Neutron Spin Echo Spectroscopy Research Articles

Steric interactions in cellular structures formed in a water/oil/surfactant/cosurfactant mixture.

Dynamics of dense microemulsion droplets forming ordered cellular phases as temperature increases are investigated by neutron spin echo spectroscopy (NSE). The shape fluctuations of droplets are shown using a specific contrast. Their relaxation time tau(f) is obtained by analysis of the NSE curves, which also reveals the short-range Brownian motion (diffusion constant D0) of the "caged" droplets. The thermal dependence of D0 and tau(f) supports the notion of entropically driven interdroplet steric repulsion stabilizing the cellular phases.

Form fluctuations of carbosilane dendrimers in dilute solution: a study using neutron spin echo spectroscopy

A series of carbosilane dendrimers with perfluorinated end groups has been prepared. The structure of these molecules in dilute solution is studied using small angle neutron scattering. For generations g<3 we find a non-spherical shape of the particles and a tendency for aggregation. This result is supported by the analysis of the diffusion coefficient obtained from photon correlation spectroscopy. The overall shape of the molecules is that of a core-shell particle. The generation 4 molecule is obtained as a compact sphere. Neutron spin echo spectroscopy reveals a relaxation time which is attributed to the form fluctuations of this particle.

Phase Separation of a Binary Liquid System in Controlled-Pore Glass

ABSTRACTThe phase separation behaviour and the dynamics of concentration fluctuations of binary liquid mixtures are strongly influenced by confinement effects. We have investigated this confinement effect for the binary system isobutyric acid iBA + D2O imbibed into a mesoporous silica glass (CPG-10). The dynamics of the mixture are studied in the one-phase region as well as in the phase-separated state by means of neutron spin echo spectroscopy (NSE). Moreover, the averaged structures of the liquid are explored by means of small angle neutron scattering (SANS) leading to a length scale 4 &lt; ξ &lt; 9 nm for the fluctuations. The associated effective diffusion coefficient Deff as obtained for NSE is found to decrease with temperature ny nearly one order of magnitude in the temperature range from 75°C to 25°C.

Self and collective dynamics of ordered star polymer solutions

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 Deff is well described by the term D0/S(Q), with D0 the diffusion coefficient at infinite dilution. At Qm, 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 Qm the time range of NSE was extended for the first time up to 350 ns using long wavelengths, λ=19 A, 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 1/2 of star cores, which is related to the blob size ξ of the surrounding star polymer solution obtained by dynamic light scattering.

Neutron spin-echo investigation of the microemulsion dynamics. in bicontinuous, lamellar and droplet phases

Using neutron spin-echo (NSE) spectroscopy in combination with dynamic light scattering (DLS), we performed an extensive investigation of the bicontinuous phase in ternary water–surfactant–oil microemulsions, with extension to lamellar and droplet phases. The dynamical behavior of surfactant monolayers of decyl-polyglycol-ether (C10E4) molecules, or mixtures of surfactant with long amphiphilic block-copolymers of type poly-ethylene propylene/poly-ethylene oxide (PEP–PEO) was studied, under comparable conditions. The investigation techniques provide access to different length scales relative to the characteristic periodicity length of the microemulsion structure. Information on the elastic bending modulus is obtained from the local scale dynamics in view of existing theoretical descriptions and is found to be in accordance with small angle neutron scattering (SANS) studies. Evidence for the modified elastic properties and additional interaction of the amphiphilic layers due to the polymer is more pronounced at a larger scale. Experiments on oriented lamellar phases, and also droplet phases, with anchored block-copolymers offer more insight into the effective layer–layer interaction and efficiency boosting associated with the polymers.

Neutron scattering study on the structure and dynamics of oriented lamellar phase microemulsions.

Lamellar phases consisting of water and oil with intervening surfactant monolayer interfaces (eventually containing low molecular weight amphiphilic block-copolymers) are studied. Structural and dynamical investigations of oriented lamellar phases at the length scale of the intermembrane distance and beyond are performed using small-angle neutrons scattering and neutron spin-echo spectroscopy. The data analysis in terms of static and dynamic structure factors for a stack of elastic interfacial membranes yields information of the membrane curvature elasticity and membrane interactions in terms of phenomenological parameters such as the bending elastic modulus kappa and compression modulus (-)B, and on the dissipation related to the viscosity eta. The influence of the block-copolymers anchored to the surfactant monolayers is studied for a series of lamellar phase microemulsions containing equal volume fraction of surfactant and polymer, by varying only the polymer molecular weight. Based on numerical evaluations and fits to the data the conditions of applicability of the available theoretical concepts are discussed.

Dynamics of star-burst dendrimers in solution in relation to their structural properties

We have measured both the static and dynamic structure factors of a single dendrimer with small-angle x-ray scattering (SAXS) and neutron spin-echo spectroscopy under good solvent conditions with the aim of finding a consistent correlation between the structural properties of dendrimers and their dynamic behavior. The samples under investigation were star-burst polyamidoamine dendrimers with generations g=0 to 8 in dilute methanol solutions. A model independent approach employing inverse Fourier transformation and square root deconvolution methods has been used to analyze the SAXS data to obtain the pair distance distribution function p(r) and the radial excess electron density profile Δρ(r). In addition, we formulated a model that takes both the colloidal (globular, compact shape with form polydispersity or fuzzy surface) as well as the loose, polymeric (self-avoiding random walk) character of dendrimers into account. With this model we were able to describe the spectra of all dendrimer generations consistently. Parameters discussed as a function of the dendrimer generation are, among others, the correlation length of the density fluctuations (blob radius) ξ, the radius of gyration Rg, the sphere radius Rs, the form polydispersity σs or analogously, the width of the fuzzy surface region 2σf. Both the model-independent approach and the model fits reveal that at least down to the third generation the dendrimers exhibit a rather compact, globular shape. These findings are in agreement with the dynamic results obtained by NSE spectroscopy which probes length scales both larger and much smaller than the dimension of a single dendrimer. The method reveals that the dynamics throughout is dominated by the center-of-mass diffusion—the internal dynamics is suppressed. The diffusion coefficients obtained are close to the values calculated from the Stokes–Einstein relation using the sphere radius Rs determined from the SAXS spectra. Dynamically, the dendrimers behave like “hard”, solid spheres.

Neutron spin-echo spectroscopy for diffusion in crystalline solids

Neutron spin-echo spectroscopy (NSE) offers unprecedented opportunities in the investigation of diffusion in crystalline systems due to its outstanding energy resolution. NSE not only enables measurements at lower diffusivities than the established techniques of neutron spectroscopy, but it also gives a very immediate access to the different time scales involved in the diffusion process. This is demonstrated in detail on the example of the binary alloy NiGa where the Ni atoms hop between regular sites on the Ni sublattice and anti-sites on the Ga sublattice. Experiments on two different NSE instruments are compared to measurements using neutron backscattering spectroscopy. The potential of NSE for the investigation of jump diffusion and experimental requirements are discussed.

Water structure and dynamics in a fully hydrated sodium vermiculite clay

Abstract We have investigated the structure and dynamics of confined water in a fully hydrated sodium vermiculite clay by neutron diffraction and neutron spin-echo (NSE) spectroscopy. All measurements were performed with the Q vector parallel to the clay layers. In the case of the diffraction experiments we used both protonated and deuterated water in order to elucidate the interlayer water structure. The structural results support a picture where the intercalated water is generally ‘liquid like’ with only a small degree of orientational ordering of the water molecules relative to the clay surfaces and other water molecules. The intermediate scattering function S(Q, t) measured by NSE can be described by the Kohlrausch-Williams-Watts stretched-exponential function, probably associated with a broad distribution of relaxation times owing to widely different local environments for the water molecules. Some water molecules are strongly interacting with the clay surfaces or the intercalated Na+ ions, whereas the remaining molecules are interacting only with other water molecules. The Q dependence of the average relaxation time is in good agreement with the Gaussian jump-length distribution model for translational diffusion, suggesting that most of the motions on the experimental time scale (3-3000 ps) have a translational character.

Water structure and dynamics in a fully hydrated sodium vermiculite clay

Abstract We have investigated the structure and dynamics of confined water in a fully hydrated sodium vermiculite clay by neutron diffraction and neutron spin-echo (NSE) spectroscopy. All measurements were performed with the Q vector parallel to the clay layers. In the case of the diffraction experiments we used both protonated and deuterated water in order to elucidate the interlayer water structure. The structural results support a picture where the intercalated water is generally ‘liquid like’ with only a small degree of orientational ordering of the water molecules relative to the clay surfaces and other water molecules. The intermediate scattering function S(Q, t) measured by NSE can be described by the Kohlrausch-Williams-Watts stretched-exponential function, probably associated with a broad distribution of relaxation times owing to widely different local environments for the water molecules. Some water molecules are strongly interacting with the clay surfaces or the intercalated Na+ ions, whereas t...

Interaction between Polymer and Surfactant Mesophases

The interaction of polymers with flexible surfactant layers may modify strongly the elasticity and fluctuations of the layers. This effect has been studied in two surfactant mesophases. Neutron Spin Echo (NSE) spectroscopy was used to probe the modification of membrane fluctuations first in the case of dilute microemulsion droplets both in the presence and absence of polymer. A second example studied more recently was hydrophobically modified polymers (polysoaps) interacting with fluctuating lamellar bilayers of a nonionic surfactant.

Internal dynamics in colloidal PNIPAM microgel particles immobilised in mesoscopic crystals

Abstract Internal dynamics of colloidal microgels based on N -isopropyl-acrylamide is investigated by means of neutron spin-echo spectroscopy. To avoid interference of the relaxation of interest with the centre of mass diffusion, the microgels were immobilised in a colloidal crystal. Due to the q -range used in these quasi-elastic neutron scattering experiments, it is possible to obtain nearly completely decaying ensemble averaged intermediate scattering functions 〈ISF〉 E . These can be fitted by single exponential forms, simplifying largely the data analysis compared with photon correlation spectroscopy. This is due to the fact that for light q 2 〈 ξ 2 〉→∞ can not be reached, whereas for neutrons this condition is approximately fulfilled. The obtained network diffusion coefficients decrease linearly with increasing cross-linker concentration. This is in contrast to previous results from photon correlation spectroscopy obtained for macroscopic poly( N -isopropyl-acrylamide) gels [Macromolecules, 29 (1996) 8746].

Probing jump diffusion in crystalline solids with neutron spin-echo spectroscopy.

Neutron spin-echo spectroscopy has been applied to jump diffusion in a crystalline solid. Definite advantages of spin-echo spectroscopy over quasielastic neutron scattering are the direct visual discrimination of fast and slow diffusion processes and the high resolution in the time-energy domain. The potential of the method is demonstrated by way of Ni diffusion in the intermetallic alloy NiGa.

Neutron scattering and the glass transition in polymers – present status and future opportunities

We present results obtained by the exploration of molecular motions by neutron scattering and outline future opportunities resulting from methodological progress in high-resolution neutron spectroscopy. Starting on methododical grounds, we will commence with possible future developments in neutron spin echo spectroscopy (NSE), the prime technique for the observation of molecular processes in the nanosecond domain. After describing the scope of neutron scattering for experiments on glass-forming materials, we will present measurements of the self-correlation function, which include the β-relaxation in polyisobutylene, and address the problem of the heterogeneity of the α-process. Thereafter, we deal with different aspects of the pair correlation function, starting from coherent scattering experiments on polybutadiene, where the choice of the momentum transfer is selective for the observation of the α- or β-relaxation. Then, we emphasize results on the structural relaxation obtained for the different polymers at the first structure factor peak. Finally, by example, we will address the capability of neutron scattering, to deal with motions in polymer blends, where by judicious labeling the dynamics of the different components can be observed separately.

Polarimetric neutron spin echo

Neutron spin echo (NSE) spectroscopy inherently involves polarization analysis features. Thus, the very principle of paramagnetic NSE used for the study of magnetically isotropic systems is based on the vector properties of the neutron polarization change in the scattering, which implies that the incoming and outgoing neutron spin directions are not parallel. Another, conventional polarization analysis feature is the opposite sign of the NSE signal for nuclear and nuclear-spin incoherent scattering, or the signal parallel and perpendicular magnetic fluctuations in the presence of a strong magnetizing field. In these cases the measured NSE spectra are the algebraic sums of two components. In its actual form, however, NSE does not allow one to investigate spin-flip and non-spin-flip scattering channels independently from each other. At the expense of rather moderate loss of signal it is, however, possible to generalize the NSE technique for the study of a given channel of spin-dependent scattering processes in the most general sense of neutron polarimetry. This means to perform inelastic NSE analysis on the partial scattering cross-section corresponding to the transition from any incoming spin direction P to any outgoing spin direction P′ . A method of combining NSE with neutron polarimetry will be described together with a simple example of experimental realization.

HMI holds workshop on neutron spin echo spectroscopy

An international workshop on “Neutron Spin Echo Spectroscopy—Future Aspects and Applications” was held from October 16 to 17, 2001 at HMI, Berlin, Germany, with almost 60 participants coming from all around the world. The workshop was supported financially by the Neutron Round Table and the HMI.

The dynamics in polyethyleneoxide–alkali iodide complexes investigated by neutron spin-echo spectroscopy and molecular dynamics simulations

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide (PEO) and PEO–alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics (MD) computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm−1 to approximately 20 nm−1. We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt (1 metal ion per 15 monomers) leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data.

Collective dynamics of a photosynthetic protein probed by neutron spin-echo spectroscopy and molecular dynamics simulation

We have studied the collective dynamics of an hydrated soluble protein, the C-phycocyanin by using neutron spin-echo spectroscopy. Molecular dynamics simulations, performed with the same system, showed that the main dynamic contribution comes from backbone and side-chain motions. Both the experimental and spectra exhibit a dynamic relaxation with a characteristic time of about 10 ps.

The dynamics in polyethyleneoxide-alkali iodide complexes in short and intermediate time ranges

The dynamics of liquid polyethyleneoxide (PEO) and PEO–NaI complexes were investigated by means of time-of-flight spectroscopy (TOF), neutron spin-echo spectroscopy (NSE) and molecular dynamics simulations (MDS) in a time range of 0.1 ps–1 ns and in a wave vector range of 4–25 nm −1.

Reptation in polyethylene-melts with different molecular weights

It is known since a decade that polyethylene shows a characteristic deviation from the Rouse dynamics, i.e. pronounced entanglement effects, now observable in neutron spin-echo (NSE) spectroscopy as a plateau at high Fourier times t. Earlier spin-echo data were limited to t=20–40 ns and could only determine entanglement distances but not decide between competing models. The now available time range up to 200 ns and more at the IN15 (ILL) NSE spectrometer allowed to distinguish the various models and to decide in favour of de Gennes’ Reptation model. In this model the restriction of motion originates from a trapping of the chains in a virtual tube. We present a series of new NSE-data from PE-melts at 509 K with different molecular weights between M w =12 000 g/ mol and M w =190 000 g/ mol extending to 170 ns that allow for the first time a systematic investigation of the reptation dynamics of PE-melts over a wide range of molecular weights.