[Show abstract][Hide abstract] ABSTRACT: The crystal and molecular structure of p-N,N 0-tetraacetylodiaminodurene (TADD) is reported based on the X-ray diffraction studies. The N-acetyl moieties are planar and all N-acetyl groups are perpendicular to the ring plane. Methyl groups both of acetyl moieties and of durene form a number of non-conventional hydrogen bonds with nitrogen and oxygen atoms. The vibrational spectra very well reflect the structure of molecules and their contacts. They are compared with calculated data by using various theoretical approaches. The neutron scattering spectra show two tunnel lines of low energy values (at ±0.9 and ±2.3 leV at 4 K), which can be ascribed to methyl groups of N-acetyl moieties, which behave more freely than those attached to the phenyl ring.
[Show abstract][Hide abstract] ABSTRACT: We revisit the nonuniversal aspect of polymer dynamics by considering both new and existing data on the zero-shear viscosity and linear viscoelastic response of various polymers, each with a wide range of molecular weights. Analysis of the zero-shear viscosity data in terms of the packing length p, whose role in entanglements has been discussed previously by Fetters and co-workers, reveals a behavior that is irreconcilable with our current understanding based on the tube model. Specifically, we find that the transition regime between Rouse and pure reptation dynamics, currently understood as the regime where contour length fluctuations are active, systematically shrinks as the packing length of the polymer increases. Further, we find that the slope of the loss moduli in the high-frequency wing of the terminal peak of well-entangled systems also decreases from the common −0.25 to −0.125 with increasing p. This is contrary to the single expected value of −0.25 from tube models which include contour length fluctuations or −0.5 from pure reptation. These findings hint on possible missing ingredients in our current understanding of polymer dynamics.
[Show abstract][Hide abstract] ABSTRACT: Large-scale domain motions in alcohol dehydrogenase (ADH) have been observed previously by neutron spin-echo spectroscopy (NSE). We have extended the investigation on the dynamics of ADH in solution by using high-resolution neutron time-of-flight (TOF) and neutron backscattering (BS) spectroscopy in the incoherent scattering range. The observed hydrogen dynamics were interpreted in terms of three mobility classes, which allowed a simultaneous description of the measured TOF and BS spectra. In addition to the slow global protein
diffusion and domain motions observed by NSE, a fast internal process could be identified. Around one third of the protons in ADH participate in the fast localized diffusive motion. The diffusion coefficient of the fast internal motions is around two third of the value of the surrounding D2O solvent. It is tempting to associate the fast internal process with solvent exposed amino acid residues with dangling side chains.
The Journal of Chemical Physics 08/2015; 143(7):075101. DOI:10.1063/1.4928512 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paired helical filaments (PHF) formed by the intrinsically disordered human protein tau are one of the pathological hallmarks of Alzheimer disease. PHF are fibers of amyloid nature that are composed of a rigid core and an unstructured fuzzy coat. The mechanisms of fiber formation, in particular the role that hydration water might play, remain poorly understood. We combined protein deuteration, neutron scattering, and all-atom molecular dynamics simulations to study the dynamics of hydration water at the surface of fibers formed by the full-length human protein htau40. In comparison with monomeric tau, hydration water on the surface of tau fibers is more mobile, as evidenced by an increased fraction of translationally diffusing water molecules, a higher diffusion coefficient, and increased mean-squared displacements in neutron scattering experiments. Fibers formed by the hexapeptide 306VQIVYK311 were taken as a model for the tau fiber core and studied by molecular dynamics simulations, revealing that hydration water dynamics around the core domain is significantly reduced after fiber formation. Thus, an increase in water dynamics around the fuzzy coat is proposed to be at the origin of the experimentally observed increase in hydration water dynamics around the entire tau fiber. The observed increase in hydration water dynamics is suggested to promote fiber formation through entropic effects. Detection of the enhanced hydration water mobility around tau fibers is conjectured to potentially contribute to the early diagnosis of Alzheimer patients by diffusion MRI.
Proceedings of the National Academy of Sciences 05/2015; 112(20):6365-6370. DOI:10.1073/pnas.1422824112 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is well-known that the packing model (Fetters and co-workers) shows the relationship between microscopic chain properties (chain stiffness, monomer bulkiness) with macroscopic rheological behavior via the entanglement molecular weight (Me). However, what is less recognized is that the critical molecular weight (Mc) and reptative molecular weight (Mr) which denote transitions between unentangled (Rouse) and entangled (tube) dynamics and reptation-contour length fluctuations (CLF) to pure reptation dynamics respectively, also show dependence on the packing length (p). In this contribution, we highlight two aspects of this dependence:
1. Based on zero-shear viscosity data for six different polymers, from our own measurements and from literature, we show that there's a packing length dependence of the two crossovers (Mc, Mr) and that the trend is for the two to merge at a certain critical value of the packing length (p*). Further, analysis of the loss moduli (G'') for several well-entangled polymers with different packing lengths reveals a p-dependence of the high-frequency slope of the terminal peak contrary to the predicted value of -1/4 by tube models with CLF.
2. Based on preliminary measurements of the dynamic structure factor by neutron spin echo spectroscopy, we show that the onset for the effect of entanglements on the dynamics (i.e. deviation from Rouse behavior) starts in fact from Me and manifests by suppression of long- wavelength modes. From measurements on a series of polymers with different molecular weights (around Mc) and with different packing lengths, we examine how this deviation from Rouse behavior might proceed differently depending on the packing length of the polymer.
Both aspects hint at possible missing ingredients in the current modeling framework (tube model) for entangled polymer melts, particularly on the understated importance of the packing length (p).
10th Annual European Rheology Conference, Nantes, France; 04/2015
[Show abstract][Hide abstract] ABSTRACT: It is well-known that the transition between unentangled to entangled dynamics occurs at a critical molecular weight (Mc), typically assumed to be twice the entanglement molecular weight (Me). Recently, we have reiterated a previous finding that the ratio between Mc and Me doesn't assume a universal value of 2 for all polymer melts but in fact shows a dependence on the packing length (p). Not withstanding, the physical picture behind the independent existence of Me and Mc remains unclear.
Here, we reinvestigate the problem by probing the microscopic dynamics of polybutadiene melts near the transition by neutron spin echo spectroscopy. We analyze the dynamic structure factor within the framework of the Rouse model with modified mode spectrum. We find that suppression of long-wavelength modes in the spectra already occurs for melts with molecular weight above Me, even if they are still below Mc. We rationalize these results based on earlier ideas on entanglement formation. We also apply this framework in confronting our viscosity data for various polymers.
79th Annual Meeting of the DPG and DPG Spring Meeting, Berlin, Germany; 03/2015
[Show abstract][Hide abstract] ABSTRACT: The mechanism of proton conductivity in high temperature polymer electrolyte fuel cells (HT-PEFCs) has been investigated with macroscopic conductivity measurements and on a microscopic scale with quasielastic neutron scattering techniques. Polybenzimidazole membranes, which are used in HT-PEFCs, are doped with phosphoric acid (PA) to achieve the desired proton conductivity. Neutron spin echo experiments showed that the polymer matrix is very rigid incoherent scattering experiments, but incoherent scattering revealed rather fast diffusion processes, compatible with macroscopic conductivity measurements. The measured diffusion is faster than anticipated from the conductivity of a phosphoric acid doped PBI membrane, but slower than that expected for pure phosphoric acid. Over larger distances the fractal polymer membrane network slows down the locally fast diffusion to the macroscopic values. With elastic fixed window scans on a backscattering spectrometer an activation energy of 7.6 kJ/mol is obtained at typical conditions in an HT-PEFC.
International Journal of Hydrogen Energy 12/2014; 39(36):21657-21662. DOI:10.1016/j.ijhydene.2014.09.018 · 3.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the possibility of using the prompt γ rays emitted by aluminum windows in order to monitor the neutron flux of the beam. A Nal scintillation detector is used to detect the prompt γ rays. No additional material apart from the unavoidable Al windows along the flight path is placed in the beam. The performance of the monitor is compared to that of a standard BF3-monitor placed in the beam. Influences of a magnetic field on the photomultiplier of the Nal monitor is discussed, as well as the influence of activation gammas. At an instrument using a beam chopper the time behaviour is discussed.
Journal of Physics Conference Series 07/2014; 528(1):012038. DOI:10.1088/1742-6596/528/1/012038
[Show abstract][Hide abstract] ABSTRACT: The functional efficacy of colocalized, linked protein domains is dependent on linker flexibility and system compaction. However, the detailed characterization of these properties in aqueous solution presents an enduring challenge. Here, we employ a novel, to our knowledge, combination of complementary techniques, including small-angle neutron scattering, neutron spin-echo spectroscopy, and all-atom molecular dynamics and coarse-grained simulation, to identify and characterize in detail the structure and dynamics of a compact form of mercuric ion reductase (MerA), an enzyme central to bacterial mercury resistance. MerA possesses metallochaperone-like N-terminal domains (NmerA) tethered to its catalytic core domain by linkers. The NmerA domains are found to interact principally through electrostatic interactions with the core, leashed by the linkers so as to subdiffuse on the surface over an area close to the core C-terminal Hg(II)-binding cysteines. How this compact, dynamical arrangement may facilitate delivery of Hg(II) from NmerA to the core domain is discussed.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the thermal behavior, local structure, and dynamics in a system where 25 wt % PEO [poly(ethylene oxide)] linear chains are mixed with 75 wt % PMMA [poly(methyl methacrylate)] soft nanoparticles. Calorimetric and wide-angle X-ray scattering experiments point to a weak penetration of the PEO chains in the nanoparticles, qualifying the mixture as a nanocomposite. Quasi-elastic neutron scattering (QENS) experiments on partially deuterated samples has selectively revealed the component dynamics in the system. The α-methyl group dynamics of PMMA, which fall within the QENS time scale in the temperature range investigated, are hardly affected by the presence of PEO except for hints of a more heterogeneous environment in the nanocomposite than in bulk PMMA. The effects on the dynamics of PEO are more interesting. The observation of dynamics in the microseconds range for the PEO component of the nanocomposite at temperatures at which the calorimetric experiments indicate the freezing of its segmental relaxation provides evidence for confined dynamics below the main glass transition of the mixture—attributable to the effective glass transition of the slow component. A parallel study on an equivalent blend of PEO and linear PMMA chains shows that these effects are independent of the topology of the PMMA. However, well above the effective glass transition of the slow component, the dynamics of PEO differ in both systems. In the linear blend, PEO segments move with the typical features of supercooled polymers in metastable equilibrium, while in the nanocomposite PEO dynamics exhibit an anomalously strong deviation from Gaussian behavior. This deviation grows with increased mobility of the nanoparticles. PEO segments are seemingly trapped in effective cages imposed by the nanoparticles for a very long time—more than 2 orders of magnitude longer than in bulk or when surrounded by linear PMMA chains—before the subdiffusive process leading to segmental relaxation sets in. We speculate that local loops in the surface of the nanoparticles may play an important role in this trapping mechanism.
[Show abstract][Hide abstract] ABSTRACT: We have selectively studied the component dynamics in a nanocomposite where 25 wt % of PMMA [poly(methyl methacrylate)] soft nanoparticles (SNPs) are dispersed in PEO [poly(ethylene oxide)] by means of quasi-elastic neutron scattering (QENS) experiments on partially deuterated samples. We have covered a time range from subpico to nanosecond regime and a momentum transfer range 0.5 ≤ Q ≤ 1.8 Å–1 by combining three different spectrometers. Complementary diffraction measurements with polarization analysis have facilitated the data analysis, by providing the coherent and incoherent contributions to the scattered intensities. Regarding the SNPs, the α-methyl group dynamics of PMMA—to which the QENS experiments are most sensitive in the temperature range investigated—turn to be faster than in bulk PMMA. This could be due to the plasticization effect induced by the fast PEO chains. In fact, calorimetric measurements show the coexistence of two glass-transition temperatures in the system, associated with each of the components, but modified with respect to those in the neat materials. The QENS results on the PEO component for large length scales reveal Rouse-like dynamics slowed down by the presence of the SNPs with respect to that in the bulk. With decreasing temperature indications for distributed chain mobilities are found, probably due to the enhancement of the concentration fluctuations. At local scales, deviations from Rouse-like dynamics occur, that could be attributed to an extra-friction related to the local potentials, and also to non-Gaussian effects arising from the discrete character of the elementary processes underlying the subdiffusive dynamics in the polymer. The deviations take place in a very similar way as in bulk PEO.
[Show abstract][Hide abstract] ABSTRACT: The resolution of neutron backscattering spectrometers deteriorates at small scattering angles where analyzers deviate from exact backscattering. By reducing the azimuth angle range of the analyzers, the resolution can be improved with little loss of peak intensity. Measurements at the spectrometer SPHERES are in excellent agreement with simulations, which proves the dominance of geometric effects.
The Review of scientific instruments 11/2013; 84(11):115108. DOI:10.1063/1.4831815 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By means of quasielastic neutron scattering we have investigated the hydrogen dynamics in poly(alkylene oxide)s (PAOs) with different side-chain lengths at temperatures below as well as above the glass-transition. The combination of results from three different spectrometers (a time-of-flight and two backscattering instruments) has allowed covering almost 4 orders of magnitude in time—from the ps to ns range—with spatial resolution. The results evidence the simultaneous occurrence of vibrations and localized side-group motions at low temperatures and additional diffusive-like (segmental) dynamics at high temperatures. The localized processes of the side groups show (i) stretching of the scattering function, (ii) associated activation energies similar to those found for single and cooperative bond rotations of polyethylene, and (iii) spatial extents that increase with increasing temperature. Compared with poly(ethylene oxide) (PEO), the diffusive segmental process in PAOs presents (i) the same spectral shape, (ii) slower characteristic times—antiplasticization—(iii) similar deviations from Gaussian behavior. For comparison, we also report on backscattering results on the side-group dynamics of poly(n-hexyl methacrylate) in the same temperature range, that show evidence for confinement effects. We suggest that the dynamic asymmetry in systems with intrinsic dynamic heterogeneities between constituent parts is the key ingredient leading to both plasticization and confinement effects.
[Show abstract][Hide abstract] ABSTRACT: The diffusive motions of covalently tethered 1, 3 diphenylpropane (DPP) via a silyl-aryl-ether linkage in the mesopores of MCM-41 were studied by quasielastic neutron scattering. The geometric effect of pore radius was investigated with samples having pores that ranged from 1.6 to 3.0 nm in diameter and highest achievable DPP grafting density. The effect of molecular crowding was investigated in 3.0 nm diameter pores for surface coverage ranging from 0.60 to 1.61 DPP/nm2. Temperature dependence was determined for large pore diameter samples from 240 K to 370 K. As the DPP molecules remain attached over this entire temperature range, data were analyzed in terms of a model of localized diffusion inside a sphere. Only the motions of the DPP hydrogen atoms were considered because of the high sensitivity of neutron scattering to the presence of hydrogen. As atoms far from the attachment point have a greater range of motion than those nearer the tether, the radius of the sphere limiting the motion of individual hydrogen atoms was allowed to increase based on the atom s distance from the tether point. Both smaller pore diameters and higher DPP surface coverage resulted in larger amplitude motion while the diffusion coefficient was greatest in the largest pores at highest surface coverage. These observations support a model where the DPP molecules prefer an orientation allowing close proximity to the MCM-41 pore surface and are forced into the pore interior by either the steric effect of small pore diameter or by increased competition for surface area at high molecule surface coverage.
The Journal of Physical Chemistry C 01/2012; 116(1):923-932. DOI:10.1021/jp209458a · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transitions within the tunneling multiplet of CH(4) in phase II have been measured in an experiment at the backscattering instrument BASIS of the Neutron Source SNS. They all involve transitions from or to T-states. A statistical model is put forward which accounts for local departures from tetrahedral symmetry at the sites of ordered molecules. Different from previous work, in which discrete sets of overlap matrix elements have been studied, now large numbers of elements as well as the ensemble of T-states are considered. The observed neutron spectra can be explained rather well, all based on the pocket state formalism of A. Hüller [Phys. Rev. B 16, 1844 (1977)]. A completely new result is the observation and simulation of transitions between T-states, which give rise to a double peaked feature close to the elastic position and which reflect the disorder in the system. CH(2)D(2) molecules in the CH(4) matrix are largely responsible for the disorder and an interesting topic for their own sake. The simple model presented may lend itself to a broader application.
The Journal of Chemical Physics 12/2011; 135(22):224509. DOI:10.1063/1.3664726 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The transition metal based oxide YBaCo3FeO7 is structurally related to the
mineral Swedenborgite SbNaBe4O7, a polar non-centrosymmetric crystal system.
The magnetic Co3Fe sublattice consists of a tetrahedral network containing
kagome-like layers with trigonal interlayer sites. This geometry causes
frustration effects for magnetic ordering, which were investigated by
magnetization measurements, M\"ossbauer spectroscopy, polarized neutron
diffraction, and neutron spectroscopy. Magnetization measurement and neutron
diffraction do not show long range ordering even at low temperature (1 K)
although a strong antiferromagnetic coupling (~2000 K) is deduced from the
magnetic susceptibility. Below 590 K, we observe two features, a spontaneous
weak anisotropic magnetization hysteresis along the polar crystallographic axis
and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the
interlayer sites remain idle. Below ~50 K, the onset of a hyperfine field shows
the development of moments static on the M\"ossbauer time scale also for the Fe
interlayer sites. Simultaneously, an increase of spin correlations is found by
polarized neutron diffraction. The relaxation part of the dynamic response has
been further investigated by high-resolution neutron spectroscopy, which
reveals that the spin correlations start to freeze in below ~50 K. Monte Carlo
simulations show that the neutron scattering results at lower temperatures are
compatible with a recent proposal that the particular geometric frustration in
the Swedenborgite structure promotes quasi one dimensional partial order.
[Show abstract][Hide abstract] ABSTRACT: We use quasi-elastic neutron scattering (QENS) to study the dynamics of water confined inside reverse micelles. As a model system we use a water-in-oil droplet microemulsion based on the anionic surfactant AOT (sodium bis[2-ethylhexyl] sulfosuccinate), that forms spherical water droplets coated by a monolayer of AOT dispersed in the continuous oil matrix. Combining neutron time-of-flight (TOF) and backscattering (BS) spectroscopy, we access the dynamical behaviour of water over three decades in time from pico- to nanoseconds. We investigate the influence of reverse micelle size on the water dynamics by comparing two sample systems with bigger and smaller water core radii of about R(c) approximate to 12 angstrom and 7 angstrom. The temperature is varied over a range where both microemulsion systems are stable, from room temperature down to the region where the confined water is supercooled: 260 K <= T <= 300 K. Taking explicitly into account the previously measured diffusion of entire reverse micelles in the oil matrix we find the average mobility of the confined water to be considerably slowed with respect to bulk water. The translational diffusion decreases with decreasing reverse micelle size. Dependent on the reverse micelle size we can interpret our data by assuming two dynamically separated water fractions. We identify the faster one with bulk-like water in the middle of the core while the slower one seems to be surfactant bound water. We find that 4 molecules of water per AOT molecule are immobilized on the timescale of QENS, i.e. shorter than nanoseconds.
[Show abstract][Hide abstract] ABSTRACT: A combined quasi-elastic neutron scattering (QENS) and high-resolution solution NMR spectroscopy study was conducted to investigate the internal dynamics of aqueous (D 2 O) G5 PAMAM dendrimer solutions as a function of molecular protonation at room temperature. Localized motion of the dendrimer segments was clearly exhibited in the QENS data analysis while the global, center-of-mass translational diffusion was measured by NMR. Our results unambiguously demonstrate an increased rapidity in local scale ($ 3 A) motion upon increasing the molecular protonation. This is contrary to an intuitive picture that increased charge stiffens the dendrimer segments thereby inhibiting local motion. These charge-induced changes may be a result of interactions with the surrounding counterions and water molecules as the segments explore additional intra-dendrimer volume made available by slight electrostatic swelling and redistribution of mass in the dendrimer interior. This observation is relevant to development of a microscopic picture of dendrimer-based packages as guest-molecule delivery vehicles because reorganization of the confining dendrimer segments must be a precursor to guest-molecule release.
[Show abstract][Hide abstract] ABSTRACT: It seems to be increasingly accepted that the diversity and composition of lipids play an important role in the function of biological membranes. A prime example of this is the case of lipid rafts; regions enriched with certain types of lipids which are speculated to be relevant to the proper functioning of membrane embedded proteins. Although the dynamics of membrane systems have been studied for decades, the microscopic dynamics of lipid molecules, even in simple model systems, is still an active topic of debate. Neutron scattering has proven to be an important tool for accessing the relevant nanometre length scale and nano to picosecond time scales, thus providing complimentary information to macroscopic techniques. Despite their potential relevance for the development of functionalized surfaces and biosensors, the study of single supported membranes using neutron scattering poses the challenge of obtaining relevant dynamic information from a sample with minimal material. Using state of the art neutron instrumentation we were, for the first time, able to model lipid diffusion in single supported lipid bilayers. We find that the diffusion coefficient for the single bilayer system is comparable to the multi-lamellar lipid system. More importantly, the molecular mechanism for lipid motion in the single bilayer was found to be a continuous diffusion, rather than the flow-like ballistic motion reported in the stacked membrane system. We observed an enhanced diffusion at the nearest neighbour distance of the lipid molecules. The enhancement and change of character of the diffusion can most likely be attributed to the effect the supporting substrate has on the lipid organization.
[Show abstract][Hide abstract] ABSTRACT: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48 Å and porosity ∼60% was investigated as a function of pressure at T = 298 K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100 K (i.e. ∼90 K below the liquid–gas critical temperature TC ≈ 191 K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of ∼2.5 from 3.47 ± 0.41 × 10−10 m2 s−1 at 0.482 MPa to D = 8.55 ± 0.33 × 10−10 m2 s−1 at 2.75 MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites.