Quasi-elastic Neutron Scattering Measurements Research Articles

Fast and Slow Dynamics of Isotactic Polypropylene Melts

We report a quasi-elastic neutron scattering (QENS) study of local dynamics in isotactic polypropylene (iPP) melts. QENS measurements were carried out on the spectrometer IRIS (ISIS, UK), using a series of selectively deuterated iPP samples. Analysis of the intermediate incoherent scattering functions was conducted using two different models: (a) a two-component model involving a fast process and conformational transitions and (b) a three-component model including methyl group reorientations. We show that consistent results between the selectively deuterated samples are only obtained when methyl group reorientations are explicitly taken into account. The results presented here are consistent with the simulated intermediate scattering functions of atactic polypropylene [Ahumada, O.; Theodorou, D. N.; Triolo, A.; Arrighi, V.; Karatasos, C.; Ryckaert, J.-P. Macromolecules 2002, 35, 7110] which revealed contributions from torsional transitions of the methyl groups and segmental motion, in addition to a fast process.

Molecular dynamics of the self-organising strong hydrogen bonded 3,5-dimethylpyrazole

The family of pyrazoles containing only H and CH3 substituents displays a wide variation in physical properties which can be directly related to the manner in which the molecules self-organise in the solid state. Hydrogen-bonded multimeric motifs of the substituted pyrazoles are a recurring feature of this family. We have previously reported the use of quasielastic neutron scattering (QENS) to study 3,5-dimethylpyrazole which showed that the hydrogen-bonded amide protons within individual trimer units undergo a short range hopping motion between two equivalent sites straddling the direct N⋯H hydrogen-bond axis. This work was the first report of such a novel dynamic process in this family of materials. The current work extends the earlier study with additional QENS measurements of other isotopic variants, providing information on the methyl group dynamics, which lead us to the conclusion that the short-range amide motions are decoupled from the methyl torsions. Whilst the methyl groups were found to undergo 3-fold diffusive hopping motions on the QENS timescale (∼1011–1212 s−1), an appreciable non-mobile fraction of methyl groups was also detected at all temperatures studied.

α,α-Trehalose−Water Solutions. VIII. Study of the Diffusive Dynamics of Water by High-Resolution Quasi Elastic Neutron Scattering

The present paper shows high-resolution quasi-elastic neutron scattering (QENS) findings on homologues disaccharides (i.e. trehalose, maltose, and sucrose)-water mixtures as a function of temperature. The QENS measurements were performed on both partially deuterated disaccharides in D2O and on hydrogenated disaccharides in H2O to separate the solute dynamics from that of the solvent. The results highlight a noticeable disaccharide kosmotrope character, with results more marked for trehalose. Such evidence accounts for its higher bioprotective effectiveness.

Molecular view of water dynamics near model peptides.

Incoherent quasi-elastic neutron scattering (QENS) has been used to measure the dynamics of water molecules in solutions of a model protein backbone, N-acetyl-glycine-methylamide (NAGMA), as a function of concentration, for comparison with results for water dynamics in aqueous solutions of the N-acetyl-leucine-methylamide (NALMA) hydrophobic peptide at comparable concentrations. From the analysis of the elastic incoherent structure factor, we find significant fractions of elastic intensity at high and low concentrations for both solutes, which corresponds to a greater population of protons with rotational time scales outside the experimental resolution (>13 ps). The higher-concentration solutions show a component of the elastic fraction that we propose is due to water motions that are strongly coupled to the solute motions, while for low-concentration solutions an additional component is activated due to dynamic coupling between inner and outer hydration layers. An important difference between the solute types at the highest concentration studied is found from stretched exponential fits to their experimental intermediate scattering functions, showing more pronounced anomalous diffusion signatures for NALMA, including a smaller stretched exponent beta and a longer structural relaxation time tau than those found for NAGMA. The more normal water diffusion exhibited near the hydrophilic NAGMA provides experimental support for an explanation of the origin of the anomalous diffusion behavior of NALMA as arising from frustrated interactions between water molecules when a chemical interface is formed upon addition of a hydrophobic side chain, inducing spatial heterogeneity in the hydration dynamics in the two types of regions of the NALMA peptide. We place our QENS measurements on model biological solutes in the context of other spectroscopic techniques and provide both confirming as well as complementary dynamic information that attempts to give a unifying molecular view of hydration dynamics signatures near peptides and proteins.

Influence of extra-framework cations on the diffusion of alkanes in silicalite: Comparison between quasi-elastic neutron scattering and molecular simulations

Quasi-elastic neutron scattering (QENS) has been used to study the diffusion of n-octane in silicalite. A larger diffusivity is found when compared with previous experiments performed in Na–ZSM-5, by a factor of 4. MD simulations have been performed for n-butane, n-hexane, and n-octane in silicalite and in a model zeolite with atomic defects to mimic the steric hindrance due to extra-framework cations in ZSM-5. The new QENS measurements and the MD simulations explain to a large extent the discrepancy previously observed for n-octane in MFI-type zeolites.

Quasi-elastic Neutron Scattering Measurements of Liquid Germanium

Quasi-elastic neutron scattering measurements of liquid germanium were carried out at 1000 and 1500°C, using the inverted geometry inelastic neutron spectrometer, LAM-40. The dynamic structure factor, S(Q,E), is obtained over a wide momentum transfer

Transport Diffusivity of N2 and CO2 in Silicalite: Coherent Quasielastic Neutron Scattering Measurements and Molecular Dynamics Simulations

A direct comparison of experimental measurements (coherent quasielastic neutron scattering) and simulation (molecular dynamics) is carried out for the first time to investigate the concentration dependence of transport diffusivity of nitrogen and carbon dioxide in silicalite-1. Theoretical elaboration of the simulation results on the basis of the quasichemical mean field approximation and a model of surface diffusion due to D. A. Reed and G. Ehrlich (Surf. Sci. 1981, 102, 588) leads to the conclusion that the corrected (Darken) diffusivity D0 can exhibit significant dependence on the loading θ of the sorbed phase, varying with the strength of interactions between the sorbate molecules. As a consequence of sorbate−sorbate interactions being considerably more attractive in CO2/silicalite-1 than in N2/silicalite-1, D0(θ) is a decreasing function in CO2/silicalite-1 but displays a weak maximum in N2/silicalite-1.

Local Dynamics of Polyethylene and Its Oligomers: A Molecular Dynamics Interpretation of the Incoherent Dynamic Structure Factor

We present a detailed study of the local dynamics of short polyethylene (PE) chains using molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) data. QENS measurements were carried out on two samples having different chain lengths: C44H90 and PE2K (Mw = 2150 g/mol). The incoherent dynamic structure factors obtained from experiments at T = 450 K and MD simulations carried out at the same temperature are compared in the range 0.5 ≤ Q ≤ 2.0 A-1. Agreement between experimental data and simulations is quantitative. Attempts are made to characterize the momentum transfer and the chain length dependence of the I(Q,t) and S(Q,ω) data on the basis of the Kohlrausch−Williams−Watts (KWW) function and two exponentials relaxation functions in time. As MD simulation data cover a broader frequency window and have a very low statistical noise, a more refined analysis of the intermediate scattering functions in terms of a continuous linear combination of exponentials weighted by a distribution o...

Water diffusion in fully hydrated porcine stratum corneum

The microscopic mechanisms of water diffusion in fully hydrated porcine stratum corneum (SC) have been studied by a combination of incoherent quasielastic neutron scattering (QENS) and pulsed field gradient-nuclear magnetic resonance (PFG-NMR) for two sample orientations. The presence of three types of water in fully hydrated SC is inferred on the basis of water sorption isotherm data, i.e., (a) bound and (b) weakly bound hydration water forming layers between adjacent lipid bilayers of SC, as well as (c) bulk water probably located in the corneocytes and in intercellular regions. Water self-diffusion coefficients for motions parallel and perpendicular to the membrane plane of D∥=3.30×10−10 m2/s and D⊥=1.56×10−10 m2/s, respectively, were determined by PFG-NMR and assigned to the translational diffusion of weakly bound water. QENS measurements have been carried out using different samples hydrated with H2O and D2O, respectively, in order to separate the contribution of SC from that of the water. The QENS data for both sample orientations and two different energy resolutions can be fitted by a model which accounts for the microscopic dynamics of all three aforementioned types of water. This analysis establishes rotational diffusion coefficients for bound and weakly bound hydration water of 0.025 and 0.030 meV, respectively. Furthermore, the QENS data prove the presence of bulk water in fully hydrated SC samples.

Effect of tacticity on the local dynamics of polypropylene melts

We present a quasielastic neutron scattering (QENS) study of the effect of tacticity on the local dynamics of polypropylene (PP). QENS measurements were carried out on different spectrometers. On IN10 (ILL, France) we have measured the decrease of the elastic intensity as a function of temperature for atactic (a-PP), isotactic (i-PP), and syndiotactic (s-PP) PP. The results show that the polyproylene sub-Tg dynamics is independent of tacticity. Measurements of the dynamic incoherent structure factor were carried out on the spectrometers IRIS and OSIRIS (ISIS, UK) and, after Fourier transform, the intermediate scattering functions were computed and analyzed. For all samples investigated, the intermediate scattering functions I(Q,t) show good overlap using shift factors that are close to those reported in the literature. Detailed analysis of the incoherent dynamic structure factor in terms of fast and slow decay processes indicates that in the subpicoseconds regime molecular motion is independent of tacticity. The slower segmental process depends on the sample stereoregularity and, consistently with C13 NMR measurements and molecular dynamics simulations, isotactic PP relaxes faster than the other polymers, while s-PP is the slowest. Correlation times display a non-Arrhenius temperature dependence that is described by a Vogel–Fulcher–Tamman relationship, with parameters that depend on tacticity. Thus the ratio between the correlation times is temperature dependent, and while the dynamic behavior of the samples is very similar at 460 K, considerable differences are observed at lower temperature.

Rotation of propane molecules in supercages of Na–Y zeolite

Rotational dynamics of propane in Na–Y zeolite at room temperature is studied by molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) measurements. It is found that propane undergoes very fast rotational motion in the cages of Na–Y zeolite as first observed in the MD study and later confirmed by the QENS measurements. The dynamics of propane is described by isotropic rotational diffusion on a sphere of radius 1.88 (±0.05) Å, which is also the average distance of the hydrogen atoms from the center of mass of the propane molecule. The rotational diffusion constant as obtained from the QENS and MD simulation is in close agreement. This is the first ever study of the rotational dynamics of propane in a zeolite.

Rotational dynamics of propane in Na-Y zeolite: a molecular dynamics and quasielastic neutron-scattering study.

We report results from molecular dynamics (MD) simulations and quasielastic neutron-scattering (QENS) measurements on the rotational dynamics of propane in Na-Y zeolite at room temperature with a loading of four molecules per alpha cage. Rotational part of the intermediate scattering function F(Q,t) obtained from the MD simulation suggests that rotational motion is faster relative to the translational motion. Various rotational models fitted to the MD data suggest that rotation is isotropic. It is found that the hydrogen atoms lie, on the average, on a sphere of radius 1.88+/-0.05 A, which is also the average distance of the hydrogen atoms from the center of mass of the propane molecule. Results from QENS measurements are in excellent agreement with those obtained from MD, suggesting that the intermolecular potential employed in the MD simulation provides a realistic description of propane motion within faujasite. The rotational diffusion constant D(R) is 1.05+/-0.09 x 10(12) sec(-1) from the QENS data, which may be compared with that obtained from the MD data (0.82+/-0.05 x 10(12) sec(-1)).

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.

Temperature dependence of the segmental dynamics in polyisobutylene melts

We report a study of the local dynamics in polyisobutylene (PIB) melts using quasi-elastic neutron scattering (QENS). QENS measurements were carried out on the high resolution back-scattering spectrometer IRIS (ISIS) in the temperature range 300–460 K. The QENS data were analysed in the time domain, in order to extract information on the α-process in PIB. We show that time–temperature superposition is obeyed. The temperature dependence of the shift factors derived by QENS is in good agreement with 13C NMR and ESR literature data. However, the temperature dependence of the molecular motion observed by QENS differs considerably from the one determined from viscosity and neutron spin echo measurements.

Water mobility in Calico Hills tuff measured by quasielastic neutron scattering

Quasielastic neutron scattering (QNS) was used to measure the mobility of water in tuff at low saturation. The tuff sample is from a surface outcrop of the Calico Hills formation, which lies below the proposed location for a high‐level nuclear waste repository, at Yucca Mountain, NV. The sample is a devitrified tuff composed mainly of zeolites and having clinoptilolite (approximately 70%), mordenite, and opal‐CT as major constituents. The sample is an air dry, intact core sample. QNS measurements were made in the range 200 K to 423 K. Rotational motion of adsorbed water is easily observable and translational motion is observable at higher temperatures. The tuff data are analyzed to determine the rotational diffusion coefficient and the translational diffusion coefficient. The results yield a translational diffusion coefficient in the tuff that is significantly lower than that of bulk water but higher than for water in montmorillonite or vermiculite clays.

Nickel diffusion inB2−NiGastudied with quasielastic neutron scattering

The weak concentration dependence of Ni diffusivity in NiGa has formerly been interpreted as evidence for Ni diffusion via next-nearest-neighbor jumps, which seems reasonable in the light of the comparably high enthalpy of formation found in this alloy. Quasielastic neutron scattering (QNS) at the backscattering spectrometer IN16 at ILL has been used to study the elementary diffusion jump of Ni in NiGa single crystals near the stoichiometric composition ${\mathrm{Ni}}_{50}{\mathrm{Ga}}_{50}$ as well as in polycrystals with $57 \mathrm{at}.% \mathrm{Ni}$ and $62 \mathrm{at}.% \mathrm{Ni}.$ While the weak concentration dependence of the Ni diffusion coefficient has been confirmed over a wide concentration range on the Ni-rich site of the NiGa phase diagram, the diffusive jump of Ni atoms unequivocally turned out to be a jump via nearest-neighbor sites, i.e., antistructure sites. For the near-stoichiometric compositions it was possible to determine the residence time of the Ni atoms on the antistructure sites directly from the QNS measurements. These results indicate very high defect concentrations near the melting temperature.

Orientational effects on low-energy modes in amorphous poly(ethylene terephthalate) fiber

Inelastic and quasielastic neutron-scattering measurements have been performed on an amorphous poly(ethylene terephthalate) (PET) fiber in parallel and perpendicular scattering geometries, i.e., when the fiber axis f is parallel and perpendicular to the scattering vector Q, respectively. The so-called boson peak is observed at around −1.4 meV in neutron energy transfer at low temperatures below about 150 K in both the geometries; as temperature increases, the fast process of picosecond order appears at a certain temperature below the glass-transition temperature Tg(=348 K). Although no drastic differences in the dynamics can be observed, a closer look at the spectra revealed some interesting features of the low-energy modes. From the mean-square displacements evaluated with two different energy resolutions, it was found that the fast process appears only in the direction perpendicular to the polymer chain (not the fiber axis f) in the energy region between −1 and −0.2 meV. In the inelastic scattering spectra, we found that the boson peak is stronger in intensity for the parallel direction while the intensity of the fast process is larger for the perpendicular one. The quantitative analysis based on the recent vibration–relaxation model revealed that the fast process can mainly be explained by the softening of the vibrational modes; the softening occurs at lower temperatures in the perpendicular geometry than in the parallel one. On the other hand, the conventional single-Lorentzian fit to the fast process, in which the fast process is assumed to be a relaxational process and described by a Lorentzian, showed that the onset temperature of the fast process is lower in the perpendicular geometry than in the parallel one. These have been tentatively attributed to weaker force constants, for motions perpendicular to a polymer chain, such as torsional and librational motions, than those along the chain.

A quasi-elastic neutron-scattering study of hydrogen diffusion in C15 Leaves-phase compounds HfV 2H 0.1

Recently, the AB 2 C15 Laves-phase compounds have been of interest for metal hydride batteries due to their ability to absorb large amounts of hydrogen. Quasi-elastic neutron-scattering measurements were performed on the C15 Laves-phase compounds HfV 2H 0.1 using the IN10 spectrometer at the ILL, Grenoble, France. Quasi-elastic broadening was measured for momentum transfers between 0.27 and 1.96 åA −1 in the temperature range 250–380 K. The activation energy obtained from this data is discussed and compared to previous QENS measurements on the same compound for high hydrogen concentration.

Quasi-elastic neutron-scattering measurements on the ZrTi 2H 3.6 C-15 Laves phase compound

Quasi-elastic neutron-scattering measurements (QNS) on the ZrTi 2H 3.6 C-15 Laves phase compound have been carried out on the back-scattering spectrometer IN10 at the Institute Laue Langevin (ILL) in Grenoble. A single Lorentzian has been found to fit the data for all temperatures and Q-values. The Q-dependence of the data can be fitted with a basic Chudley-Elliot model, averaging over all directions due to the polycrystalline nature of the sample, with a jump length of 2.8 Å. This figure is very close to the distance between nearest-neighbors (n.n.) e-sites (1Zr-3Ti), 2.9 Å, showing this site as the preferred one for hydrogen in the C-15 Laves phase ZrTi 2H x .

Raman spectroscopic study of water in the poly(ethylene glycol) hydration shell

Raman scattering spectra of polymeric aqueous solutions of poly(ethylene glycol) vs. concentration and temperature are presented. We show that the shape of the OH band can be reproduced by a suitable superposition of the spectra of bulk and hydration water. The relative amount of the two contributions changes with concentration and temperature, furnishing information on the number of water molecules bonded to the polymer chain. The results show that, when water concentration increases, water molecules saturate the two lone pairs of each oxygen atom of the polymer in agreement with previous adiabatic compressibility and Incoherent Quasi Elastic Neutron Scattering measurements.