TOFTOF: Cold neutron time-of-flight spectrometer

TOFTOF, operated by the Technische Universität München, is a direct geometry disc-chopper time-of-flight spectrometer located in the Neutron Guide Hall West. It offers an excellent signal-to-background ratio, high energy resolution and high neutron flux. Adaptable for a wide range of sample environments, TOFTOF is ideal for investigations of fundamental concepts and challenges in physics and materials science

Dynamics of tetrahydrofuran as minority component in a mixture with poly(2-(dimethylamino)ethyl methacrylate): A neutron scattering and dielectric spectroscopy investigation

We have investigated a mixture of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and tetrahydrofuran (THF) (70 wt. % PDMAEMA/30 wt. % THF) by combining dielectric spectroscopy and quasielastic neutron scattering (QENS) on a labelled sample, focusing on the dynamics of the THF molecules. Two independent processes have been identified. The >fast> one has been qualified as due to an internal motion of the THF ring leading to hydrogen displacements of about 3 Å with rather broadly distributed activation energies. The >slow> process is characterized by an Arrhenius-like temperature dependence of the characteristic time which persists over more than 9 orders of magnitude in time. The QENS results evidence the confined nature of this process, determining a size of about 8 Å for the volume within which THF hydrogens' motions are restricted. In a complementary way, we have also investigated the structural features of the sample. This study suggests that THF molecules are well dispersed among side-groups nano-domains in the polymer matrix, ruling out a significant presence of clusters of solvent. Such a good dispersion, together with a rich mobility of the local environment, would prevent cooperativity effects to develop for the structural relaxation of solvent molecules, frustrating thereby the emergence of Vogel-Fulcher-like behavior, at least in the whole temperature interval investigated.Financial support from the Project Nos. MAT2012-31088 (Spanish-MINECO and EU) and IT-654-13 (Basque Government) is acknowledged. This work is based on experiments performed at TOFTOF and DNS (Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany) and has been supported by the European Commission under the 7th Framework Programme through the “Research Infrastructures” action of the “Capacities” Programme, NMI3-II Grant No. 283883.Peer Reviewe

A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth borohydrides LiBH 4 and Mg(BH 4 ) 2 and the mixture LiBH 4 + Mg(BH 4 ) 2

Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH₄)₂ in the α- and β-modifications, LiBH₄ in the low and high temperature crystal structure, and an 1 : 1 molar mixture of LiBH₄ + α-Mg(BH₄)₂. All investigated compounds show a rich dynamic behaviour below an energy range of ΔE = 10 meV with the superposition of rotational dynamics of the constituent [BH₄]¯ anions and low lying lattice modes. For Mg(BH₄)₂, the rotational diffusion of the [BH₄] units was found to be much more activated in the metastable β-polymorph compared to the α-phase, and the low lying lattice modes are even softer in the former crystal structure. In Mg(BH₄)₂, the structural phase transition is mainly governed by the lattice dynamics, while alkaline LiBH₄ exhibits a transition of the [BH₄] rotations around the phase transition temperature. Ball milled LiBH₄ + α-Mg(BH₄)₂ remains a physical mixture of the parent compounds and each component retains its characteristic dynamic signature up to the melting temperature

Influence of solvent on poly(2-(dimethylamino)ethyl methacrylate) dynamics in polymer-concentrated mixtures: a combined neutron scattering, dielectric spectroscopy and calorimetric study

We have investigated the dynamical processes-α-relaxation, local motions of the side-groups, and methyl group rotations-in poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in the dry state and in mixtures (at 70 wt% polymer concentration) with tetrahydrofuran (THF) and water, to address the question as to how these polymer motions are affected by plasticizers interacting in different ways with the polymer. Differential scanning calorimetry, dielectric spectroscopy, and neutron scattering techniques on labeled samples (with deuterated solvents to isolate the signal of the polymer component) have been combined. The α-relaxation is drastically affected, with similar shifts of the glass-transition temperature for both solvents. Effects of compositional heterogeneities and reduction of the fragility are also observed. On the contrary, methyl-group dynamics are unaffected by the presence of solvent. Regarding side-group local motions (β-relaxation), two kinds of components-a slow and a fast one-could be identified in the dry state. On the basis of the spatial information provided by neutron scattering, a model for the geometry of the motions involved in the fast component has been proposed. Adding solvent, this process would remain essentially unaltered, but the population involved in the slower one would be reduced. With THF as solvent, this reduction would be complete, but with water it would be only partial. This could be attributed to rather heterogeneous distribution of water molecules in the polymer likely associated with the presence of water clusters. Such a scenario would also explain the much more pronounced broadening of the glass-transition region observed for the polymer in the aqueous mixture with respect to that induced by THF.Financial support from the Projects MAT2012-31088 (Spanish MINECO and EU) and IT-654-13 (Basque Government) is acknowledged. This work is based on experiments performed at FOCUS (SINQ, Paul Scherrer Institute, Villigen, Switzerland), and at TOFTOF and SPHERES (Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany), and has been supported by the European Commission under the seventh Framework Programme through the “Research Infrastructures” action of the “Capacities” Programme, NMI3-II Grant Number 283883.Peer Reviewe

Self-diffusion in single component liquid metals: a case study of mercury

We report the temperature dependent atomic dynamics in mercury investigated with quasi-elastic neutron scattering between 240 and 350 K. The self-diffusivity follows an Arrhenius behavior over the entire investigated temperature range, with an activation energy of 41.8 ± 1.4 meV. The standard deviation is in the order of 5%, significantly more precise than previously reported measurements in the literature. Similar to alkali metal melts, the self-diffusion coefficient close to the melting point can be predicted with an effective atom radius of 1.37 Å. This shows a dominant contribution from the repulsive part of the interatomic potential to the mass transport. We observed deviations from the Stokes/Sutherland–Einstein relation and indications of an increasing collective nature of the dynamics with decreasing temperature. Thus, a transport mechanism of uncorrelated binary collisions cannot fully describe the temperature dependence of the self-diffusion

Probing Ab-/desorption of Deuterium in Hydrogen Storage Materials with Neutron Diffraction In-situ

Solid state hydrogen storage is an attractive option for a clean energy carrier. Neutron diffraction during deuterium absorption/desorption offers the unique opportunity of probing in real time and under experimental conditions the phase transformation behavior of the investigated compounds. Deuterium sorption of mixtures of magnesium amide (MgND2)2 and lithium hydride (LiD) were investigated at 220°C and different deuterium pressures up to 70 bars. An intermediate reaction step was identified and the structures of the corresponding phases were refined. The results are important for optimizing this promising hydrogen storage system.JRC.DDG.F.2-Cleaner energ

Multiscale water dynamics in model Anion Exchange Membranes for Alkaline Membrane Fuel Cells

Ionic conductivity and water transport through alkaline Anion Exchange Membranes (AEM) are key properties for application in Alkaline Membrane Fuel Cells (AMFC), Redox-Flow-Cells, or Alkaline Membrane Electrolysis. AEMs consist of a polymer domain with cationic side groups and a pervading water domain through which anions are conducted. In this study, Quasielastic Neutron Scattering (QENS) is employed to study water rotational and diffusive dynamics in the hydroxide form of a model AEM (Fumatech FAA-3) at water contents relevant for application. Two distinct diffusion time- and length-scales are accessed: “Localized” diffusion at the lower Ångstrom/tens of picosecond scale and “extended” diffusion at the higher Ångstrom/hundreds of picosecond scale. The localized diffusion length scale is smaller than the membrane's water domain thickness and its diffusion coefficients remain close to the value of bulk water even at decreasing water content. Extended diffusion approaches the thickness of the water domain and its diffusion coefficients decrease strongly with decreasing water content. A master curve links water domain thickness to the extended water diffusion coefficients for the alkaline hydrocarbon model AEM and literature data on acidic per-fluorinated Proton Exchange Membranes

Homogeneous and heterogeneous dynamics in native and denatured bovine serum albumin

A characteristic property of unfolded and disordered proteins is their high molecular flexibility, which enables the exploration of a large conformational space. We present neutron scattering experiments on the dynamics of denatured and native folded bovine serum albumin (BSA) in solution. Global protein diffusion and internal macromolecular dynamics were measured using quasielastic neutron time-of-flight and backscattering spectroscopy on the picosecond to nanosecond time- and Ångstrom length-scale. Internal protein dynamics were analysed in a first approach using stretched exponential functions. In denatured BSA predominantly slow heterogeneous dynamics dominates the observed macromolecular motions. Reduction of disulphide bridges in denatured BSA does not significantly alter the visible motions. In native folded BSA fast homogeneous dynamics and slow heterogeneous dynamics were observed. In an alternative data analysis approach, internal protein dynamics was interpreted using the analytical model of the overdamped Brownian oscillator, which allowed us to extract mean square displacements of protein internal dynamics and the fraction of hydrogen atoms participating in the observed motions. Our results demonstrate that denaturation modifies the physical nature of internal protein dynamics significantly as compared to the native folded structure