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.     

Investigating one-dimensional diffusion by quasielastic neutron scattering: a theoretical approach

Scattering functions and full widths at half maximum for quasielastic neutron scattering (QENS) are calculated for diffusion in systems of one-dimensional channels. The self-correlation function for diffusion in isotropically oriented channels is given and it is found that this function diverges at the origin. The calculations are carried out for both normal and single-file diffusion and the influence of the ballistic phase is investigated. It is found that the ballistic phase influences the scattering functions very strongly for large diffusion coefficients. QENS data from the literature are analyzed with respect to this influence. The influence of three different resolution functions (triangular, Gaussian, and Lorentzian) is considered.     

Sodium self-diffusion coefficient in sodium silicate glass by quasielastic neutron scattering

Using pulsed neutrons of 19.8 Å wavelength a quasielastic line broadening as low as 0.03 eV (FWHM) has been observed due to Na⁺ diffusion in the glass Na₂O·2SiO₂. From the linewidths a Na⁺ self-diffusion coefficient of 3.1·10^–8 cm²/s at 420°C was obtained in excellent agreement with the diffusion coefficient determined for the same sample batch using²²Na radioactive tracers. The experimental Q dependence of the quasielasic linewidths gives a hint for deviations from a purely random walk in an ionic glass.     

Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

Quasielastic neutron scattering (QENS) has been applied to study the sodium mobility on nanosecond time scales in the perovskite fluoride cryolite, Na₃AlF₆, at high temperatures. Up to T = 1153 K the diffusion of Na ions is well described by a diffusion process of jumps between six and eight-fold coordinated sites. Above this temperature, where a step-like increase in the electrical conductivity occurs, the jump length increases, which indicates additional jumps over larger distances. The electrical conductivity derived from the self-diffusion coefficient via the Nernst–Einstein relation and the corresponding activation energy are in excellent agreement with the previous conductivity measurements. We conclude that the jump diffusion of sodium ions is the dominant mechanism for the electrical conductivity in cryolite at high temperatures up to T = 1153 K.     

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

The anomalous Debye-Waller factor of hydrogen in niobium as determined by quasielastic neutron scattering

The integrated intensity of quasielastic neutron scattering by protons in polycrystalline NbH_0.16 and in a single crystal of NbH_0.045 was investigated as a function of the scattering vector Q. Strong deviations from a harmonic Debye-Waller factor behavior were observed at elevated temperatures. The results show a temperature dependent delocalization of the proton extending as far as the neighboring sites of the interstitial lattice. Experiments on the single crystal indicate a directional dependent mean-square amplitude of the proton even at room temperature.     

Investigation of diffusion processes in liquid sodium and sodium-hydrogen melt by quasielastic neutron scattering

The quasielastic neutron scattering experiments on liquid sodium (at T = 378, 573, and 693 K) and sodiumhydrogen melt (T = 693 K, hydrogen concentration ～ 0.4 at %) were performed with the DIN-2PI time-of-flight spectrometer. The characteristics of the diffusion mobility for particles comprising the liquids studied are extracted from the experimental results and analyzed with the help of the phenomenological and “relaxing cage” models. The self-diffusion coefficient in liquid sodium obtained for all temperatures is in the agreement with the values known from literature. The diffusion mobility in pure sodium and in sodium-hydrogen melt were found to be coinciding; it leads to the conclusion that in our experimental conditions hydrogen exists and diffuses in melt in the form of hydride NaH.     

Effect of dispersion state of carbon nanotube on the thermal conductivity of poly(dimethyl siloxane) composites

Herein, the effect of dispersion uniformity of multi-walled carbon nanotube (MWCNT) on the thermal conductivity of poly(dimethyl siloxane) (PDMS) composites was investigated by comparing experimentally obtained and calculated results based on simple models. Two different MWCNTs, i.e., raw and oxidized/masterbatched MWCNTs, were used and compared. For raw MWCNT, the dispersion in PDMS was poor, resulting in the significant reduction in the aspect ratio of MWCNT. However, for composites using masterbatched MWCNT, the thermal conductivity was always about 10% greater than those prepared with raw MWCNT and the aspect ratio calculated by the model equation was also 1.7 times greater. Above 1.5 phr masterbatched MWCNT concentrations, the aspect ratio of 430 was maintained. Finally, the results suggest that the thermal conductivity can be correlated with the degree of dispersion and aspect ratio obtained from the model equation used.     

Zero- and first-sound damping in NaF observed by inelastic neutron scattering

The frequency and temperature dependence of the damping of transverse acoustic modes with 0.06 ? q/q_max ? 0.25 in NaF was determined by means of inelastic neutron scattering. The results clearly show a transition from the Akhiezer to the Landau-Rumer regime.     

Collective excitations in solid hydrogen as observed by incoherent neutron scattering

Incoherent neutron scattering experiments are reported at a number of scattering angles for solid H₂ using the time-of-flight technique. The samples had an ortho concentration ofX0.91 and the temperatures were 4.2, 2.2 and 1.2 K. The recorded scattering functions exhibit the elastic line, the ortho-para conversion line, the phonon spectrum on the energy-gain side of the neutrons and finally the phonon spectrum on the energy-loss side. Analysis of the data shows multiple scattering to be very important especially at small scattering angles. Accordingly, the results of an extensive calculation are reported that include up to four-fold scattering. The phonon spectrum that brings calculation and experiment in agreement at all angles is presented. It is concluded from the comparison between the spectra at various temperatures that unexpectedly the samples have always remained in the hexagonal close-packed phase even below the usual transition temperature into the cubic phase. Comparison is made with the spectrum (also in thehcp phase) obtained from coherent neutron scattering. The high-energy phonon tail, to be expected for quantum crystals, is observed and is qualitatively consistent with that in a similar spectral density derived theoretically from ortho-para conversion data under pressure. No evidence of a libron spectrum atT=2.2 K for a sample withX=0.91 could be found, which is consistent with the absence of the cubic orientationally ordered phase in these experiments.Supported in this research by a grant from the National Science Foundation     

Ultra-small-angle neutron scattering studies on phase separation of poly(vinyl alcohol) gels

Time-resolved light scattering measurements during the gelation process of a poly(vinyl alcohol) (PVA) solution in a mixture of dimethyl sulfoxide and water (H2O) have shown that a spinodal decomposition (SD) type phase separation takes place in the early stage of gelation. In this case the kinetics of SD is valid only before macroscopic gelation occurs because the growth rate is slowed down by gelation. Such SD type phase separation makes the solution opaque as it proceeds, and hence the structural change can no longer be followed by light scattering. To investigate the structure of the opaque PVA gel as well, we have employed an ultra-small-angle neutron scattering technique using a Bonse-Hart camera. These observations reveal that even after the macroscopic gelation the structure due to the microphase separation on a spatial scale of several &mgr;m continues to grow against the elasticity. This may be because at first the gel structure is too soft to suppress the growth of the microphase separation, but within 24 h after the quenching the growth terminates. On the basis of the results, we will discuss a possible mechanism of the microphase separation after gelation.     

Lattice dynamics of cubic PbTiO3 by inelastic neutron scattering

High-temperature dispersion relations of the phonon modes in a cubic PbTiO₃ single crystal have been investigated along the [ξ 0 0] and [ξ ξ 0] directions by inelastic neutron scattering. Above T _c, the phonon dispersions are only temperature-dependent close to the Brillouin zone centre where the mode softening comes through. The measurements indicate large cubic anisotropy of the elastic tensor and relatively low anisotropy of the soft mode dispersion. The differences from an earlier inelastic neutron scattering study are discussed.     

Collective dynamics of lipid membranes studied by inelastic neutron scattering

We have studied the collective short wavelength dynamics in deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayers by inelastic neutron scattering. The corresponding dispersion relation variant Planck's over 2pi omega(Q) is presented for the gel and the fluid phase of this model system. The temperature dependence of the inelastic excitations indicates a phase coexistence between the two phases over a broad range and leads to a different assignment of excitations from that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. 86, 740 (2001)]]. As a consequence, we find that the minimum in the dispersion relation is actually deeper in the gel than in the fluid phase. Finally, we can clearly identify an additional nondispersive (optical) mode predicted by molecular dynamics simulations [Phys. Rev. Lett. 87, 238101 (2001)]].     

Conformation of poly(L-lysine)-graft-poly(ethylene glycol) molecular brushes in aqueous solution studied by small-angle neutron scattering

Small-angle neutron scattering (SANS) has been employed for the analysis of conformations of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g -PEG) molecular bottle brushes in aqueous solutions. The degree of polymerisation of the PEG chains was systematically varied in order to unravel dependence of the conformational properties of the bottle brushes on the molecular weight of the grafted chains. The grafting density was kept constant and high enough to ensure strong overlap of the PEG chains. The scattering spectra were fitted on the basis of the model of an effective worm-like chain with the account of average radial distribution and local fluctuations of the PEG density in the bottle brush. The results of the fits indicate that molecular brushes retain weakly bent configuration on the length scale of the order of (or larger than) the brush thickness. This finding is in agreement with earlier simulation and recent theoretical results.