The second harmonic of the local deuterium vibration in NbDp 85 has been observed for the first t... more The second harmonic of the local deuterium vibration in NbDp 85 has been observed for the first time. Its energy fico = 170 meV is 7 meV below the harmonic value of twice the lower local-mode energy, 2%co,. . The measured isotope effect for the lowest local-mode energy is co", /cool = 1.365+0.005 which differs from the harmonic value of (MD/MH)'"-= Q2. The temperature dependence of the local modes for both the deuteride and hydride system exhibit pronounced precursor effects prior to the P~a' order-disorder phase transition.
The localized vibrations of hydrogen in various LaNiSH, samples were studied using inelastic neut... more The localized vibrations of hydrogen in various LaNiSH, samples were studied using inelastic neutron scattering (neutron spectroscopy). The pronounced vibrational peaks obtained for "virgin" strain-free a-LaNi,HO.lS indicate the existence of two different hydrogen sites. During the activation for hydrogen absorption, substantial distortion is introduced into the LaNi, lattice which is locally probed by the vibrating hydrogen atoms.
Nb ,oo_,V,H, is a random b.c.c. alloy with a hydrogen sublattice disordered with respect to site ... more Nb ,oo_,V,H, is a random b.c.c. alloy with a hydrogen sublattice disordered with respect to site energies. We have determined the energetically most favourable hydrogen sites by means of neutron vibrational spectroscopy, which is a powerful tool for this purpose when the translational symmetry is strongly reduced. In Nb,,V,H,v, Nb~V,~H~ and Nb,V,H, both octahedral and tetrahedral site occupation was observed.'On the basis of ~peetros~opi~ and statistical arguments we assign the low temperature vibrational peaks in the niobium-rich Nb -V-H alloys at low hydrogen content to Nb,V, octahedral and NbV, tetrahedral sites.
The diffusion rate of positive muons in high purity Cu was measured by means of the zero field mu... more The diffusion rate of positive muons in high purity Cu was measured by means of the zero field muon spin relaxation method for temperatures from 69 mK to 135 K. We found that with decreasing temperature the rate reaches a minimum at around 20 K and increases down to 0.5 K, where it clearly levels off. The origin of the observed temperature dependence is discussed in the light of recently developed theories taking into account the bt+-conduction electron interaction in metals, where the basic coupling constant K is determined to be 0.32.
Real-time SANS and 1H-NMR studies during “living” anionic polymerization of butadiene in hydrocarbon media
Physica B: Condensed Matter, 2004
Small angle neutron scattering (SANS) and high-resolution proton nuclear magnetic resonance spect... more Small angle neutron scattering (SANS) and high-resolution proton nuclear magnetic resonance spectra (1H-NMR) studies have been carried out in situ on the organolithium polymerization of butadiene in hydrocarbon media. The time resolved NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The combination of SANS and NMR methods offers a new way to characterize the aggregation
Tuning of structure and kinetics of chain exchange in star-like PEP-PEO block copolymer micelles
Physica B: Condensed Matter, 2004
Abstract A study of structural and dynamical properties of star-like polymeric micelles is presen... more Abstract A study of structural and dynamical properties of star-like polymeric micelles is presented. The micelles are formed by highly asymmetric poly(ethylene-alt-propylene)–poly(ethylene oxide)(PEP1–PEO20)(the numbers indicate the molecular weight in kg/mole) block copolymers. Using small angle neutron scattering (SANS) we have studied the global aggregation behaviour of these micelles in different water/dimethyl formamide (DMF) solvent mixtures. Using Pendant drop tensiometry as a complementary technique we measured the interfacial tension, γ, between the insoluble PEP block and the corresponding solvent mixtures. The results reveal a good agreement between the proposed power law valid for star-like micelles: P∼γ6/5, where P is the aggregation number. Furthermore, we demonstrate using a special time-resolved SANS technique that the kinetics of chain exchange can be studied. The kinetics was found to be very slow; however, it can be tuned by γ into an appropriate time scale so that it can be followed by SANS. The obtained relaxation curves show a multiexponential decay that suggest that the exchange kinetics is quite complex.
Entanglement constraints in polymer melts. A neutron spin echo study
Macromolecules, 1992
The melt dynamics of polyisoprene (PI), saturated polybutadiene (PEB-2), and poly(ethylene-propyl... more The melt dynamics of polyisoprene (PI), saturated polybutadiene (PEB-2), and poly(ethylene-propylene) alternating copolymer (PEP) was studied using neutron spin echo spectroscopy. The experiments on all three polymers show that beyond a characteristic length d and after a crossover time τ e the relaxational density fluctuations within a given chain are strongly impeded. The presence of an intermediate dynamic length scale in the chain dynamics establishes the microscopic existence of a well-defined entanglement distance confirming thereby one of the essential assumptions of the reptation concept
The fast relaxation process near the glass transition in amorphous polymers with different microstructure
Journal of Non-Crystalline Solids, 1994
ABSTRACT
Segmental mobilities in an athermal diblock copolymer melt far above T g by incoherent neutron scattering
The Journal of Chemical Physics, 1996
We report on incoherent elastic and quasielastic neutron scattering from a symmetric poly(isopren... more We report on incoherent elastic and quasielastic neutron scattering from a symmetric poly(isoprene-1,2 butadiene) (PIP-PVE) almost athermal diblock copolymer at different wave vectors Q in the range 0.18 to 1.8 Å−1. A fixed window temperature scan between 20 K and 300 K revealed the presence of segmental dynamics above 200 K and hence line shape analysis of the incoherent quasielastic spectra Sinc(Q,ω) was performed at high temperatures far above the single glass transition Tg(=221 K) to obtain the characteristic time scales. A two step nonexponential relaxation function was found adequate to describe consistently Sinc(Q,ω) with the faster relaxation time being comparable with the segmental relaxation time in bulk PIP at similar Q. This dynamic incompatibility may arise from dynamic heterogeneities as a consequence of composition fluctuations. The observation of a bimodal relaxation distribution function in PIP–PVE with low Tg contrast at temperatures far above the single Tg emphasi...
Mobility of muons in Cu below 2 K
Hyperfine Interactions, 1984
ABSTRACT
Neutron Spin Echo Study of Membrane Undulations in Lipid Multibilayers
Europhysics Letters (EPL), 1993
The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (... more The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (19 weight % of water; water thickness dw ≈ 10 ) was studied by spin echo spectroscopy in the wave vector range 0.03 -1 < q < 0.125 -1, i.e. at qdw ⩽ 1. The linewidth of the excitations exhibits a scaling law Γq ∝
Zeitschrift f�r Physik B Condensed Matter and Quanta, 1978
We have made a model calculation for the decay of phase coherence of muon spin rotation in the ca... more We have made a model calculation for the decay of phase coherence of muon spin rotation in the case of muon diffusion in the presence of traps. First we have developed a random-walk theory of the decay function P(t), for diffusion in homogeneous systems, and obtained an explicit expression in the frequency domain previously found by Kubo. We have derived P(t) in the time domain by numerical inversion of the Laplace transform applying the recently developed Korrektur method. The result coincides with the usual Gaussian-Markovian theory for slow and fast modulation, and deviates at most by 8~ in the intermediate region. We have extended our theory to a two-state description for capture and release processes on traps and found an analytic expression equivalent to the NMR lineshape I(co). The results for the decay function agree with experimental results for muon diffusion in metals with traps. Finally we have compared our theory with approximate calculations of the decay function and the damping rate.
We investigated the dynamics of 18-arm polyisoprene star polymer solutions well above their overl... more We investigated the dynamics of 18-arm polyisoprene star polymer solutions well above their overlap concentration c * . Combining neutron spin echo spectroscopy (NSE) and selective H/D labelling, we were able to separate inter-(collective) and intra-star (self ) dynamics. Only at low Q-vectors do self and collective dynamics become discernible. Here, collective dynamics are found to be consistent with a colloidal approach resulting from star-star interactions. The collective short time diffusion coefficient D eff is well described by the term D 0 /S(Q), with D 0 the diffusion coefficient at infinite dilution. At Q m , the peak position in the structure factor S(Q), no difference is observable between collective and self dynamics. For covering the slowed-down dynamics at Q m the time range of NSE was extended for the first time up to 350 ns using long wavelengths, λ = 19 Å, at IN15 (ILL, Grenoble). We found that S(Q, t)/S(Q, 0) relaxes into a concentration-dependent plateau. The plateau height gives the mean-square displacement < r 2 > 1/2 of star cores, which is related to the blob size ξ of the surrounding star polymer solution obtained by dynamic light scattering.
By means of quasielastic neutron scattering we show that the H-diffusion mechanism in Pd85Si~5H75... more By means of quasielastic neutron scattering we show that the H-diffusion mechanism in Pd85Si~5H75 is characterized by two well-separated regimes of jump rates, in clear contradiction to present ideas on diffusion in amorphous substances. A semiquantitative interpretation of the data is possible in terms of a two-state diffusion-trapping model. %e find no evidence for the hypothesis that the network of diffusive paths in an amorphous substance has a fractal dimension.
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