with the back-scattering spectrometer at ILL, Grenoble. Detailed measurements have been made Ca2+–montmorillonite equilibrated at seven different water-vapour partial-pressure (p/p0) values and for Ca2+, Mg2+ and Na+ vermiculite samples at fixed p/p0.The QENS data are fitted to a model consisting of an elastic and a single Lorentzian quasi-elastic component plus a flat background. The quasi-elastic linewidth and the relative intensities of the three components are adjustable parameters. The resultant quasi-elastic broadenings were approximately five times smaller than the values derived previously from measurements at lower energy resolution. Also, the quasi-elastic scattering intensity increased with p/p0 and decreased with increasing scattering angle, the latter being the reverse of the behaviour observed at the broader energy resolution.The low- and high-resolution measurements taken together indicate the occurrence of two different phases of translational motion of water molecules not directly coordinated to the exchangeable (Ca2+) cations. These are attributed (i) to rapid localized motions in ‘cages’ bounded by the silicate sheets and the hydrated cations and (ii) to slower, longer-range inter-cage diffusion. In addition, the relative intensities of the quasi-elastic scattering at high and low wavevector transfer suggest that the high-resolution measurements also observe rotations of the complete six-fold coordinated hydration shells of the cations about one of the C3 axes of the octahedron. To explain the observed width, this rotation must have a correlation time of ca. 10–10 at ambient temperature.The present QENS data yielded effective diffusion coefficients for water in Ca2+–montmorillonite which increased rapidly from < 1.0 × 10–10 m2 s–1 at p/p0= 0.15 to ca. 4.5 × 10–10 m2 s–1 at p/p0= 0.33. At higher values of p/p0 the effective diffusion coefficients remained approximately constant within the limits of experimental error. For water adsorbed by vermiculite having Na+, Ca2+ or Mg2+ as the exchangeable cation the diffusion coefficient measured at p/p0= 0.76 was found to be significantly smaller than the corresponding value for Ca2+–montmorillonite. This may be attributed to the hindering effect of the more densely packed network of hydrated cations in the more highly charged vermiculite.

An incoherent neutron quasi-elastic scattering study of two nematogens

High-resolution quasi-elastic neutron-scattering measurements have been made on two nematogens: DMBCA with a nematic range 108 to 119°C, and 5CB and a tail-deuteriated sample (D5CB), having a nematic range 22·6 to 35·1°C. Results on 5CB in the crystal phase at ∼18°C showed no significant quasielastic broadening, which means that any random motions of the alkyl chain are slower than about 5 × 109 rad s-1. Measurements were made at a single temperature in the nematic phases on specimens aligned in a magnetic field of 0·25T; for DMBCA with scattering vector Q⊥n (n is the nematic director) and for 5CB and D5CB with Q⊥n and Q∥n and also on the isotropic liquid phase of D5CB at 45°C. Analysis of the coherent scattering from nematic D5CB at Q = 1·2 A-1 and 25°C gave an order parameter =0·55, close to the simple mean field value for this temperature. The coherent scattering from DMBCA is too weak to allow this experiment to be performed. The most remarkable qualitative feature of the results is the close sim...