Using neutron spin echo and dielectric spectroscopy we have studied the molecular motions of 1-4 polybutadiene in the alpha-beta relaxation regime. At the first peak of the static structure factor the relaxation times follow the temperature dependence of the viscosity, while near the second peak the Arrhenius law of the beta relaxation is observed. Considering localized motions on a length scale of 1.5 Angstrom with the barrier distribution from dielectric spectroscopy the dynamic structure factor in the beta relaxation regime can be described quantitatively.

The static scattering function for a regular star-branched polymer is calculated in the full excluded-volume limit with renormalization group techniques, and a closed-form expression for the scattering intensity I(k) is given. The influence of the excluded-volume interaction between monomers becomes more noticeable as the functionality f of the star increases, qualitative differences appearing between I(k) and the Gaussian limit I0(k). A simplified formula for I(k) is provided, which can be used for the treatment of experimental data. The scattering of one labeled arm of the star is also investigated and the radius of gyration of the arm is calculated, giving a quantitative expression for the stretching of the branches.

In their Comment [E. Donth, K. Schroter, and S. Kahle, preceding Comment, Phys. Rev. E 69, 1099 (1999)] on our paper [A. Arbe, D. Richter, J. Colmenero, and B. Farago, Phys. Rev. E 54, 3853 (1996)], Donth et al. conclude that the procedure followed by us for analyzing data of glass-forming polymers, which is based on a Williams-like ansatz, is not suited for the merging region of the alpha and beta dielectric processes. They claim that a simple addition ansatz is the correct procedure. In this Reply, by considering new dielectric data of six different polymers, we show that our procedure seems to be certainly suitable for describing the merging region without any ad hoc assumption. Moreover, we also show that an unbiased application of the addition ansatz in fact gives support to the procedure followed by us. [S1063-651X(99)03007-X].

Equilibrium and dynamical properties were obtained by light scattering methods for solutions of multiarm polyisoprene star polymers in a good solvent, cyclohexane, over a wide concentration range. Results for 8-arm and 18-arm stars were compared with the behavior of linear polymers in the dilute and semidilute regimes and were analyzed in terms of recent theories based on the Daoud-Cotton model. In dilute solutions the temporal fluctuations on distance scales smaller than the overall dimensions were found to be markedly slower for stars than for linear chains of the same size. Evidence for star- star repulsion and liquidlike ordering near the overlap concentration was also obtained. Such differences between linear polymers and stars vanish in the semidilute region, however, with the osmotic pressure derivative and mutual diffusion coefficient becoming power law functions of the polymer concentration alone. The scaling law prefactors depend on the arm number but less strongly than predicted by theory.

In this paper we show that the concentration dependence of the plateau shear elastic modulus and of the viscosity can be understood if two lengths, the correlation length of concentration fluctuations and the tube diameter, are considered. To interpret the temperature variation of the viscosity and of the self-diffusion coefficient with the solvent quality, one has to take into account the local viscosity to which monomers are sensitive. This concentration dependent local viscosity, deduced from neutron spin-echo measurements, is independent of the polymer molecular weights, solvent viscosities and Theta temperatures. The local viscosity decreases to reach the solvent viscosity value at high temperature, that is, under good solvent conditions. The role of the macroscopic concentration on a local scale, seems to indicate that, in semidilute Theta solutions, the polymers are transparent to each other, as far as dynamic properties are concerned.

Larmor precession of neutrons due to dynamical diffraction effect through magnetic multilayer of permalloy45 (Fe55Ni45:(55) Angstrom)-titanium(Ti:54 Angstrom) 15 bilayers has been observed using a forward diffraction (transmission) neutron spin echo technique. The observed transmission probabilities of up arrow and down arrow spin neutrons are well reproduced by a calculated up arrow and down arrow spin wave functions of neutron through one-dimensional periodic potential of rectangular type for each spin direction. The shift of Larmor precession oscillates sinusoidally in the rocking curve of the transmitted beams around a magnetic Bragg condition, which is also well reproduced by the calculated phase difference of up arrow and down arrow spin neutron wave functions based on one-dimensional Schrodinger equation. (C) 1998 Elsevier Science B.V. All rights reserved.

Larmor precession of neutron transmitted through a helical magnetic crystal of holmium has been firstly observed using a forward diffraction (transmission) neutron spin-echo technique, with an external magnetic field parallel to the helical axis. The shift of the Larmor precession as a function of rocking crystal angle is considerably enhanced at the magnetic Bragg condition of (0 0 0)(+/-) satellites up to 0.05 turns with a neutron path length of 1.6 mm. For the reflected neutrons of (0 0 0)(+/-) satellites, the spin echo signal has completely disappeared. For the helical magnetic satellites, only one of the spin states of the Larmor state sees the magnetic potential and the O-wave of the forward diffracted up arrow spin state recombines with the simply transmitted down arrow spin state giving extra Larmor precession. (C) 1998 Elsevier Science B.V. All rights reserved.

Here we report results obtained by neutron spin echo on semidilute theta solutions of polystyrene in deuterated cyclohexane. It is shown that the pertinent length for the diffusion coefficient is not the correlation length xi(theta) of concentration fluctuations but rather the distance between binary contacts. At length scales smaller than this distance, the dynamics is linked to the local viscosity, which is found to be surprisingly concentration and temperature dependent.

The local dynamics of 1,4 polybutadiene below and above the merging of the alpha- and beta-relaxations have been investigated by means of Neutron Spin Echo. At temperatures below the merging, the dynamic structure factor can be described by considering localized motions on a length scale of 1.5 Angstrom for the beta-process with the barrier distribution obtained from dielectric spectroscopy. Above the merging of the alpha- and beta-relaxations, the experimental data have been successfully described by assuming that both processes are statistically independent.

The local dynamics of 1,4 polybutadiene below and above the merging of the alpha and beta relaxations have been investigated by combining neutron spin echo (NSE) and dielectric spectroscopy. The study of the dynamic structure factor measured by NSE over a wide momentum transfer range allows us to characterize the alpha relaxation as an interchain process while the beta relaxation originates from mainly intrachain motions. At temperatures below the merging, the dynamic structure factor can be described by a superposition of elemental processes for the beta relaxation as obtained from dielectric spectroscopy. The elemental motions behind this process can be related to rotational jumps of the chain building blocks around their center of mass. Furthermore, we have been able to consistently describe the dynamic structure factor above the merging of the alpha and beta relaxations by assuming that both processes are statistically independent. In the framework of this scenario a procedure for analyzing the dielectric response in the alpha-beta merging region has been developed. Its application to the dielectric data allows us to describe the dielectric response in this region on the basis of the low temperature behavior of the alpha and beta processes and without considering any particular change in the relaxation mechanism of these processes. The temperature dependence found for the relaxation time of the alpha process follows now the viscosity, a masked feature in the experimental data due to the merging process. In this way, we have been able to consistently describe the relaxation of both, the polarization and the density fluctuations, by using the same scenario, i.e., independent alpha and beta processes, and considering the same functional forms and temperature dependences of the characteristic times of the two processes.

We have, for the first time, observed neutron spin precession due to a dynamical diffraction effect through a pair of ferromagnetic crystals of permalloy across a Bra,og condition of (200) by means of the neutron spin echo and phase echo principles. In spite of the imperfect crystal, the shift of Larmor precession is enhanced in the rocking curve of the transmitted beams at the magnetic Bragg condition. What was observed is a longer dwelling phase of up arrow spin neutron wave function due to the multiple reflection process of O-wave at Bragg reflection, against the rather freely passing opposite spin wave function. (C) 1999 Elsevier Science Ltd. All rights reserved.

This paper discusses the dynamic-scattering properties of an (A-B)r ring diblock-copolymer chain in semi-dilute solutions. The expressions of the frequencies describing the dynamic behaviour of such systems are given in both models: the Zimm model, where the hydrodynamic effects are important and the Rouse model, where these effects are neglected. The results show sensitive differences with respect to the corresponding linear diblock copolymer (A-B)l. This is mainly due to the effect of connectivity of the chain extremities A and B, leading to different conformations for both types of diblock- copolymer chains. Although the description we discuss in this paper is valid for any cyclic diblock-copolymer chain (i.e. at any composition), explicit equations are given for the symmetrical case (50/50 composition) where the effects are more pronounced. Experimental investigations using neutron spin echo (NSE) are particularly useful in order to test these theoretical predictions.

Recent developments in sputtering techniques allow the fabrication of multilayers with a high degree of perfection over large areas. We show, that using reactive sputtering, it is possible to adjust the index of refraction for neutrons, n(i) of the individual layers. This property is particularly important for polarizing mirrors, where n(nm) for the non- magnetic layers can be matched to n(m) of the magnetic layers such that neutrons for one spin-eigenstate are not reflected by the coating, whereas the reflectivity is high for the other spin-eigenstate. In addition, by using anisotropic sputtering conditions it is possible to orient the easy axis of magnetization within the plane of the mirrors in any particular direction resulting in a simultaneous appearance of a pronounced remanence and coercivity. Remanent polarizers can be used as broad band spin selectors at continuous and in particular at pulsed neutron sources thus eliminating the need of spin flippers, whose performance depends on the wavelength of the neutrons and is often strongly influenced by stray magnetic fields from the sample environment. The possibility to operate remanent supermirrors in arbitrary small fields leads to attractive applications of polarizing devices in low held environments such as they occur in neutron-spin-echo or in spin selective neutron guides. We present applications, where several tasks like polarizing, focusing and spin selection are performed in one single device thus reducing the problem of phase space matching between different neutron optical components. (C) 1999 Elsevier Science B.V. All rights reserved.

Dynamical properties of vulcanized polymer networks are addressed via a Rouse-type model that incorporates the effect of permanent random cross-links. The incoherent intermediate scattering function is computed in the sol and gel phases, and at the vulcanization transition between them. At any nonzero cross-link density within the sol phase Kohlrausch relaxation is found. The critical point is signaled by divergence of the longest time scale, and at this point the scattering function decays algebraically, whereas within the gel phase it acquires a time-persistent part identified with the gel fraction. [S0031-9007(97)04323-8].

This paper deals with mixtures of homopolymers and copolymers in solvents under the zero average contrast condition. This condition is chosen in order to decouple the correlations due to polymer composition fluctuations from those due to polymer concentration fluctuations. This makes the measurement of these correlations much easier using techniques based on static light scattering, photoncorrelation spectroscopy, small angle neutron scattering, neutron spin echo or X-ray scattering. In the first part of the paper, a simple theoretical framework is developed to present the guidelines for the interpretation of the scattering data. In the second part, some experimental examples are discussed using the scattering techniques mentioned above. Data obtained from various systems involving alike (deuterated and ordinary) homopolymers and diblock copolymers as well as unalike species are considered. An example of mixtures of polyelectrolytes in aqueous solutions is also discussed within the framework of the zero average contrast condition. (C) 1997 Elsevier Science Ltd.

Experimental heat capacity, dielectric, and shear spectroscopy data are reported on the alpha beta splitting region of poly(n- octyl methacrylate). New facets of a major difference between the high-temperature alpha relaxation at frequencies above the alpha beta splitting region and the ordinary alpha relaxation below are obtained: An alpha cooperativity onset in the splitting region and dominance of Rouse-Zimm-like modes R for the alpha relaxation. The alpha and alpha relaxations are not related by temperature-time superposition.

This article reviews the understanding of static and dynamic scattering properties of multicomponent polymer systems in solution achieved during the past decade. We shall describe particularly ternary polymer systems which include polyelectrolyte solutions (polyion/counter-ions/water), mixture of homopolymers (A and B)/solvent and the case of diblock copolymer (A-B, linear or cyclic) solutions. The purpose of this paper is not an extensive survey of theoretical and experimental results obtained on the scattering behavior of these systems but rather an updating of recent results. For polyelectrolyte systems we shall focus, by means of scattering techniques, on the conformation of the polyelectrolyte chain and on the structure of the system induced by the dominant electrostatic interactions in solution involving polyions, counter-ions and solvent. As for the mixture of homopolymers in solution and diblock copolymer/solvent systems, we shall mainly discuss their dynamic behavior and show that using linear response theory and the Random Phase Approximation (RPA), two relaxation modes describe the autocorrelation functions as revealed using dynamic light scattering (DLS) or/and Neutron Spin Echo (NSE) techniques: the first mode characterizes the concentration fluctuations and the second one the composition fluctuations. We shall discuss the scattering properties of these systems on the basis of recent developments with emphasis on possible coiling of the polyelectrolyte chain at low charge density and also how important parameters such as the mobility of the chain (diffusion process) and the interaction parameter (compatibility), which control the dynamics and the thermodynamics in homopolymer mixtures and diblock copolymer systems, could be deduced from scattering experiments.

A brief review is given of applications of Monte Carlo simulations to study the dynamical properties of coarse-grained models of polymer melts, emphasizing the crossover from the Rouse model toward reptation, and the glass transition. The extent to which Monte Carlo algorithms can mimic the actual chain dynamics is critically examined, and the need for the use of coarse-grained rather than fully atomistic models for such simulations is explained. It is shown that various lattice and continuum models yield qualitatively similar results, and the behavior agrees with the findings of corresponding molecular dynamics simulations and experiments, where available. It is argued that these simulations significantly enhance our understanding of the theoretical concepts on the dynamics of dense macromolecular systems. (C) 1997 John Wiley & Sons, Inc.

Neutron spin echo studies on an amorphous strong random magnetic anisotropy system reveal nonexponential spin relaxation to temperatures greater than 10T(g).

The initial decay rate Omega((k) over bar) of the dynamical scattering factor of a single regular star polymer is investigated with the chain conformational renormalization group method up to first order in epsilon. The reduced relaxation rate Omega((k) over bar)/k(3) is expressed as the ratio of the mobility to the static structure factor, and we have analyzed the two contributions separately. At the Gaussian fixed point we have found a qualitatively different result to that of the standard calculation in three dimensions, and the difference comes from the effect of the epsilon-expansion on the mobility. The renormalization group result at the self- avoiding fixed point describes qualitatively well the behavior of Omega((k) over bar)/k(3) observed in neutron scattering experiments for 12-arm-stars in a good solvent.

The results of neutron spin echo (NSE) experiments conducted in the direct beam geometry using the BeO filtered incident spectrum have been discussed in the light of the results published in the literature. The design details of the NSE spectrometer to be installed at the Dhruva reactor are reported.

Recently reported light-scattering studies of CaKNO3 and Salol are reanalyzed, using the extended version of the mode-coupling theory of the liquid-glass transition including activated transport or hopping effects. Problems found in the original fits due to the neglect of hopping terms are largely corrected, and quantitative predictions for the susceptibility minimum below the crossover temperature T(c) are found. The results are also shown to accurately explain the CaKNO3 neutron spin-echo data reported by Mezei [J. Non-Cryst. Solids 131-133, 317 (1991)].

The results of the quasi-elastic neutron scattering experiments carried out on dilute aqueous solutions of tetramethylurea (TMU) are reported. The concentration dependence of the translational diffusion coefficients of TMU and water is interpreted in terms of solute-solute interactions derived from previous small-angle neutron scattering studies.

We have measured the remanent magnetization of single-crystal EuS from 10(-4) to 10(4) sec after removing an applied field. Using a recent model for magnon relaxation on finite domains, the data yield size-scaling exponents of theta = -0.10 +/- 0.02 and zeta = 0.669 +/- 0.004, in excellent agreement with exact theoretical predictions. As a function of temperature, the remanent magnetization is a maximum above the Curie transition, and, for some thermal histories, becomes negative in the ferromagnetic regime. The behavior is consistent with a percolation backbone that is negatively coupled to the finite domains.

Relaxation times of concentration fluctuations in semi-dilute solutions of polydimethylsiloxane, have been measured at several values of the reciprocal wave vector q, with the neutron spin echo spectrometer of the Laboratoire Leon Brillouin. The experiment has been carried out successively on a solute of identical chains, and on a solute divided in equal parts between labelled and non labelled chains at zero average contrast. We report observations of the dispersion relation associated with the Brownian motion of the polymer chains, in the vicinity of the inverse mesh size 1/xi. For values of q which are greater than 1/xi, the two experiments give identical results but when q decreases below 1/xi, the dispersion curves associated with each experiment are different. A bifurcation occurs at 1/xi and two distinct transport processes become observable, which are related to cooperative diffusion and to inter-diffusion respectively. The observed coefficents are compared with predictions of the effective medium and the dilute solution theories.

In this work, we present calculations of the dynamic structure factor for monodisperse melts of linear polymers. Our calculations are based on a model in which a freely jointed chain encounters fluctuating obstacles whose relaxation is self-consistently determined from that of the chain conformation. The calculation of the structure factor is related to the solution of a one-dimensional random walk with dynamical disorder, which is treated within the dynamical effective medium approximation. This model has been applied previously to the calculation of the self-diffusion coefficient and the mean-squared displacement of a chain segment. We present calculations of the structure factor over a wide range of wave vectors, times, and entanglement molecular weights. Our predictions are compared with calculations from the theories of Ronca and of de Gennes, and with the experimental results for polyethylene-butylene-2 obtained by Richter et al. from neutron spin echo measurements. Our calculations show semiquantitative agreement with the experimentally determined structure factors for the time range accessible to these experiments.

Using the technique of neutron spin-echo spectroscopy, we have conducted a direct measurement of the superparamagnetic relaxation of nanoscale, magnetic monodomain iron particles in the time range between 0.01 and 160 nsec and for momentum transfers q between 0.035 and 0.15 Angstrom(-1) Using a phenomenological model which includes the effect of a particle size distribution, we are able to determine the temperature T and q dependence of the longitudinal superspin relaxation time spanning 4 orders of magnitude. We find that for T less than or similar to 40 K, interparticle correlations affect the longitudinal superspin Fluctuations. [S0031-9007(98)08356-2].

Neutron spin-echo spectroscopy was used to resolve for the first time the slow relaxation component associated with the fluctuations of the whole spin particle in monodomain Fe particles in an alumina matrix. These fluctuations are progressively freezing with decreasing temperature. A distribution of relaxation times was observed most likely due to a finite particle size distribution. As a result, one will have to measure at significantly longer Fourier times in order to quantify the freezing behaviour. (C) 1998 Elsevier Science B.V. All rights reserved.

The main transition of amorphous polymers is analyzed with respect to a fine structure by means of new experimental dynamic shear, dielectric, and heat capacity data for the following polymers: poly(n-alkyl methacrylate)s with alkyl = methyl, ethyl, propyl, butyl, and hexyl, polystyrene, poly(vinyl acetate), a series of weakly vulcanized natural rubbers, a series of butyl rubbers with different carbon black content, polyisobutylene, and bromobutyl rubber. The components of the fine structure are assumed to be a proper glass transition at short times, followed by a confined flow zone, and, at large times, a hindering zone caused by entanglements at large times. Two lengths are assumed to correspond to the first and third components, respectively, the characteristic length to the proper glass transition and the entanglement spacing to the hindering zone. The confined flow will be described by a dispersion law (general scaling) across the main transition. The characteristic length of the glass transition for the poly(n-alkyl methacrylate)s-only of order 1 nm as determined by calorimetry-is confirmed by backscaling from the entanglement spacing by means of a Rouse dispersion law for shear. The fate of the Rouse modes below the alpha beta splitting of the glass transition is discussed for the other amorphous polymers. Finally, a speculative molecular picture of the different modes in the main transition is described. The new element is a low-viscosity longitudinal motion of individual chain parts in the confined flow zone. A simple rheological model for the confined flow is also presented.

We have developed a new experimental method for measuring extremely small changes in kinetic energy in a flux of atoms. Analogously to the well-known technique of neutron spin echo, we have succeeded in manipulating the nuclear spins in an atomic beam of He-3. The result, He-3 spin echo, is a universal,high intensity, high resolution tool for a wide range of areas in physics and molecular sciences. We describe the experimental setup and show first spin echo curves. In addition, we demonstrate the power of the method by resolving a change in energy of the He-3 atoms of ca. 33 neV.

In very concentrated aqueous electrolyte solutions, the competition between dipolar and ionic forces and most of the system properties, are finely monitored by the concentration. For LiCl, RH2O (R being the number of water moles per mole of salt) two domains of concentration exist around even hydration numbers R = 6 and R = 4, which can be reversibly undercooled from the liquid to the glassy state. At other concentrations, for R<12, metastability is achieved only in a restricted temperature range. The collective behaviour as well as individual atomic motions, have been studied by neutron inelastic scattering in the metastable state of LiCl, 6H2O and LiCl, 4H2O down to the glass transition. The study of the structural relaxation by neutron spin echo (NSE) allows us to show that the temperature dependence of the response of the system follows a scaling relation with a cross-over at a phase transition present in the liquid phase.

We have investigated the dynamical behavior in the incommensurate phase of NaNO2 by means of the high resolution neutron spin echo (NSE) technique. The results strongly suggest the existence of a mode of phason type and a simple model with both phason and amplitudon relaxational modes provides a satisfactory description of our NSE data.

The dynamical behavior of sodium nitrite in the vicinity of the paraelectric-incommensurate phase transition has been reinvestigated by means of the high-resolution neutron spin- echo technique, in particular, in the modulated phase. It is shown that the results obtained in the incommensurate phase are consistent with a model featuring two slow relaxation mechanisms corresponding to fluctuations in the amplitude and phase of the modulation and characterized by relaxation times tau-A (q) and tau-phi (q), respectively, compatible with the relaxation time tau(q) determined in the paraelectric phase.

The evaluation method of dielectric data based on a Williams product ansatz [A. Arbe, D. Richter, J. Colmenero, and B. Farago, Phys. Rev. E 54, 3853 (1996)] is not suited to the crossover region of the dynamic glass transition. This method is insensitive to the shape, position, or even existence of at least one of the relaxation functions if, in the extrapolation from below to above the crossover region, the two processes cross or approach one another. Since a crossover region without any particular change in the relaxation mechanism of these processes can in fact be adjusted by this method, better dielectric data for polybutadiene in the gigahertz frequency region are needed to determine whether or not there are peculiarities in the dielectric function. It would be an odd assumption that the many particularities in the crossover region in other instances are not reflected in the dielectric response. [S1063-651X(99)02607-0].

Using neutron spin echo spectroscopy, the dynamics of high molecular weight poly(dimethylsiloxane) in the melt (T = 473 K) was investigated. In order to decide whether the lateral constraints are uniform or nonuniform with respect to the contour of a test chain, chains were labeled either completely or partially by a corresponding proton-deuterium exchange. It was found that the central and the terminal chain sequences experience the same amount of lateral confinement, which in the framework of the reptation or tube model - can be characterized by a tube diameter d(t) of about 70 angstrom. Nearly the same d(t) value is obtained from the plateau modulus of rheological measurements, too. Thus, these experiments show unambiguously that the lateral constraints, responsible for the reptational chain dynamics, are quite homogeneous, even in the case of extremely large tube dimensions.

Neutron spin echo (NSE) spectroscopy, an advanced high- resolution quasi-elastic neutron scattering technique, provides the unique opportunity to investigate long-range relaxation processes of macromolecules simultaneously in space and time on nano-scales. In particular, information on the single-chain behavior is not restricted to dilute solutions, but may also be obtained from concentrated solutions and melts, if labelling by proton deuterium exchange is used. Thus, this method facilitates a direct microscopic study of molecular models developed to explain the macroscopic dynamic properties of polymers, e.g. transport and viscoelastic phenomena. This article gives a short outline of the method and reviews the relevant experimental results obtained from polymer melts and networks and from dilute and semi-dilute solutions of chain molecules with different architectures since the first successful NSE work on polymers was published in 1978. The experimental observations are compared with the predictions of the related microscopic models and other theoretical approaches, which are briefly introduced and adapted accordingly.

Using the neutron spin echo spectroscopy, the internal segmental diffusion of chain molecules in polymer melts and concentrated solutions was studied. These investigations show that beyond a characteristic length d(t) and after a cross over time tau(e)(d(t)) the segmental diffusion of the single chains is strongly impeded and deviates from the Rouse dynamics. d(t) is polymer specific and depends on the temperature as well as on the polymer concentration. Within the framework of the reptation concept, where d(t) is identified with the mean distance between intermolecular entanglements or with the tube diameter, the microscopically determined d(t)-values agree quite well with those derived from related macroscopic measurements of the plateau modulus. A similar good agreement is also found with respect to the segmental friction coefficients obtained either from the Rouse regime of the NSE spectra or from theological data of corresponding short chain systems, where entanglements are not yet effective.

Semidilute polymer solutions can be considered as transient networks of lifetime, tau(g) and of average meshsize xi. xi depends on the polymer concentration, c, and the solvent conditions. If the segmental diffusion of such systems is studied on timescales t << tau(g), using the neutron spin echo technique, different relaxation modes and continuous transitions between them are identified by varying the magnitude of the scattering vector, q. Under good solvent conditions, a crossover from single chain Zimm dynamics, as valid in dilute solutions on the entire intramolecular length scale, to the collective many chain diffusion is found at q xi(c) approximate to 1. The corresponding relaxation rates, Omega(q), vary from Omega(q) similar to q(3) to Omega(q) similar to q(2). In Theta-solvents, the many chain regime was not accessible. However, due to the existence of self- entanglements, which introduce an additional length scale xi(i)(c) < xi(c), the crossover from disentangled (Omega(q) similar to q(3)) to entangled single chain relaxation (Omega(q) similar to q) becomes visible.

We propose three kinds of modified neutron spin echo methods using cold neutron spin interferometry, which is based on the coherent superposition principle of the neutron spin. Two kinds of the modified methods are based on a novel quantum precession of neutron spin by multilayer spin splitters and another method on time dependent spin interferometry using RF flippers, The performances of the last type are discussed and the firmer two methods are described in other proceedings. (C) 1999 Elsevier Science Ltd. All rights reserved.

We present a new model of rubber elasticity where linear forces act to constrain the fluctuations of the eigenmodes of the phantom model. The model allows us to treat the constrained junction and the tube model within the same, transparent formalism! does not require any further approximations, and is particularly suited for the analysis of simulation data for (strained) model polymer networks. As an interesting side result we show that in order for the model to be consistent, the constraints (but not the mean polymer conformations!) have to deform affinely, a severe restriction that might also apply to other models. Complementary, we prove in analogy to the derivation of the virial theorem that introducing constraints into the phantom network Hamiltonian leads to extra terms in addition to the usual Doi-Edwards formulas for the polymer contribution to the stress tensor which vanish only for affinely deforming constraints.

In 1997 the instrumental basis for neutron spin echo (NE) investigations of polymer dynamics was considerably enlarged as new and improved spectrometers became available. The scientific highlights include NSE studies on the segmental motion of polymers with various chain architectures in the liquid as well as in the solid state and a direct comparison of NSE findings with the results of molecular dynamics simulations.

We propose a modified neutron spin echo method using a novel spin precession caused by multilayer spin splitters. This precession is free from magnetic field and needs a very short length (e.g., 20 nm) for one rotation. The method will bring us. a compact spin echo spectrometer applicable to a pulsed neutron source. (C) 1999 Elsevier Science B.V. All rights reserved.

Neutron spin echo has brought us a way to circumvent intensity limitations in high energy resolution neutron spectroscopy. Nevertheless, the goal of improvements stays always twofold: better resolution, more intensity. With a given neutron source and after the best possible beam conditioning the only way of further progress is the use of multidetectors. We will present here a report on the progress of the previous years on this two fronts. (C) 1999 Elsevier Science B.V. All rights reserved.

In recent years about four new NSE spectrometers became or are becoming available all over the world. Most of them an optimized for the small angle range where high scattered intensity and high resolution go nicely together. There was, however, a non-negligible demand for beam time on relatively high q range 0.3-2.4 Angstrom(-1). In this q range neutrons are sparse and even giving up on the resolution side there is still a substantial area between TOF, back scattering and state-of- the-art NSE resolution. This motivated the development of a relatively large-area detector extension of IN11. The design and construction is 90% completed, the very first test results are presented. (C) 1998 Elsevier Science B.V. All rights reserved.

Neutron spin echo (NSE) can be employed to study the dynamics of a variety of microemulsion phases. Starting from the droplet phase, we will show what can be measured by coherent quasi- elastic neutron scattering with special emphasis on model independent information. Combining the well known contrast variation method of small angle neutron scattering (SANS) with the unique energy resolution of NSE we can separate motion which are otherwise undetectable. Combining the static and dynamical experiments, microscopical parameters like bending rigidity (K) can be deduced via model fitting.

Inelastic x-ray measurements of polybutadiene are reported, performed over a wide temperature range covering both the glass and the liquid phase. At each temperature, the frequency position Omega and the width Gamma of the inelastic peaks of the spectra have been obtained for different values of the scattering vector Q. A linear behavior of Omega(Q) for Q<4 nm(- 1) has been revealed, allowing the determination of the unrelaxed sound velocity upsilon(infinity). Consistently with the results obtained in different glass-forming systems, the Q dependence of Gamma is well represented by a Q(2) law. For Q>5 nm(-1) the values of Gamma overtake those of Omega and the acousticlike excitations progressively loose their propagative nature. In the glass, upsilon(infinity)(T) compares well with previous Brillouin Light Scattering (BLS) determinations, while in the liquid the BLS sound velocity shows a steeper temperature dependence related to the structural relaxation. The temperature behavior of the nonergodicity factor has been derived both from upsilon(o) and upsilon(infinity) (in the liquid phase) and from the ratio between elastic and inelastic intensities of inelastic x-ray scattering spectra (in the whole investigated temperature range). Both temperature and Q behavior of this quantity might be consistently interpreted in the framework of the mode coupling theory.

We have studied the magnetic correlations in the heavy-fermion superconductor URu2Si2 using high-resolution neutron spin-echo techniques. The measurements confirm previous results that the weak ordered moment flips the neutron spin, and hence is of dipolar origin. The spin-echo signal, with an energy resolution two orders of magnitude better than earlier neutron experiments, confirms the static nature of the moment, in agreement with muon measurements. In addition, we found evidence for the presence of rapidly fluctuating magnetic moments in the ordered phase, which an most likely at the origin of the enhancement of the linear term in the specific heat at low temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.

The rheological properties of star-shaped polyisoprenes having a wide range of arm numbers and arm molecular weights are reported. In contrast to linear polymers, stars have a broad relaxation spectrum and a viscosity that increases exponentially with arm molecular weight. A comparison of eight pairs of samples having 3 and 4 arms and identical arm molecular weights showed that the viscosity of 3-arm stars is approximately 20% lower. For higher degrees of functionality, 4 less-than-or-equal-to f less-than-or-equal-to 33, the effect of functionality saturates and the viscosity is determined by arm molecular weight only. The nonlinear properties of one sample were studied using step-strain tests and found to be essentially identical to those of linear polymers. The predictions of a molecular theory for star polymers based on an extension of the reptation model are reviewed and shown to be in good agreement with the experimental data.

There are several ways of investigating flux lines with polarised neutrons. We shall describe our recent work on IN 15 at ILL, investigating spatially varying fields perpendicular to the fluxoid axes, which are present with tilted fields in anisotropic superconductors. These field components may be detected by polarisation analysis of neutrons diffracted by the flux line lattice, and allow a detailed investigation of flux structure in such materials as YBCO and NbSe2. A further application of polarised neutrons is the use of neutron spin- echo techniques to measure the speed of moving flux lines. We have recently demonstrated this in an Nb-Ta alloy on instruments IN11 and IN15, by measuring the energy change of neutrons diffracted by flux lines, which are moving under the influence of a transport current. We shall also comment on others' work on the search for interference terms between flux lattice and nuclear diffraction signals, and the use of neutron depolarisation for investigation of flux distributions in superconductors. (C) 1999 Elsevier Science B.V. All rights reserved.

Neutron spin echo (NSE) was employed to study the dynamics of a variety of microemulsion phases. Starting from the droplet phase, we present very recent results on the lamellar and sponge (L3) phases. Combining the well-known contrast-variation method of small-angle neutron scattering (SANS) with the unique energy resolution of NSE we can separate motions which are otherwise undetectable. Combining the static and dynamical experiments, microscopic parameters like the bending rigidity (K) could be deduced. We find, for example, that the local dissipation is 4-6 times higher than would be predicted from the solvent viscosities.

Polystyrene(PS)-polyisoprene(PI) diblock copolymers dissolved in n-decane aggregate into spherical micelles with a PS core and a corona of tethered PI chains. The PI corona may be considered as a brush on the surface of a sphere or as the outer part of a many-armed polymer star. The thermal density fluctuations of the PI corona have been observed by neutron spin-echo spectroscopy. The peculiar multi-decay time relaxation behavior can be described remarkably well by a model based on an idea of de Gennes for treating the corona as a semidilute polymer solution with varying concentration profile.

The relaxation behaviour of two molecular glass-forming systems, namely sorbitol and maltitol, are investigated in the large temperature range relevant to the glass-transition. These data are obtained by combining three techniques, i.e. low- frequency mechanical spectroscopy medium and high frequency dielectric spectroscopy, and viscosity measurements. This procedure allows to determine the relaxation map of these polyols on a wide time range [10(-9)-10(7) s]. Two different relaxation processes can be observed. The principal alpha- relaxation process exhibits a complex behaviour: comprising a non-Arrhenius temperature dependence above T-g, (supercooled liquid state), and an Arrhenius behaviour below T-g (glassy state). A secondary beta-relaxation is observed at higher frequencies with an Arrhenius temperature dependence. The secondary process appears in the same time-temperature range in both polyols. Consequently the molecular root of this relaxation is most likely the same in these complementary chemical systems. On the other hand, the time scale on which the alpha and beta processes cross is very different for these two polyols. We relate this feature to the differences in the relative contributions of intra and inter-molecular interactions due to the different chemical architecture of these polyols.

Brillouin-light-scattering measurements have been performed on both bulk and thin supported polybutadiene (PB) films, in the frequency range 1-300 GHz. The results obtained on the relaxation behavior of PB have been interpreted in the framework of the mode-coupling theory and compared with neutron spin-echo data relative to the same system. For temperatures higher than the critical temperature T(c), a quantitative agreement of the mean relaxation time with the viscosity time scale has been found, together with the validity of a time- temperature scaling law. For temperatures approaching the glass transition, a departure of relaxation time from the viscosity time scale has been observed. This phenomenon has been attributed to a decoupling between structural and secondary relaxation processes.

Neutron spin-echo and small-angle neutron-scattering techniques were employed to study both the size and the shape fluctuations of the di-2-ethylhexyl sulfosuccinate water-in-oil microemulsion system. We observe a softening of the bending rigidity of the interface with the addition of butanol as cosurfactant. Comparison of both techniques with the theory implies that for the simultaneous description of the shape and size fluctuations both the splay and saddle-splay bending moduli are the key parameters, and the spontaneous curvature has little influence.

Neutron spin echo (NSE) [F. Mezei, ed., Neutron Spin Echo, Lecture Notes in Physics Vol. 128 (Springer, Berlin, 1980)] is a technique for neutron scattering in which the energy resolution can be much narrower than the spectral width of the incident beam and it allows the direct measurement of the Fourier transform of the energy transfer spectrum, i.e. the time dependence of the density-density correlation function of the scattering system. It is normally discussed in terms of neutron Larmor precession in magnetic fields and the scattering process is described in terms of S(Q, omega). We show how a quantum mechanical treatment of the NSE technique offers new insight into the scattering process and explains how NSE directly yields the intermediate scattering function S(Q, t) or pair distribution function G(r). In addition it gives a basic picture of the physics of 'phonon focussing' in NSE and reveals the link between the spin echo time r, the resolution of the measurement and the details of the scattering process. In this paper we present three different approaches to understanding the system: (i) the usual classical Larmor precession, (ii) a semi-classical ray tracing model and (iii) a full quantum mechanical treatment. We apply these models to a spin echo spectrometer as used for various kinds of measurements.

RF-flipper coils can be combined to set up different types of new high resolution spectrometers, which share several properties with the neutron spin echo technique. We want to explain the principles of these methods and discuss the present state of the art.

Measurements of swelling pressure, neutron spin-echo scattering, and dynamic light scattering were made in an end- linked poly(dimethylsiloxane) (PDMS) gel swollen to equilibrium in a good solvent (toluene) and also in the equivalent solution. The macroscopic osmotic modulus is depressed in the gel. Neutron spin echo observations at intermediate and high values of the scattering vector Q reveal that the mobility of the monomers is unaffected by cross-linking. Elastic neutron scattering at small Q detects non-uniformities in the polymer concentration distribution, which are absent from the solution. These non-uniformities play a major role in the dynamic response of the system at lower Q, and and appear to be the cause of the observed reduction in osmotic pressure.

Concentration fluctuations in swollen gels are controlled by a collective diffusion coefficient D(c). Two experimental observations are described in which a swollen gel is mechanically deformed from its equilibrium configuration. In the first, the anisotropy of D(c) in a uniaxially deswollen polyacrylamide-water gel is investigated using dynamic light scattering. No anisotropy is detected. In the second, D(c) is measured by the neutron spin echo technique in a stretched poly (dimethylsiloxane) gel swollen in toluene. The measured anisotropy is small and at the limit of the experimental error.

The static and dynamic structure factors for star polymers in THETA solutions are derived for semiflexible models and the partially stretched (PS) arm model. The PS model for the static structure factor is found to be in fairly good agreement with SANS experiments with exclusion of the high-k region, where only a qualitative agreement is obtained. The nonpreaveraged first cumulant for the PS model reproduces fairly well the peculiar effects found by neutron spin-echo experiments even though those experiments were carried out in good solvents. The effects of preaveraging and screening of the hydrodynamic interaction are considered. Results for the full dynamic structure factor are presented only in the preaveraging approximation.

A nonperturbative approximation and a fortuitous cancellation of errors lead to an accurate, simple approximation for the dynamic single-chain structure factor-or coherent intermediate scattering function-of a Rouse polymer fluid in the short to intermediate time and wave vector region. With this approximation the breakdown at small wave vector of the wave vector to the fourth scaling of the ''Rouse frequency'' is illustrated, and compared to recent neutron spin-echo measurements on polydimethylsiloxane. Experimental study of this breakdown might provide information about the relation between static structure and dynamic behavior in polymer melts.

A molecular simulation of many-arm star polymers in solvents of varying quality is presented. The number of arms f in each star covered the range 3 less-than-or-equal-to f less-than-or-equal- to 80 with N = 50 and 100 monomers per arm. The resulting equilibrium structures are compared to scaling predictions based on the blob model and to experimental results. The monomer density rho(r) from the center was found to fall off as a power law, r(-a), for both a good and THETA solvent in agreement with the scaling predictions. The free ends were found to be excluded from a region near the center of the star and the distribution of center-to-end distances R was found to be well approximated by a Gaussian for many-arm stars (f greater than or similar to 20) for all solvents. For small f, there were some deviations from a Gaussian form, particularly for small R. For both a good and THETA solvent, the relaxation time for the shape fluctuations depended imperceptibly on the number of arms f.

Simulations of stress relaxation in a model polymer melt of freely-jointed chains with N = 300 bonds are performed with the use of a nonequilibrium molecular dynamics algorithm. After a deformation is applied in a short loading period, special attention is paid to the decay of bond orientation, P-2(t;1), and the relation of this quantity to the stress sigma(t) computed by the atomic virial stress formula. It is found that the ratio P-2(t;1)sigma(t) has a low value in the early glassy period and then undergoes a transition to a higher value that remains substantially constant. An explanation on the atomic level for the behavior of this ratio, which bears a close relation to the stress-optical coefficient is given. Various modes of coarse-graining the model melt are considered by subdivision of each chain into segments, each with NR bonds. A second, molecular, calculation of the stress is made for the coarse-grained melt by use of the entropic spring stress formula and denoted by sigma(e)(t;N-R). At early times sigma(t) > sigma(e)(t;N-R) for all N-R. At later times, the value of N-R for which sigma(t) = sigma(e)(t;N-R) increases from N-R = 5 to N-R = 50. In these simulations, no value of N-R is found for which sigma = sigma(e) for an extended period. Conceptual difficulties, suggested by these simulations, with the use of Rouse dynamics for the calculation of the plateau onset and plateau modulus are discussed.

We have succeeded in measuring precisely the spin precession angle of neutrons tunneling through a Permalloy45 (Fe55Ni45) ferromagnetic film as a function of incident angles. The angles of additional spin precession are directly observed as a shift of neutron spin echo (NSE) signals by inserting the film in one of the Larmor precession fields of a transverse NSE spectrometer, and are well reproduced by the relative phase difference between up arrow and down arrow spin neutron wave functions, which are derived by solving the one-dimensional Schrodinger equation for the optical potentials in the film. The Larmor times are extracted from the additional spin- precession angles for nontunneling and tunneling cases. [S1050- 2947(99)07403-X].

A new principle of neutron-spin-echo (NSE) method is proposed for neutron optics and neutron forward-scattering experiments. The essential of this method is to set a sample in one of the Larmor precession fields, This sample geometry gives new physical information, in particular the refractive index of neutrons for a Si single crystal which was determined to be 1 - (1.85 +/- 1.16) x 10(-5) at 5.7 Angstrom.

Spin-incoherent scattering reduces the polarization in a conventional spin echo spectrometer by a factor of 3, which is a serious drawback for incoherent measurements. With a special option of the zero-held spin echo technique called MIEZE1 this problem can be overcome. In a MIEZE1-spectrometer the analyzer is upstream of the sample, leaving the signal indifferent to magnetic fields and spin flips in the sample. Special care has to be taken in order to achieve equal flight lengths for all scattered neutrons. Using reflection geometry, measurements at all scattering angles are possible. Recent results From the NRSE instrument at the LLB/Saelay are presented.

A review is given on the state of the art of high resolution neutron spectroscopy techniques including backscattering and neutron spin echo. New methods in high resolution neutron spectroscopy such as double focusing, gradient, ultrasound excited, active, multiplexing, offset and other complex monochromators will be discussed. Sub-neV tunneling spectroscopy with NSE might become feasible soon. Projects on future neutron sources are mentioned. On these powerful sources new concepts for instruments will emerge from the symbiosis of ultra high flux, sharp time structures and modern neutron optics. These developments will lead high resolution neutron spectroscopy into exciting new applications in the 21st century.

Combined measurements are described involving elastic and quasi-elastic neutron scattering, quasi-elastic light scattering, nuclear magnetic resonance, and swelling pressure on an end-linked poly-(dimethylsiloxane) (PDMS) gel swollen to equilibrium in a good solvent (toluene) and the equivalent solution. The factors affecting the collective diffusion coefficient are considered. The swelling pressure measurements show that the osmotic modulus is appreciably depressed in the gel. The neutron spin-echo measurements reveal no difference in the dynamic response at intermediate and high values of the scattering vector Q. Elastic neutron scattering at small Q detects nonuniformities in the network structure, which are absent from the solution. These nonuniformities play a major role in the dynamic response of the system at lower Q and appear to be the cause of the observed reduction in osmotic pressure. The NMR measurements show a small increase of the solvent mobility in the gel, which is consistent with the appearance of structural nonuniformities in the system.

By using neutron spin echo and synchrotron radiation small- angle X-ray scattering, we have studied the structure and dynamics of water/sodium bis(2-ethylhexyl)sulfosuccinate/d(16)- n-heptane microemulsion with varying water/surfactant molar ratio, w(0). We have found the anomalous q-dependence of the effective diffusion coefficient D-eff at low w(0) whose peak- position at low w(0) does not satisfy the predicted relation of qR = pi (R, microemulsion radius). This would relate to the presence of the oligomeric phase at low w(0) observed by SAXS. (C) 1999 Elsevier Science Ltd. All rights reserved.

Neutron polarization analysis experiments of the past 25 years are reviewed. In that time the technique has progressed from a curiosity to being a useful tool to be used when needed. In early experiments, the polarization of the scattered beam was analysed in the same direction as the polarization of the incident beam but, in some later experiments, full three- dimensional polarization analysis has been employed. This article starts by writing down the interactions which the neutron has with condensed matter and deriving the cross- sections for scattering and final polarizations of the scattered beam. This is done displaying the spin state functions of the neutron explicitly. A variety of experiments is then reviewed, commencing with the elastic and inelastic scattering experiments performed by Moon, Riste Koehler in the late 1960s. Elastic scattering experiments where it is important to separate nuclear and magnetic cross-sections such as antiferromagnetic defect scattering are reviewed together with separation out of the nuclear spin scattering for various purposes. Of particular interest are the fully three- dimensional analysis experiments which reveal more about the structure and domain populations of certain antiferromagnets. Inelastic experiments for which polarization analysis is vital are those on paramagnets at high temperatures where it is necessary to discriminate against phonon scattering. Spin glasses are treated as frozen paramagnets Polarization analysis also has another role to play in the separation of magnetic modes in both paramagnets and ordered magnets, and several of these experiments are reviewed. Finally it is possible to tag the polarization of a neutron beam in time and space and to measure the result at another time and place and this through various techniques yields information about the change in neutron energy on scattering. The techniques of pseudo-random flipping time of flight, neutron spectral modulation and neutron spin-echo spectroscopy are briefly reviewed but the techniques of polarized-neutron-beam management are left to another review.

Recently, an alternative approach to the determination of the bending elastic constants of the surfactant film in droplet microemulsions was developed, which is based on a combination of neutron spin-echo spectroscopy (NSE) and dynamic light scattering (DLS). In this work the approach will be applied to investigate another C10E5-based microemulsion containing the oil n-dodecane. The determined values for kappa and <(kappa)over bar> compare well to values determined for the n- octane containing system. Small-angle neutron scattering (SANS) data will also be presented and discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

The bending elastic energy of a surfactant monolayer at the oil-water interface in a microemulsion system can be described by the spontaneous monolayer curvature and two elastic constants kappa and <(kappa)over bar>. Here we present the determination of these constants by a combination of dynamic light scattering and neutron spin-echo spectroscopy for the octane-C10E5-water microemulsion on the water continuous side of the phase diagram. Small-angle neutron scattering data are also presented and compared to the light scattering results.

We have observed, for the first time, inelastic neutron scattering from the phonons externally excited in single crystals. We have used ultrasonic transducers (50-200 MHz) for the excitation and neutron spin-echo spectroscopy for observing acoustic phonons. This allowed us to trace evolution of the elastic Bragg intensity I(0) and of the phonon harmonics up to the seventh order as a function of the excitation power. In contrast to the common belief, we have found that in perfect Si ultrasound does not seriously increase I(0), but an apparent increase in the Bragg intensity is entirely due to the onset of inelastic scattering. At the highest power we observed a cross- over to a chaotic phonon behavior. In deformed Si I(0) strongly decreases at a low acoustic power. The same effect could be observed in KBr and graphite crystals where the ultrasonic field made the entire mosaic blocks vibrate. (C) 1998 Elsevier Science B.V. All rights reserved.

Based an considerations of how a neutron interferometer operates in the velocity space, a new type of high-resolution spectrometer for quasielastic neutron scattering is proposed. It has an extremely high energy resolution (in the picoelectronvolt region), which is a few orders of magnitude higher than the resolution of neutron spin-echo spectrometers and principally does not require polarized neutron beams.

The object of this report is to review the recent results and some perspectives on cold neutron interferometry using multilayer neutron mirrors developed in KUR. The purposes of our research are to develop new experimental techniques, new devices for controlling neutron spin states for opening up a new world of physics. A multilayer neutron polarizer working under a very low external magnetic field (low field magnetic mirror), Funahashi's interferometer, a cold neutron spin echo instrument, a multilayer neutron spin splitter and a cold neutron pulser were developed. The equivalency of the quantum precession induced by the effective gap layer of the multilayer neutron spin splitter and Larmor precession of neutron spin in a magnetic field is established. This idea allows us to develop a cold neutron phase-spin-echo interferometer (a neutron spin interferometer). The neutron spin interferometer and a cold neutron pulser enabled us to carry out the delayed choice experiment for answering the question "how the neutron wave propagates after being split by the semitransparent mirror (wave splitter)". These matters are reviewed in this paper.

We describe neutron beams by their coherence properties. Examples for various instruments are given and the relations between coherence volume and resolution become obvious. Special emphasis is given to the spin-echo case.

We want to review the applications of high-frequency-flippers in neutron spin echo and related fields: Two separated flippers of this type can replace a homogenous static magnetic held in a spin-echo spectrometer. We describe the spin-echo instruments at Saclay and Berlin relying on such flippers and present some measurements. By tilting these coils with respect to the beam axis, one can determine line widths of phonons with unprecedented precision. First experiments are shown. With two flippers driven at different frequencies one can realise an instrument ("MIEZE"), where the intensity at the detector is modulated with a frequency up to several MHz. Its use for high- resolution quasielastic scattering from protonated samples is demonstrated. The high-frequency signal may be used for "neutron computer holography" - we discuss a possible realisation. (C) 1999 Elsevier Science B.V. All rights reserved.

We report results of a molecular dynamics simulation of an ''isotope'' mixture of polymer chains, which are represented by a standard bead-spring model, and whose two species differ only by their monomer masses. Detailed analysis of the Rouse modes shows that for sufficiently short (non-entangled) chains this system can be well described by the Rouse model. Each species is described by its individual monomeric friction coefficient, whose dependence on both mass ratio as well as mixing ratio is studied. The main effect of mixing is an acceleration of the slower chains and a slowdown of the faster ones, while both species remain dynamically different. Some microscopic insight into the mechanism is obtained by studying the short-time behavior of the monomeric velocity autocorrelation function. Studies in the slightly entangled regime (chain length up to N = 150, where the typical entanglement chain length is N-e approximate to 35) seem to further corroborate the hypothesis that the ''tube diameter'' of the reptation model is a quantity which results mainly from the static configurations, i.e., is an equilibrium thermal average. The usefulness of recently suggested analysis methods in this regime is briefly discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)52141- 6].

The size of glass transition cooperativity for 11 polymers and glycerol is determined from heat capacity spectroscopy (HCS) and differential scanning calorimetry (DSC) by using a calorimetric fluctuation formula. The data cover in total the 0.25 less than or equal to x less than or equal to 0.95 part of the reduced temperature interval between the Vogel temperature and a cooperativity onset temperature. A steep cooperativity increase at lower temperature is observed. The data are consistent with the hypothesis that the temperature dependence can be described by the enlargement of a minimal cooperativity at the onset.

The mode-coupling theory (MCT) of the glass transition can exhibit stretching (nonexponential) behavior observed in the primary relaxation of supercooled liquids near the glass transition. Gotze first showed that MCT can exhibit this stretching behavior in the alpha-relaxation regime by phenomenologically introducing a linear term into the mode- coupling integral for the dynamic viscosity. We show in this paper how this linear term can be generated using fundamental considerations. In particular, we extend fluctuating nonlinear hydrodynamics of compressible liquids, developed by Das and Mazenko [Phys., Rev. A 34, 2265 (1986)], to include another slow variable in addition to the conserved slow variables such as the mass and momentum densities and show that the extended theory naturally generates this linear term. The additional slow variable is defined as the distortion field of the local structure inherent in dense liquids. The suppression of the cutoff mechanism due to Das and Mazenko is also discussed.

We investigate the current status of the mode-coupling theory (MCT) of the glass transition by making a comparison with two recent sets of experiments: the scaling data resulting from the dielectric measurements of Dixon et al. [Phys. Rev. Lett. 65, 1108 (1990)] and the neutron-scattering data. Our main conclusions include the following. (a) Experiments do show the sequence of time scales in the relaxation predicted by MCT. In particular, we believe that the high-frequency tail of the data of Nagel and co-workers is the realization of the so-called von Schweidler relaxation predicted by MCT in the frequency regime. (b) The nature of scaling discovered by Dixon et al. is quite different from and more universal than that predicted by MCT, which is basically a time-temperature superposition. Furthermore, the dielectric measurements show that there exists a universal relation between the exponent b governing von Schweidler relaxation and the exponent beta-governing a stretched exponential: 1 + b = 3/4 (1 + beta). (c) Dixon et al. find no evidence for the existence of a special temperature T0 (well above the phenomenological glass-transition temperature T(g)) above and below which the relaxation dynamics is substantially different. This constitutes a discrepancy between the two sets of experiments since neutron-scattering experiments have been interpreted as confirming the existence of such a temperature well above T(g). A possible reinterpretation of MCT is suggested so as to reconcile these two sets of experiments and MCT.

The dynamical aspects of the order-disorder phase transition in KDCO3 are studied by means of thermal-neutron spin-echo triple- axis spectrometry with a high-energy resolution of similar to 0.2 mu eV. Besides the critical slowing down of the correlated fluctuations of the (DCO3)(2) dimers, temperature-dependent fluctuations of the strain field caused by the individual low- symmetry dimers are seen in the form of dynamical Huang scattering around the main Bragg peak. An interesting mechanism for the order-disorder phase transition, namely a strain- mediated phase transition, is examined from a dynamical point of view.

Neutron spin echo measurements were performed on polymeric micelles consisting of deuterated polystyrene (d-PS)- polyisoprene (PI) diblock copolymers in dilute and concentrated solutions. The scattering length density of the core of the micelles matched to the solvent so that dynamics of polymer chains in the corona were measured. The observed intermediate scattering functions in the dilute solution were analyzed by the theory for tethered chains proposed by de Gennes. In the concentrated solutions, motional slowing down was observed, which may be caused not only by increase of the viscosity but also by decrease of the osmotic compressibility. (C) 1999 Elsevier Science Ltd. All rights reserved.

We develop the theory of dynamic light scattering from vesicle- like droplets - the fluid particles with flexible interfaces immersed in another fluid. The scattering cross section is calculated assuming that the thermal fluctuations of the droplet shape can be accompanied with density changes in the surface membrane. The obtained scattering function generalizes previous expressions from the literature and corrects them in the limiting case of incompressible membrane. Its constant part, in addition to the term given by the scattering on a non- fluctuating double-layered particle, contains contributions reflecting the droplet fluctuations in shape. The time- dependent part reflects both the shape and density fluctuations of the membrane. The frequencies of the fluctuations, calculated using the Helfrich's concept of interfacial elasticity, essentially differ from those used so far in the interpretation of experimental data. The theory well describes the diffusing-wave spectroscopy experiment on emulsions by Gang et al. [Phys. Rev. E 52 (1995) 6289]. (C) 1999 Elsevier Science B.V. All rights reserved.

We report neutron spin echo measurements of the dynamic correlation function of cross-linked polydimethylsiloxane disolved in toluene. The experiments were performed on semi- dilute solutions. The main result is that at the frequencies probed by the spectrometer, there is no difference between the dynamics of cross-linked and uncrosslinked polymers. The dynamics is Zimm like and includes hydrodynamic interactions. Using a reduced time variable, it is possible to plot all the experimental data for both concentrations we studied on the same mastercurve. Moreover, previous results by Csiba et al. on uncross-linked PDMS are also located on the same curve. This shows that cross-linking has no dramatic effect on the local dynamics of the linear chains,

The results of a detailed molecular-dynamics investigation of the liquid-glass transition in a realistic model of the metal- metalloid system Ni80P20 are presented. Several static quantities, such as mean-square amplitude of vibration, volume, enthalpy, and coordination numbers, exhibit a marked change at a point we identify as the glass transition temperature, in agreement with a recent Mossbauer study of a similar system. From a dynamical viewpoint, we find, in accord with the mode- coupling theories for supercooled liquids, that relaxation proceeds in two stages: fast (or conformational) relaxation, related to local rearrangements of atoms, and slow relaxation, connected with atomic transport, i.e., diffusion; these processes are referred to as beta and alpha, respectively. Our simulations show that diffusion exists even in the glass state, where it proceeds mostly by jumps, in contrast to the liquid phase where it is continuous. This relaxation mechanism is well described by a stretched exponential (Kohlrausch) law. The fast relaxation regime, on the other hand, does not correspond to that predicted by mode-coupling theory. This may be a consequence of the presence of intense jump-diffusive motion in our system.

A detailed molecular-dynamics investigation of the liquid-glass transition is carried out for a realistic model of the metal- metalloid glass-former Ni80P20, with a view to assessing some of the predictions of the recent mode-coupling theory. This theory suggests that relaxation in glasses proceeds in two steps: fast relaxation at short times, corresponding to local atomic rearrangements and described by a power law, and slow relaxation at long times, corresponding to atomic transport (i.e. diffusion) and described by a stretched exponential (Kohlrausch law). The latter essentially disappears at the transition when the liquid becomes "structurally arrested", i.e. glassy. The transition is therefore characterized by the passage from a state of ergodicity (the liquid) to a state of non-ergodicity (the glass). Our simulations agree with the results of mode-coupling theory for the existence of two scaling regimes, but do not lead to critical exponents consistent with the predicted behaviour.

We studied the diffusion of single domain particles of Mn0.3Zn0.7(Fe2O4) (R-p approximate to 100 Angstrom) stabilized by oleic acid in dodecane near the transition temperature T-c, of particles from superparamagnetic to paramagnetic state. The structure of the ferrofluid has been studied by small angle neutron scattering. We found a strong density fluctuation near T-c, 2 20 degrees C and H = 0, induced by critical spin diffusion. In addition, we studied this system using neutron spin echo (NSE) inelastic scattering. The temperature variation of the self-correlation function G(S)(T, q, t) = exp[ - q(2)D (t/tau)(beta(T))] at momentum transfer q = 0.07 Angstrom (-1) in the time interval t = 0.5-40 ns reveals strong influence of magnetic fluctuations on the mobility: these fluctuations decrease the diffusion rate D and induce stretched relaxation (beta approximate to 0.6) at T < T-c approximate to 16 degrees C. Above T,, Stokes diffusion dominates (beta approximate to 1) (C) 1999 Published by Elsevier Science B.V, All rights reserved.

We describe a new version of Neutron Spin Echo, for the study of magnetic flux in high-T-c superconductors, which is based on the evolution of the echo group in the specimen which is installed in the third precession region. The polarization of the transmitted beam reflects the spin rotation in the internal field which can be found by Fourier transforming the data. It is required for example, in research on the flux self- organization in the critical state. Experiments on Y-Ba-Cu-O ceramics are discussed.

Neutron spin echo and triple axis spectrometry were used to study the critical dynamics of a AuFe re-entrant ferromagnet (x = 16%). Our data show an unprecedented behaviour: for the same T/T(c), kappa is almost one order of magnitude smaller than in pure Fe, and GAMMA does not depend on the temperature up to T/ T(c) less-than-or-equal-to 1.3.

We consider the neutron Dirac equation with electric moment in addition to magnetic moment, solve it rigorously in a uniform electromagnetic field, and set up the relativistic neutron spin-echo theory with a magnetic moment. We also solve the equation in an alternating magnetic field.

The tracer diffusion of a liquid in a porous media was studied using the neutron spin echo technique. The samples used consisted of porous Vycor glass saturated with a mixture of D2O/H2O (64/36) water to match out the coherent small-angle scattering component due to the neutron scattering density contrast between water and the Vycor matrix, in order to reduce the small-angle neutron scattering and the chemical diffusion contribution to the quasi-elastic scattering. A series of measurements has been made in the Q range 0.07-0.14 angstrom-1 using the spin echo spectrometer IN11 at the ILL. This experiment in principle enables us to measure the tracer diffusion coefficient of the liquid within a pore and through the local pore network and hence to investigate the interconnectivity of these pores through a determination of < r2(t) < over length scales to about 100 angstrom.

In the study of ferrofluid dynamics the separation of self and pair correlation is the essential problem at both intermediate and high concentrations of the magnetic phase. The neutron spin-echo (NSE) total scattering function S-t(q, t) = beta S- p(q, t) + (1 - beta)S-s(q, t) contains both contributions, where the parameter beta = sigma(p)/(sigma(p) + sigma(s)) depends on the corresponding cross sections sigma(s,p). To distinguish these different kinds of dynamics we eliminated one of them by using a mixture of magnetic particles with different scattering amplitudes and concentrations. The NSE experiments (LLB, CE-Saclay) at momentum transfer q(1) approximate to 0.4 nm similar to 1/D-p (D-p is the particle diameter) and q(2) approximate to 0.7 nm > 1/D-p have been carried out for these opposite situations. The functions S-p(q, t), S-s(q, t), from these data behave quite differently. The autocorrelations reveal a stretched relaxation at low q: S-s(q, t) = exp[ - q(2)r(t)/2], where Gamma(t) = 2Dt(alpha) is the squared particle displacement, the exponent alpha = 0.5 +/- 0.1 at q(1), and alpha = 0.8 +/- 0.1 at q(2). The magnitude alpha < 1 indicates the strong influence of dipole forces on the motion of a particle. It resembles the segmental relaxation in a polymer chain. On the other hand, the pair correlations show the oscillations (period similar to 40 ns, amplitude - 1 nm) mixed with diffusion. (C) 1999 Elsevier Science B.V. All rights reserved.

The static structure factor S(Q) and the coherent dynamic structure factor S(Q,t) are calculated from molecular dynamics simulations of polyisoprene melts and compared with neutron scattering results [R. Zorn, D. Richter, B. Farago, B. Frick, F. Kremer, U. Kirst, and L. J. Fetters, Physica B 180&181, 534 (1992)]. Both the shape and the absolute time scale of the calculated S(Q,t) are consistent with experimental results. The decay of S(Q,t) can be almost entirely attributed to intramolecular dynamics throughout the Q range studied (1.2 less than or equal to Q less than or equal to 3.0 Angstrom(- 1)), i.e., the full S(Q,t) can be approximated by considering only the self terms and the cross terms localized to within a few repeat units along the chain. It was found that the factor of 5 observed between the dynamics at the first two peaks of S(Q) is part of a general trend largely independent of whether S(Q) is at a minimum or a maximum. A comparison of S(Q,t) in the region of the first peak in S(Q) and the P2C-H bond vector orientation autocorrelation function F-C(t) suggests that the same molecular motions influence both the neutron spin echo and NMR T-1 relaxation experiments. [S1063-651X(98)12712-5].

Conventional resolution inelastic neutron scattering spectroscopy allows us to explore the behaviour of condensed matter essentially on the time scale of thermal atomic vibrations. By the application of the Neutron Spin Echo trick, which enables us to get around the Liouville theorem limitation of conventional methods, the resolution can be improved very substantially. This opened up the field for the study of a large variety of slow motion phenomena (critical slowing down, relaxation effects, disordered dynamics, soft matter), i.e. the investigation of processes on a mesoscopic time scale between microscopic collision times and macroscopic dynamics.

Diblock copolymers in the melt exhibit order-disorder phase transitions (ODT), which are accompanied by strong concentration fluctuations. These transitions are generally described in terms of the random phase approximation (RPA) of Leibler and Fredrickson, which is able to explain small angle scattering results in the neighborhood of the ODT, in particular around the correlation peak at q*. The RPA theory has been extended to include dynamical phenomena, predicting the short time relaxation of the dynamic structure factor in polymeric multicomponent systems. We report small angle neutron scattering and neutron spin echo experiments on polyethylene- block-polyethylethylene (PE-PEE) and poly(ethylene-propylene)- block-polyethylethylene (PEP-PEE) copolymers with molecular weights of 16.500 and 68.000 g/mol, which explore the structure and dynamics of these block copolymers. Studying melts with different hydrogen/deuterium labeling it was possible to observe experimentally the different relaxation modes of such systems separately. In particular the collective relaxation behavior as well as the single chain motion were accessed. The experimental results were quantitatively compared with the RPA predictions, which were based solely on the dynamical properties of the corresponding homopolymers and the static structure factors. The collective dynamics exhibits an unanticipated fast relaxation mode. This mode is most visible at low wave numbers (q greater than or equal to q*) but extends to length scales considerably shorter than the radius of gyration. Furthermore, the dynamical RPA yields expressions for the mobilities of chain segments in the block copolymer melt. These combination rules are at variance with the experimental findings for the single chain dynamics, while they hold for the collective response. (C) 1999 American Institute of Physics. [S0021-9606(99)52120-X].

We report impedance spectroscopy experiments in LiCl,xH(2)O with x similar to 6. We have found two separate relaxations at temperatures well above the glass transition temperature Tg=135 K. The low frequency contribution (f<10(6) Hz) is consistent with previously reported similar data. The high frequency contribution (10(6)

[Mon99]

M. Monkenbusch. Correction scheme for divergent beams in zero-field spin-echo spectrometers. Nuclear Instruments & Methods in Physics Research Section A-, 437(2-3):455-458, 1999.

A scheme to correct for path length differences due to divergent beams that cause resolution limitation for the zero- field spin-echo technique (Golub and Gahler, Phys. Lett. A 123 (1987) 43, Gahler and Golub, Physica B 97 (1992) 234) is proposed. The ability to use divergent beams will allow for the use pf larger area detectors and for a larger incoming divergence. Thereby, the effective count rate of zero-field spin-echo spectrometers may be significantly enhanced. (C) 1999 Elsevier Science B.V. All rights reserved.

[Mon90]

M. Monkenbusch. On solenoid design for neutron spin-echo spectrometers. Nuclear Instruments & Methods in Physics Research Section A-, 287(3):465-475, 1990.

[MSB+83]

C. Mazureespejo, M. Schlenker, J. Baruchel, J. P. Guigay, and F. Mezei. Local measurement of magnetization in a gallium-substituted yig crystal by neutron spin-echo. Physica B & C, 120(1-3):66-69, 1983.

[MSR97]

M. Monkenbusch, R. Schatzler, and D. Richter. The julich neutron spin-echo spectrometer - design and performance. Nuclear Instruments & Methods in Physics Research Section A-, 399(2-3):301-323, 1997.

The new neutron spin-echo (NSE) spectrometer that recently went into operation in the guide hall ELLA of the DIDO reactor FRJ-2 in Julich is described. Design consideration and measured performance data are reported. The innovation of magnetic decoupling by rigidly coupled compensation loops yields a number of benefits as ease of operation, stability and immediate access to high scattering angles. A new technical realization of the correction elements yields large area coverage and high transparency. Together with an area multidetector this results in a highly efficient neutron usage. The operation program allows for ''set-and-run'' operation without tuning increasing again the overall neutron economy.

[MSR+95]

M. Monkenbusch, D. Schneiders, D. Richter, B. Farago, L. Fetters, and J. Huang. Dynamics of polymer brushes - what can neutron spin-echo spectroscopy contribute. Physica B, 213:707-711, 1995.

Polymer brushes forming the outer shells of micellar aggregates of A-B diblock copolymers in a selective solvent allow for the observation of collective relaxational motions via neutron spin-echo spectroscopy. New neutron spin-echo investigations on planar FEB-brushes on the surface of crystalline PE-cores of platelet-like aggregates are reported. The data are discussed in comparison to a continuum model of the brush assuming a parabolic density profile, semidilute solution scaling relations for the elastic properties and friction coefficient. For the planar brush a full 3D solution of the model is accessible. The observed scattering from an isotropic sample is compatible with the continuum model with an added static fluctuation contribution.

[MSR+94]

M. Monkenbusch, D. Schneiders, D. Richter, B. Farago, L. Fetters, and J. Huang. Aggregating block-copolymers as model systems to study polymer brush dynamics. Nuovo Cimento Della Societa Italiana Di Fisica D-condensed, 16(7):747-755, 1994.

A-B block copolymers in a selective solvent-good for the B- species and bad for the A-species-form micellar aggregates with a compact A-core with a corona (brush) of B <> reaching into the solvent. Whereas polystyrene(PS)-polyisoprene(PI) in decane forms spherical micelles with a PS core of about 10 nm radius, polyethylene(PE)-polyethylenepropylene(PEP) forms micellar platelets, the shape of which is goverend by the habitus of PE crystallites forming the core. These planar aggregates have large (several hundred nanometers) lateral extension and a core thickness in the range of 10 nm. Both systems are model systems for polymer brushes, either on a spherical surface or planar. Neutron spin-echo experiments allow for the investigation of the dynamics of the brushes which reflects their viscoelastic properties. Results of neutron small-angle and spin-echo investigations are reported. The brush dynamics is explained using a model based on an idea of de Gennes describing the brush properties in terms of scaling relations for osmotic pressure and viscosity of a semi- dilute solution with inhomogeneous density.

[MWP+98]

A. Meyer, J. Wuttke, W. Petry, O. G. Randl, and H. Schober. Slow motion in a metallic liquid. Physical Review Letters, 80(20):4454-4457, 1998.

Liquid Zr46.8Ti8.2Cu7.5Ni10Be27.5 has been studied by quasielastic neutron scattering. The microscopic dynamics of this bulk glass forming alloy deviates qualitatively from simple metallic liquids; it shows two-step structural relaxation like other Liquids in their supercooled state. Unexpectedly for a nonfragile system, fast beta relaxation is in full accord with the mode-coupling theory of the liquid to glass transition. The long-time alpha relaxation suggests failure of the Stokes-Einstein relation.

[NH93]

D. A. Neumann and B. Hammouda. Ultra-high resolution inelastic neutron-scattering. Journal of Research of the National Institute of Standards and, 98(1):89-108, 1993.

Two types of ultra high energy resolution neutron scattering instruments, the backscattering spectrometer and the spin echo spectrometer, are described. Examples of the types of research which can be done with these instruments are given and plans for a cold neutron backscattering spectrometer which will be built in the NIST Cold Neutron Research Facility (CNRF) are discussed. It is hoped that this information will be of use to researchers considering neutron scattering experiments at NIST.

[NRH+86]

B. Nystrom, J. Roots, J. S. Higgins, B. Gabrys, D. G. Peiffer, F. Mezei, and B. Sarkissian. Dynamics of polystyrene sulfonate ionomers in solution - a neutron spin-echo study. Journal of Polymer Science Part C-polymer Letters, 24(6):273-281, 1986.

[OERF88]

R. Oeser, B. Ewen, D. Richter, and B. Farago. Dynamic fluctuations of crosslinks in a rubber - a neutron- spin-echo study. Physical Review Letters, 60(11):1041-1044, 1988.

[OPH+99]

G. S. Oakley, S. Pouget, A. Harrison, J. Frunzke, and D. Visser. Neutron spin echo study of magnetic fluctuations in the kagome antiferromagnet (d3o)fe-3(so4)(2)(od)(6). Physica B, 268:145-148, 1999.

The kagome lattice with antiferromagnetic nearest-neighbour exchange provides a simple example of a geometrically frustrated magnet with a highly degenerate ground state. Theoretical work indicates that the static and dynamic correlations of this magnet are unconventional, with strong fluctuations down to very low temperatures. Deuteronium Jarosite ((D3O)Fe-3(SO4)(2)(OD)(6)) provides a rare example of a suitable model material to test such theory. DC susceptibility studies show strong antiferromagnetic exchange between the S = 5/2 moments (theta(CW) congruent to - 700 K), and a spin-glass transition at T-f congruent to 13 K. We report on neutron spin-echo measurements made on this compound using the polarised neutron spectrometer IN11 at ILL taken with momentum transfer Q = 1.35 Angstrom(-1) for times ranging from 9.7 to 870 ps. The magnetic fluctuations were observed to slow down gradually on cooling through T-f, until they appeared to be static at the shortest experimental timescale at 1.5 K. (C) 1999 Elsevier Science B.V. All rights reserved.

[PA99]

S. Pouget and M. Alba. Nse investigation of the spin dynamics in the nearly percolating frustrated insulating compound cdcr2(1-x)in2xs4 (x=0.10). Physica B, 268:304-307, 1999.

Neutron spin echo spectroscopy was used to investigate the dynamics of the disordered frustrated magnetic system CdCr1.8In0.2S4. This dynamics can be described as the sum of two exponential relaxation processes. While the fast one does not depend on the temperature down to 25 K, the slow relaxation evidences a continuous slowing down on decreasing the temperature. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

[PDJ94a]

B. Prevel, J. Dupuyphilon, and J. F. Jal. H-bond and glassy state formation - a structural and dynamic study applied to aqueous-solutions. Journal of Molecular Structure, 322:141-149, 1994.

Aqueous electrolytic solutions MX.RH2O, especially with LiCl as solute, are good at forming a glass. At some specific concentrations, compound formation can be bypassed on cooling the system. This occurs when the hydration number is such that the solution has lost the characteristic behaviour of water. Structural relaxation can be analysed easily in the three thermodynamic states: liquid, metastable (supercooled liquid) and out of equilibrium (glass). The local structural relaxation is given by neutron spin echo spectroscopy. The structural evolution can be followed efficiently also using neutron scattering. Indeed using isotopic substitution on nearly all the elements of the material Li, Cl and D we can derive the evolution of the short, medium and long range order in the three thermodynamic states. Correlated to the strong slowing down of the structural relaxation, is the ability of the supercooled and glassy states to construct characteristic H- bonded local configurations.

[PDJ+94b]

B. Prevel, J. Dupuyphilon, J. F. Jal, J. F. Legrand, and P. Chieux. Structural relaxation in supercooled glass-forming solutions - a neutron spin-echo study of licl, 6d2o. Journal of Physics-condensed Matter, 6(7):1279-1290, 1994.

We have performed neutron spin-echo measurements on the supercooled glass-forming system (LiCl)-Li-7, 6D2O above its glass transition temperature (T(g) = 135 K). This investigation has permitted us to determine the time dependence of the density correlation function phi(Q,t) between 2 ps and 2 ns at the maximum of the structure factor(Q = 1.9 angstrom-1). We have shown for the viscosity, the existence of two temperature domains limited by two critical temperatures (fixed points). This crossover from the stable liquid to the metastable liquid might be related to a change in the multiplicity of the microscopic interactions. In this study we show that the dynamics of the structural relaxation is consistent with the two scaling laws of the viscosity. Both the average relaxation time and the stretched exponential behaviour indicate a crossover regime around 210 K. However, the scaling of the density correlation function differs, in the lower-temperature regime, from the scaling of the shear viscosity. For the supercooled liquid dynamics, the results are more or less correlated to the mode-coupling theory.

[PKB+99]

C. Pappas, G. Kali, P. Boni, R. Kischnik, L. A. Mertens, P. Granz, and F. Mezei. Performance of the multidetector nse spectrometer span at bensc. Physica B, 268:285-288, 1999.

The neutron spin-echo spectroscopy (NSE) precession field, which is created by three pairs of coils with diameters 1, 3 and 4.8 m, respectively, is the peculiarity in the design of SPAN. Each pair is mounted in a Helmholtz-like fashion, with the electric currents oriented oppositely to each other, one coil above and one coil below the horizontal scattering plane. The centers of all coils are placed on the same vertical axis, which crosses the sample position. The NSE field is horizontal with axial symmetry. Particular care has been taken to shape the magnetic field around the sample, which is weak, vertical and very homogeneous. We present a first series of NSE measurements, which show that only little held adjustment is required to satisfy the exact echo condition (phase) simultaneously for a range of scattering angles and confirm the high axial symmetry of the precession field. The results show that the new magnetic field configuration of SPAN allows for multidetector NSE over a wide angular range. (C) 1999 Elsevier Science B.V. All rights reserved.

[PKL+93]

W. Pfeiffer, S. Konig, J. F. Legrand, T. Bayerl, D. Richter, and E. Sackmann. Neutron spin-echo study of membrane undulations in lipid multibilayers. Europhysics Letters, 23(6):457-462, 1993.

The undulatory excitations of highly ordered stacks of lipid bilayers at a high hydration level (19 weight % of water; water thickness d(w) almost-equal-to 10 angstrom) was studied by spin echo spectroscopy in the wave vector range 0.03 angstrom-1 < q < 0.125 angstrom-1, i.e. at qd(w) less-than-or-equal-to 1. The linewidth of the excitations exhibits a scaling law GAMMA(q) is-proportional-to q2.5+/-0.5. Undulations of a free membrane are expected to show a q3-behaviour and stacks of coupled bilayers a q2-dependence. Although both cases are still possible according to our scaling analysis, they can be ruled out as the frequencies expected from theory are a factor of 100 to 1000 higher than the observed ones. Our data seem to favour a recent theory by Seifert and Langer taking into account that the bilayer consists of two single layers coupled by friction. A new mode with a (omega is similar to q2)-dependence is predicted and its calculated frequencies are in accordance with the experimentally observed order of magnitude.

[PKR96]

P. T. Por, W. H. Kraan, and M. T. Rekveldt. Neutron spin-echo technique applied to 3-d neutron polarisation analysis in magnetised media. Nuclear Instruments & Methods in Physics Research Section A-, 374(1):79-84, 1996.

In this paper a method, based on the principle of neutron spin- echo, is described to perform 3-D neutron polarisation analysis in magnetised media. This method, the so-called rotation compensation method, allows for the determination of the magnetic properties of a magnetised sample that are contained in the parallel and perpendicular component of the polarisation, with respect to the sample's magnetisation, after interaction with the sample. This can be done without any corrections for the depolarisation of the beam due to the finite wavelength distribution of the beam.

[PPRF91]

E. A. Pavlatou, G. N. Papatheodorou, A. K. Rizos, and G. Fytas. Dynamic light-scattering and low-frequency raman-scattering study of ionic kno3.ca(no3)2 mixtures. Journal of Non-crystalline Solids, 131:88-91, 1991.

The density correlation function, C(t), in the mixed ionic glass KNO3.Ca(NO3)2 with three different compositions between 50 and 70% at temperatures near and above the glass transition, T(g), was measured by photon correlation spectroscopy. The C(t) which is of recent theoretical interest is well represented by the non-exponential form exp(-t/tau)beta with beta = 0.47 +/- 0.03 insensitive to temperature and composition variation in the investigated range. The relaxation time, tau, is independent of the wavevector, q, in the range (1.2-4.7) x 10(-3) angstrom -1 and its dependence on the composition arises mainly from the composition-dependent T(g). The relaxation time which is in agreement with recent neutron spin-echo (NSE) (time domain 2 x 10(-11)-2 x 10(-9) s) is associated with local rearrangements involving a few neighbouring ions. Information about local structure can be extracted from the low-frequency Raman spectra.

[PSY+98]

W. Paul, G. D. Smith, D. Y. Yoon, B. Farago, S. Rathgeber, A. Zirkel, L. Willner, and D. Richter. Chain motion in an unentangled polyethylene melt: a critical test of the rouse model by molecular dynamics simulations and neutron spin echo spectroscopy. Physical Review Letters, 80(11):2346-2349, 1998.

We have investigated the dynamic structure factor for single- chain relaxation in a polyethylene melt by means of molecular dynamics simulations and neutron spin echo spectroscopy. After accounting for a 20% difference in the chain self-diffusion coefficient between simulation and experiment we find a perfect quantitative agreement of the intermediate dynamic structure factor over the whole range of momentum transfer studied. Based on this quantitative agreement one can test the experimental results for deviations from standard Rouse behavior reported so far for only computer simulations of polymer melt dynamics.

[QB99]

H. Qiu and M. Bousmina. New technique allowing the quantification of diffusion at polymer polymer interfaces using rheological analysis: Theoretical and experimental results. Journal of Rheology, 43(3):551-568, 1999.

A new technique allowing the quantification of diffusion at polymer/polymer interfaces and also measurement of the self- diffusion coefficient of polymer melts using rheological tools has been developed. The technique consists of measuring the dynamic moduli as a function of time for a multilayer sandwich- like assembly at temperature slightly above Tg. The technique was tested on a polystyrene/polystyrene (PS/PS) system sheared in oscillatory mode under small amplitudes of deformation for different times of welding. The experimental results showed that the dynamic complex shear modulus increased with the elapsed time of welding. The increase was attributed to the diffusion of polymer chains at the superimposed interface layers. Based on the reptation theory, an analytical expression for the self-diffusion coefficient as a function of polymer rheological material functions was derived. Graessley's expression [Graessley (1980)] for the bulk self-diffusion coefficient is recovered as a special case. The obtained self- diffusion coefficient for polystyrene was found to be in good agreement with the literature data reporting the self-diffusion coefficient of PS measured by classical techniques. (C) 1999 The Society of Rheology. [S0148-6055(99)01303-6].

[RAC+98]

D. Richter, A. Arbe, J. Colmenero, M. Monkenbusch, B. Farago, and R. Faust. Molecular motions in polyisobutylene: a neutron spin-echo and dielectric investigation. Macromolecules, 31(4):1133-1143, 1998.

We present neutron spin-echo and dielectric results on the local dynamics of polyisobutylene. The dielectric spectra reveal the existence of a so far unknown secondary relaxation process being distinctly different to previous theoretical predictions. Neutron spectra have been taken over a large range in momentum transfer Q and temperature. At the Q value of the first structure factor maximum the dynamic pair correlation function is selective for interchain motions. There the neutron spectra display the same temperature dependence and shape as the classical rheological data taken in the terminal zone and do not follow the temperature laws based on spectroscopic results. A quantitative evaluation combining the information content of the dielectric and neutron results reveals a small stepwidth of 0.5-0.9 Angstrom involved in the secondary process. The alpha process is diffusive and follows the Gaussian approximation resulting in a sublinear time development of associated mean squared displacements.

[Rao91]

K. R. Rao. Update on reactors and reactor instruments in asia. Physica B, 174(1-4):491-498, 1991.

The 1980s have seen the commissioning of several medium flux (approximately 10(14) neutrons/cm2s) research reactors in Asia. The reactors are based on indigenous design and development in India and China. At Dhruva reactor (India), a variety of neutron spectrometers have been established that have provided useful data related to the structure of high-T(c) materials, phonon density of states, magnetic moment distributions and micellar aggregation during the last couple of years. Polarised neutron analysis, neutron interferometry and neutron spin echo methods are some of the new techniques under development. The spectrometers and associated automaton, detectors and neutron guides have all been indigenously developed. This paper summarises the developments and on-going activities in Bangladesh, China, India, Indonesia, Korea, Malaysia, the Philippines and Thailand.

[RBF+92]

D. Richter, R. Butera, L. J. Fetters, J. S. Huang, B. Farago, and B. Ewen. Entanglement constraints in polymer melts - a neutron spin-echo study. Macromolecules, 25(23):6156-6164, 1992.

Using neutron spin echo spectroscopy, we have studied the melt dynamics of polyisoprene (PI), saturated polybutadiene (PEB-2), and poly(ethylene-propylene) alternating copolymer (PEP). The experiments on all three polymers show that beyond a characteristic length d and after a crossover time tau(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 microscopically determined lengths agree well with those obtained from the plateau moduli if interpreted in terms of the reptation theory. The detailed line shape of the dynamic structure factor supports the concept of local reptation.

[RBT+91]

E. Rossler, K. Borner, J. Tauchert, M. Taupitz, and M. Poschl. Reorientational correlation-functions of simple supercooled liquids as revealed by nmr-studies. Berichte Der Bunsen-gesellschaft-physical Chemistry Chemical, 95(9):1077-1084, 1991.

P-31, H-1 and H-2 NMR investigations on supercooled simple organic liquids are presented. Combining measurements of spin- lattice relaxation, spin-spin relaxation and stimulated echo decays the reorientational correlation function F2(t) of the second Legendre polynomial is probed over a wide temperature interval. Correlation times tau in the range of 10(-11)-1s are covered, and the scaling property of the correlation function F2(t) corresponding to the alpha-relaxation, i.e., the main relaxation in the supercooled liquid, is checked. The function F2(t) can be described equally well by a Cole-Davidson distribution of correlation times or by Kohlrausch-Williams- Watts function (10(-8) < tau < 1s). Near T(g) in the highly viscous regime the correlation times become independent from the molecular size. At similar temperatures the bifurcation of a secondary process (beta-process) is observed. Combining results from neutron scattering and NMR the complete F2(t) has to be described by a three-step function near the glass transition temperature T(g) and by a two-step function at temperatures well above T(g). The transition at T(g) is accompanied by the onset of non-exponential spin-lattice relaxation and by a discontinious change of the temperature coefficient of the relaxation.

[RE88]

D. Richter and B. Ewen. Neutron spin-echo investigations on the dynamics of polymers. Journal of Applied Crystallography, 21:715-728, 1988.

[REFW89]

D. Richter, B. Ewen, B. Farago, and T. Wagner. Microscopic dynamics and topological constraints in polymer melts - a neutron-spin-echo study. Physical Review Letters, 62(18):2140-2143, 1989.

[Rek99]

M. T. Rekveldt. Transmission of polarised neutrons in magnetic materials. Physica B, 268:60-68, 1999.

An overview is given on some applications of neutron Larmor precession in magnetic materials. Neutron depolarisation has been applied to study a large variety of static and time dependent phenomena, ranging from superconductors, thin films, magnetic slurries to magnetic phase transitions. Some recent examples will be discussed in more detail. Tn addition, recent applications of Larmor precession, where neutrons are labelled with their wavelength and their angle of transmission through a precession device,will be discussed. This application is used to design a new SANS instrument based on neutron spin echo suitable for studying extremely large correlation distances and relatively short measuring times. Finally a new high-resolution diffraction instrument (Delta d/d similar to 10(-6)), based on Larmor precession is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

[Rek98]

M. T. Rekveldt. High-resolution diffraction using larmor precession and multiple narrow beams defined by multislits. Journal of Applied Physics, 84(1):31-37, 1998.

A new method is introduced to measure lattice constants by diffraction in Bragg scattering with very high accuracy. Use is made of Larmor precession of a polarized neutron beam in a magnetic field. The number of precessions is directly related to the wavelength of the neutron beam and the phase change in one precession period can be identified as a slightly changed wavelength which itself is indicative of a change in the lattice parameter. Using multislits as diaphragms in; the incident and in the diffracted beams, one selects angular regions of the neutron beam in the scattering plane with phase angles in Larmor precession that differ only by multiples of 2 pi at a certain magnetic field in the Larmor coil. Doing this, a very high resolution in lattice parameter can be obtained with an intensity, which is a multiple of the intensity in conventional diffraction methods at this high angular resolution. Use of the proposed method in materials science applications such as stresses in materials and in crystal size determination in materials is discussed. (C) 1998 American Institute of Physics.

[Rek97]

M. T. Rekveldt. Neutron reflectometry and sans by neutron spin echo. Physica B, 234:1135-1137, 1997.

The application of neutron spin echo as a small-angle neutron scattering instrument (SESANS) using DC magnetic fields is discussed. Its principle is based on the difference in Larmor precession angle in a coil system with changing transmission angle. A scatterer between two identical coils with opposite precessions causes depolarisation. The latter as a function of the precession field yields a real space correlation function. A modification of this principle of SESANS implemented into a polarised neutron reflectometer using a linear position- sensitive detector, is proposed, which enables one to measure with a full divergent beam. From two measurements with zero and maximum spin-echo field, the fraction of off-specular reflection can be determined at each point of the detector. Moreover, from a field-dependent measurement the full correlation function of the roughness perpendicular to the sample plane causing the off-specular reflection can be determined, when desired also in a direction with the momentum transfer in the plane of the sample by rotating the spin-echo coils over 90 degrees. The sensitivity of SESANS is up to the micron range.

[Rek96]

M. T. Rekveldt. Novel sans instrument using neutron spin echo. Nuclear Instruments & Methods in Physics Research Section B-, 114(3-4):366-370, 1996.

Following a recent report by Keller et al. in Neutron News 6, no. 3 (1995), the application of neutron spin echo as a small angle neutron scattering instrument (SESANS) using de magnetic fields is discussed. The principle is based on the difference in Larmor precession angle in a coil with changing transmission angle, caused by scattering from a sample. A scattering object between two identical coils with opposite precessions causes depolarisation. The latter as a function of the precession field yields a real space correlation function, A modification of this principle of SESANS is introduced enabling to suppress the contribution of inelastic scattering, which changes the precession angle too. The application of this technique is very promising for systems with large correlation lengths and may enable one to measure time dependent effects in correlation lengths with a time resolution of a few seconds. The latter corresponds to the time needed to measure a complete spectrum from which a correlation function in real space is derived.

[RFE+91]

D. Richter, B. Farago, B. Ewen, L. J. Fetters, and J. S. Huang. On the dynamics of dense polymer systems. Physica B, 174(1-4):209-217, 1991.

In this short review we discuss experimental results on molecular motion in polymer melts, obtained by neutron spin echo spectroscopy (NSE). We show that in the short-time regime the assumption of entropic restoring forces (Rouse model) describes perfectly well the space and time dependence of self- and pair-correlation functions of one chain molecule. For longer times or stronger geometrical constraints we observe systematic deviations from the Rouse model revealing the presence of a well-defined intermediate dynamical length scale beyond which density fluctuations within a given chain are strongly reduced. Its value is found to be in excellent agreement with the entanglement distance obtained from rheological measurements. Measurements of the temperature dependence of the enlargement distance, the radius of gyration and the plateau modulus give first insight into the molecular origin of entanglement constraints. The data favour packing models and contradict the topological approach.

[RFEO89]

D. Richter, B. Farago, B. Ewen, and R. Oeser. The fluctuations of cross-links in a rubber - a neutron spin- echo study. Physica B, 156:426-429, 1989.

[RFF88]

D. Richter, B. Frick, and B. Farago. Neutron-spin-echo investigation on the dynamics of polybutadiene near the glass-transition. Physical Review Letters, 61(21):2465-2468, 1988.

[RFH+91]

D. Richter, L. J. Fetters, J. S. Huang, B. Farago, and B. Ewen. Neutron spin-echo investigations on the dynamics of polymer systems. Journal of Non-crystalline Solids, 131:604-611, 1991.

Neutron spin-echo spectroscopy allows the observation of the long-range internal relaxation mechanism of macromolecules simultaneously in space and time. Thereby, it facilitates a microscopic study of molecular models applied for the explanation of macroscopic viscoelastic properties of polymer materials. After an outline of the method, experimental results on chain relaxation in polymer melts are discussed. It is shown that in the short-time regime, the Rouse model describes perfectly well the space and time dependence of the self- and the pair-correlation function. For longer times distinct deviations from the Rouse model towards slower relaxation are observed. These deviations exhibit a systematic dependence on the momentum transfer, Q, and thus show that beyond a certain length scale the relaxation of density fluctuations within a given chain is strongly reduced. This intermediate dynamic length scale shows direct evidence of the existence of a well-defined entanglement distance in polymer melts, confirming thereby the essential assumption of a reptation concept. Its value is found to be in excellent agreement with the entanglement distance obtained from the rheological measurement.

[RFH+89]

D. Richter, B. Farago, J. S. Huang, L. J. Fetters, and B. Ewen. A study of single-arm relaxation in a polystyrene star polymer by neutron spin-echo spectroscopy. Macromolecules, 22(1):468-472, 1989.

[RHE92]

N. Rosov, C. Hohenemser, and M. Eibschutz. Dynamic critical-behavior of the random-exchange ising system fe0.9zn0.1f2 determined via mossbauer-spectroscopy. Physical Review B-condensed Matter, 46(6):3452-3457, 1992.

We have made Mossbauer-effect-spectroscopy (MES) measurements of the dynamical broadening of the Mossbauer lines above T(N) for a high-quality single crystal of Fe0.9Zn0.1F2 with an inhomogeneity-induced Neel-temperature (T(N)) broadening of less than 25 mK. In the reduced-temperature range 3 X 10(-4) < t (= T/T(N)-1) < 10(-2), we find that the excess linewidth diverges as t(-w) with w = 0.79(7). On the assumption of a single divergent relaxation time, this measurement of w implies a dynamical exponent z = 2.18(10) via dynamic-scaling theory. Our value for z involves an asymptotic approach to T(N), which is an order of magnitude closer than previous MES and neutron- spin-echo measurements and agrees with the renormalization- group prediction z almost-equal-to 2.11 for the d = 3 random- exchange Ising model. The result does not agree with the predictions of conventional van Hove theory, z = 1.97, or the results of previous, fundamentally less asymptotic, measurements.

[Ric97a]

D. Richter. Neutron spin echo investigations in the alpha and beta relaxation regime of polybutadiene. Macromolecular Symposia, 121:147-161, 1997.

This short review presents quasielastic neutron scattering and dielectric experiments on the alpha and beta(slow) relaxation in polybutadiene. Exploiting the momentum transfer dependent dynamic structure factor, spatial information about the underlying molecular motions is obtained. While the beta(slow) process reveals itself as a local jump with average jump distances of about 1.5 Angstrom, the alpha relaxation is diffusive and occurs statistically independently from the beta(slow) process. With this result a consistent interpretation of dielectric spectra on the same polymer is achieved.

[Ric97b]

D. Richter. On the dynamics of polymers in dense systems - results of neutron spin echo spectroscopy. Hyperfine Interactions, 106(1-4):3-18, 1997.

One of the basic problems in the dynamics of polymers concerns the importance of geometrical or topological interactions which are directly related to the large scale molecular structures. In the famous reptation model these constraints are pictured in terms of a tube of localization following the average chain profile and confining the chain motion to the curve-linear tube. Recently studying the dynamic structure factor of a single labeled chain in a polymer melt by means of neutron spin echo spectroscopy (NSE) led to a direct observation of these tube constraints. Here I shall summarize these neutron spin echo experiments. I shall address the NSE technique, present results on the entropy driven segmental chain dynamics, discuss the dynamics of single chains in the melt where the chain length is increased through the transition to ''reptation'' dynamics and display NSE measurements on long chain systems which revealed the molecular existence of the entanglement distance. Their magnitudes agree very well with tube diameters derived from dynamical mechanical measurements on the basis of the reptation model proving thereby the basic assumption of this Nobel Price winning concept.

[Ric92]

D. Richter. Neutron spin-echo investigations on molecular-motion in polymers. Physica B, 180:7-14, 1992.

Neutron spin echo spectroscopy allows the observation of long range internal relaxation mechanisms of macromolecules, simultaneously in space and time. Thereby, it facilitates a microscopic study of the molecular origins of the macroscopic viscoelastic properties of polymer materials. The molecular understanding of polymer dynamics bases on concepts of entropic forces and topological and hydrodynamic interactions. I shall briefly outline the different concepts and then discuss experimental results on these different aspects of polymer motion on a molecular scale.

[Ric90]

D. Richter. Neutron spin-echo investigations on the dynamics of polymers. Molecular Crystals and Liquid Crystals, 180:93-100, 1990.

[RMA+99]

D. Richter, M. Monkenbusch, J. Allgeier, A. Arbe, J. Colmenero, B. Farago, Y. C. Bae, and R. Faust. From rouse dynamics to local relaxation: A neutron spin echo study on polyisobutylene melts. Journal of Chemical Physics, 111(13):6107-6120, 1999.

We investigated the single chain motions of monodisperse polyisobutylene chains in the melt by neutron spin echo spectroscopy. Thereby a wide range in momentum space over a large dynamic range was covered. Motional processes from the center of mass diffusion, the Rouse dynamics to the more local relaxation processes which limit the validity of the standard Rouse model, were elucidated. The observed dynamic structure factors were analyzed in terms of relevant theoretical approaches addressing the limiting factors of the Rouse model. We found that other than claimed in the literature effects of local chain stiffness-they were treated in terms of the all rotational states model and a bending force model-cannot account for the experimental observations. It appears that additional damping effects related to an internal viscosity of the chain have to be involved, in order to explain the experimental results. (C) 1999 American Institute of Physics. [S0021-9606(99)50537-0].

[RMA+97]

D. Richter, M. Monkenbusch, A. Arbe, J. Colmenero, and B. Farago. Dynamic structure factors due to relaxation processes in glass- forming polymers. Physica B, 241:1005-1012, 1997.

We report neutron spin echo experiments on the momentum (Q) and time (t)-dependent dynamic structure factor S(Q, t) from the glass-forming polymers polyisobutylene (PIB) and polybutadiene (PB). Performing measurements in a Q-range encompassing the first and second structure factor peaks, we are able to separate inter and intrachain relaxations and to assign the secondary Johari-Goldstein beta-relaxation to an intrachain relaxation. While for PB this process exhibits average motional amplitudes of about 1.5 Angstrom, the underlying jump distances for PIE are much smaller (0.6 Angstrom). The structural alpha- relaxation is characterized as an interchain process displaying anomalous diffusion. Its temperature dependence thereby agrees with that of the viscous flow. The data bear evidence that a polymer segment undergoes primary and secondary relaxation in a statistically independent way. (C) 1998 Elsevier Science B.V. All rights reserved.

[RRM98]

N. Rosov, S. Rathgeber, and M. Monkenbusch. Neutron spin echo spectroscopy at the nist center for neutron research. Abstracts of Papers of the American Chemical Society, 216:U865-U865, 1998.

[RS96]

E. Rossler and A. P. Sokolov. The dynamics of strong and fragile glass formers. Chemical Geology, 128(1-4):143-153, 1996.

We review recent results from Raman scattering experiments on different types of glass formers. The fast dynamics is studied in the frequency range 10(10) Hz < nu < 10(13) Hz and at temperatures near and above the glass transition temperature T- g. The Raman spectra are analysed in the spirit of mode coupling theory (MCT) which is briefly summarized. Both fragile and non-fragile glass formers exhibit similar high-temperature dynamics which qualitatively can be reproduced by MCT. Quantitatively, systematic deviations from MCT predictions are found. They are stronger the less fragile the glass former is. One possible reason for the deviations is the presence of low- frequency vibrations which also contribute to the dynamic structure factor. This boson peak is small for fragile and large for strong glasses. A change of the dynamics at a well- distinguished temperature T-c with T-c > T-g is observed in all cases. This crossover is also indicated by the a slow secondary relaxation process at T approximate to T-g upon cooling and by the breakdown of rotational translational coupling for T < T-c which has been found in simple liquids. Also, a viscosity analysis yields a master plot at T < T-c. All in all, the experiments support the idea of a change of transport mechanism in fragile and strong glasses at a characteristic temperature well above T-g.

[RS93]

S. Z. Ren and C. M. Sorensen. Relaxations in gels - analogies to alpha-relaxation and beta- relaxation in glasses. Physical Review Letters, 70(11):1727-1730, 1993.

We present dynamic light scattering data which show that aqueous gelatin gels display a power-law relaxation to a nonergodic background. In the pregel sol this power law is terminated by a stretched exponential which restores ergodicity and which has a q dependent characteristic time proportional to the viscosity. The power-law exponent is q dependent and related to a characteristic length in the gel. Except for the q dependences these behaviors are similar to the alpha and beta relaxation behavior in glasses. It is proposed that the different q dependences of the gels and glasses is a result of different characteristic length scales.

[RSEN87]

D. Richter, B. Stuhn, B. Ewen, and D. Nerger. Collective relaxation of star polymers - a neutron spin-echo study. Physical Review Letters, 58(23):2462-2465, 1987.

[RTE94]

E. Rossler, J. Tauchert, and P. Eiermann. Cooperative reorientations, translational motions and rotational jumps in viscous-liquids. Journal of Physical Chemistry, 98(33):8173-8180, 1994.

We have studied H-2 NMR relaxation of symmetric and nonsymmetric guest molecules dissolved in the glass former tricresyl phosphate (TCP). Correlation times for a liquidlike overall reorientation (tau(iso)) and for a solidlike rotational jump (tau(j)) are extracted and compared to the corresponding time constants of the host TCP which has been studied by P-31 NMR. While the liquidlike motion of the guests exhibits a strong slowing-down upon supercooling, the jump process stays fast and persists also in the glassy state. At high temperatures in the fluid liquid, guest and host molecules exhibit correlation times tau(iso) which are well distinguished: the smaller is the guest, the faster is its reorientation. However, in the highly viscous regime the time constants of guest and host molecules approach each other independent of molecular size. In this regime the guest molecules probe the glass transition of the host. Furthermore, the NMR results are compatible with assuming isotropic reorientation in the viscous and anisotropic motion in the fluid liquid. For some guests also translational correlation times have been obtained, and they tend to separate from those of the reorientational motion upon cooling; i.e. translational rotational coupling breaks down in the highly viscous regime. All in all, these observations support the idea of a change of transport mechanism in the supercooled liquid at a temperature 15-20% above the glass transition temperature.

[RWR+99]

S. Rathgeber, L. Willner, D. Richter, A. Brulet, B. Farago, M. Appel, and G. Fleischer. Polymer dynamics in bimodal polyethylene melts: a study with neutron spin echo spectroscopy and pulsed field gradient nuclear magnetic resonance. Journal of Chemical Physics, 110(20):10171-10187, 1999.

We have investigated the dynamics of polymers in bimodal polyethylene (PE) melts in the transition region from Rouse- to reptationlike behavior by varying the mass fraction Phi(t) of long tracer chains (N approximate to 3N(e) or 4N(e)) in a short-chain matrix (N approximate to N-e=entanglement segment number) over the full concentration range. At short times (ns) the dynamic structure factor for single-chain relaxation was investigated by neutron-spin-echo (NSE) spectroscopy. To obtain information about the long-time (ms) dynamics the tracer diffusion coefficient (D-NMR) was measured by pulsed-field- gradient (PFG)-NMR. We discuss our NSE data within a mode analysis which includes the relaxation rates W-p of the independent normal modes of the internal chain dynamics and the center-of-mass diffusion coefficient D-NSE as model parameters. Only modes exceeding the Phi(t)-dependent length of a single entanglement strand N-e(Phi(t)) are found to be strongly hindered by topological constraints. The D-NSE are Phi(t)- independent and systematically faster than the strong concentration-dependent D-NMR, suggesting an effective time- dependent diffusion coefficient. The Hess model, which we have generalized for polydisperse melts, provides a time-dependent diffusion coefficient. Taking chain-end effects into account we get an excellent description of the NSE data. The mobility of the chain ends is much higher than the mobility of the inner segments resulting in an entanglement segment number which increases with decreasing tracer concentration. The concentration dependence of N-e(Phi(t)), as obtained from the mode analysis and the Hess model, is in agreement with our calculation within a self-consistent modification of the model by Kavassalis and Noolandi for entanglement formation. (C) 1999 American Institute of Physics. [S0021-9606(99)50520-5].

[RWZ+94]

D. Richter, L. Willner, A. Zirkel, B. Farago, L. J. Fetters, and J. S. Huang. Polymer motion at the crossover from rouse to reptation dynamics. Macromolecules, 27(25):7437-7446, 1994.

Employing neutron spin-echo spectroscopy we have studied the dynamic structure factors for the relaxation of a single. chain;in polymer melts. We have varied the molecular weights through the transition region from unrestricted Rouse dynamics to entanglement controlled behavior. Investigating the dependence of the dynamic structure factor on the momentum transfer and, it is possible to access the different relaxation modes separately. We found that, depending on their spatial extension in relation to the entanglement distance, larger scale relaxations are successively slowed down compared to Rouse relaxation. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency. Furthermore, we solve explicitly the generalized Rouse model by Hess(12) and compare its. predictions to our data. Fitting only two parameters, all the Q and molecular weight dependent structure factors can be well reproduced.

[RWZ+93]

D. Richter, L. Willner, A. Zirkel, B. Farago, L. J. Fetters, and J. S. Huang. Onset of topological constraints in polymer melts - a mode analysis by neutron spin-echo spectroscopy. Physical Review Letters, 71(25):4158-4161, 1993.

By neutron spin echo spectroscopy we have investigated the dynamic structure factors for single chain relaxation in polymer melts varying the molecular weight through the transition from unrestricted Rouse motion to entanglement controlled behavior. From an analysis of the structure factors with respect to the different relaxation modes we found that, depending on their spatial extension in relation to the entanglement length, large-scale relaxations are successively suppressed with increasing molecular weight. A comparison with macroscopic diffusion and viscosity data yields excellent internal consistency.

[RZF+93]

D. Richter, A. Zirkel, B. Farago, L. J. Fetters, and J. S. Huang. Large-scale motion in polymer melts, a neutron spin-echo study. Physica Scripta, T49A:242-246, 1993.

The plateau zone in the dynamic modulus of large chain polymer melts characterizes most strikingly their viscoelasticity reflecting the rubber-like properties of entangled chains. In this paper we shall summarize neutron spin-echo experiments which unravel the underlying microscopic large scale chain motions. NSE thereby facilitates a space-time analysis of the density fluctuations in a labeled chain among equals. The experiments show the microscopic existence of an intermediate dynamic length scale, related to the rubbery plateau. Scaling models of entanglement formation are scrutinized by systematically varying the two length scales considered to be important. Diluting with oligomers changes the contour length density. A temperature variation changes the Kuhn length. The NSE results favour binary contact models and exclude packing models. Finally, the build-up of entanglements at the crossover from Rouse to entanglement behavior is addressed by varying the chain length.

[RZF+92]

D. Richter, R. Zorn, B. Farago, B. Frick, and L. J. Fetters. Decoupling of time scales of motion in polybutadiene close to the glass-transition. Physical Review Letters, 68(1):71-74, 1992.

Studying the dynamic structure factor of polybutadiene in the first valley of S(Q) we decoupling of microscopic and macroscopic time scales about 40 K above T(g). At higher temperatures the time scale set by the viscosity scales with the microscopic scale, while at lower T strong decoupling effects occur. We present evidence that the decoupled low- temperature relaxation is distinctly different from the high- frequency "beta-process." The decoupling temperature coincides with the critical temperature of the mode coupling theory.

[RZW+97]

S. Rathgeber, A. Zirkel, L. Willner, D. Richter, A. Brulet, and B. Farago. Dynamics of bimodal polymer melts in the crossover-region from rouse-to-reptation-like behaviour - a study with nse- spectroscopy. Physica B, 234:258-259, 1997.

The single chain dynamics of bimodal saturated polybutadiene melts is addressed by neutron spin echo spectroscopy. A mode analysis of the structure factor provides an access to different relaxation modes (different spatial extension) separately. For the generalized Rouse model formulated by Hess an extension from monomodal to bimodal melts is presented.

[SAB+97]

P. Schleger, B. Alefeld, J. F. Barthelemy, G. Ehlers, B. Farago, P. Giraud, C. Hayes, A. Kollmar, C. Lartigue, F. Mezei, and D. Richter. The long-wavelength neutron spin-echo spectrometer in15 at the institut laue-langevin. Physica B, 241:164-165, 1997.

A new ultra-high resolution neutron spin-echo spectrometer at the ILL (Grenoble) extends the time range for measurements of the intermediate scattering function S(Q,t) far beyond what has previously been possible. The spectrometer design is traditional, utilizing long neutron wavelengths (8-25 Angstrom) and large, homogeneous precession coils to reach Fourier times approaching the microsecond. The standard operational mode is currently functional, providing a Fourier time range of 0.03- 180 ns, and a momentum transfer between 0.01 and 0.2 Angstrom(- 1). The other two operational modes (neutron optical focusing and time of flight) are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

[Sar96]

B. V. B. Sarkissian. Neutron spin echo determination of spin dynamics in the quasiferromagnetic regime of a au-15at.%fe alloy. Philosophical Magazine B-physics of Condensed Matter, 74(2):211-217, 1996.

Neutron spin echo (NSE) measurements have been carried out on the Au-15 at.% Fe alloy within the temperature range of 52.6 K less than or equal to T less than or equal to 73.5 K and for a Q value of 0.021 Angstrom(-1) less than or equal to 0.091 Angstrom(-1). This alloy which contains slightly less Fe solute than for the percolation threshold (C-p less than or equal to 15.7 at.% Fe) for true ferromagnetic ordering is strongly bond disordered quasiferromagnetic (composed of a large cluster of highly correlated spins) below the temperature T* (approximate to 90 K); it then becomes a spin glass below the temperature T- G = 25 K. In the NSE experiment, the spin-spin correlation function phi(Q, t) is measured directly for times 0.02 ns < t < 0.6 ns. The results obtained indicate that phi(Q, t) exhibits Kohlrausch 'stretched' exponential time decay behaviour exp[- t(/tau)(beta)], with temperature-dependent dynamical exponent beta; tau is strongly Q dependent and manifests non-Arrhenius exp(-T-2) thermally activated dynamics.

[Sar92]

B. V. B. Sarkissian. Anomalous critical spin dynamics of ferromagnetic au-17 at- percent-fe and au-19 at-percent-fe alloys - neutron-spin-echo studies. Philosophical Magazine B-physics of Condensed Matter, 65(6):1223-1229, 1992.

Measurements are presented of the temperature and wave-vector dependence of the critical spin dynamics of the ferromagnetic Au-17 at.% Fe and Au-19 at.% Fe alloys above T(c), using the neutron-spin-echo-scattering technique. The relaxation function in both alloys was found to exhibit a slow non-exponential time decay behaviour, quite unlike that observed in usual ferromagnetic systems. This anomalous behaviour of the relaxation function is discussed in terms of the combined effects introduced by the disordered fractal nature of the infinite cluster and the presence of the finite clusters that give rise to localization phenomena. An analysis of the data in terms of the dynamic scaling hypothesis suggests that the above-mentioned effects wash out the true critical region but do not cause it to revert to the hydrodynamic nature of the spin dynamics.

[Sar90]

B. V. B. Sarkissian. Cluster dynamics in au-15-percent fe alloy - neutron spin-echo studies. Journal of Physics-condensed Matter, 2(38):7873-7876, 1990.

[Sar88]

B. V. B. Sarkissian. The multidetector neutron spin-echo spectrometer in11b. Nuclear Instruments & Methods in Physics Research Section A-, 273(1):185-202, 1988.

[Sar87]

B. V. B. Sarkissian. Neutron spin-echo and neutron spin depolarization studies in y9co7. Journal of Physics F-metal Physics, 17(7):1569-1580, 1987.

[SBE+92]

U. Schmidt, T. Bitter, P. Elmuzeini, D. Dubbers, and O. Scharpf. Long-distance propagation of a polarized neutron beam in zero magnetic-field. Nuclear Instruments & Methods in Physics Research Section A-, 320(3):569-573, 1992.

A beam of fully polarized cold neutrons was transported through a zero magnetic field region of 70 m length without loss of polarization. The purpose of this exercise was twofold: firstly, to demonstrate that the new zero-field neutron spin- echo method will work also for very long neutron flight paths; secondly, to prove in the most direct way that the neutron free-flight region of the ILL neutron-antineutron oscillation experiment was indeed sufficiently field-free ("quasifree condition") by using the neutrons themselves as a magnetometer. To this purpose the residual magnetic field integrals in the long "zero-field" region were measured with a conventional neutron spin-echo method. The overall spin precession angle of the neutrons during their flight through the long zero-field region was found to be less than 2-degrees.

[SBHD97]

U. Schmidt, S. Biche, W. Haussler, and D. Dubbers. Neutron resonance spin echo with multidetector. Physica B, 234:1133-1134, 1997.

We have tested a neutron resonance spin-echo instrument (NRSE) in a multidetector arrangement. The method proposed by Golub and Gahler (Phys. Lett. A 123 (1987) 43) a number of years ago invites such development as it relies on large zero magnetic field regions as opposed to large strong magnetic field regions encountered in conventional neutron spin echo (F. Mezei (ed.), Neutron spinecho (Springer, Heidelberg, 1980)).

[SEK+99]

P. Schleger, G. Ehlers, A. Kollmar, B. Alefeld, J. F. Barthelemy, H. Casalta, B. Farago, P. Giraud, C. Hayes, C. Lartigue, F. Mezei, and D. Richter. The sub-nev resolution nse spectrometer in15 at the institute laue-langevin. Physica B, 266(1-2):49-55, 1999.

A new ultra-high-resolution neutron spin-echo spectrometer at the ILL, Grenoble, extends the time range for measurements of the intermediate scattering function S(q, t) far beyond what has previously been possible. This new dynamic range approaching the microsecond opens up the possibility for exploring dynamic phenomena which have been previously unobservable. The various design ideas for the three operational modes (standard, neutron optical focusing, and time of flight) will be presented. The standard and focusing configurations are currently operational. In the normal mode, Fourier times between 0.03 and 360 ns and momentum transfers q between 0.01 and 0.6 Angstrom(-1) are accessible. Under focusing conditions, the smallest attainable q is about 0.002 Angstrom(-1), and closes the gap to light scattering. Two examples of experiments will be presented: the reptation dynamics in an entangled linear polymer melt, and the superparamagnetic fluctuations of magnetic monodomain nano- particles. (C) 1999 Elsevier Science B.V. All rights reserved.

[SFL+98]

P. Schleger, B. Farago, C. Lartigue, A. Kollmar, and D. Richter. Clear evidence of reptation in polyethylene from neutron spin- echo spectroscopy. Physical Review Letters, 81(1):124-127, 1998.

The dynamic structure factor S(q, t) of polyethylene (PEB-2) was measured by neutron spin echo in the Fourier time range of t = 0.3-175 nsec and for momentum transfers q between 0.05 and 0.145 Angstrom(-1) to test the validity of competing phenomenological theories of relaxation in polymer melts. Previous spin-echo experiments limited to t < 25 nsec were equally well described by a variety of models. This ambiguity has now been lifted, and the experiment clearly favors the reptation model, showing that the dominant relaxation mechanism in entangled linear polymers is via reptation.

[SFZ+92]

C. Simon, G. Faivre, R. Zorn, F. Batallan, and J. F. Legrand. Neutron spin-echo study of the dynamics of undercooled selenium. Journal De Physique I, 2(3):307-314, 1992.

We present neutron spin echo measurements on the structural relaxation of undercooled liquid selenium above glass transition. We find a two-stage relaxation process essentially similar to that generally observed in liquids, except for the fact that the slow-stage characteristic time tau-1 varies with the temperature T more slowly than eta(T)/T, where eta(T) is the macroscopic viscosity. We interpret this apparent discrepancy with respect to the usually found scaling law as the manifestation of the equilibrium-polymer nature of liquid selenium. On the other hand, the data suggest that the Kohlrausch exponent of the slow stage is temperature dependent. Concerning the rapid stage of the relaxation, we show that a possible alternative to the mode-coupling interpretation is to attribute it to the existence of local inhomogeneities of the density close to the glass transition.

[SG91]

L. Sjogren and W. Gotze. Alpha-relaxation near the glass-transition. Journal of Non-crystalline Solids, 131:153-160, 1991.

The general mode-coupling equations with activated processes included are analysed in the alpha-relaxation region. This region is intimately connected with the beta-relaxation region, where the dynamics is completely ruled by generic properties of various bifurcation singularities inherent in the nonlinear equations of motion. In particular the von Schweidler relaxation, which describes the overlap region between the alpha- and beta-processes, plays a central role for the properties of the alpha-relaxation function. This satisfies the time-temperature superposition principle with a master function which is well described by a Kohlrausch law. The alpha-relaxation scale is proportional to the macroscopic viscosity, and for the temperature dependence one finds an algebraic increase above a critical temperature T(c), i.e. eta proportional (T - T(c))-gamma. For lower temperatures, there is eventually a crossover to an Arrhenius law eta proportional exp(E/k(B)T).

[Sha95]

J. S. Shaffer. Effects of chain topology on polymer dynamics - configurational relaxation in polymer melts. Journal of Chemical Physics, 103(2):761-772, 1995.

[SKM+99]

H. Seto, T. Kato, M. Monkenbusch, T. Takeda, Y. Kawabata, M. Nagao, D. Okuhara, M. Imai, and S. Komura. Collective motions of a network of wormlike micelles. Journal of Physics and Chemistry of Solids, 60(8-9):1371-1373, 1999.

Aqueous solutions of non-ionic amphiphile C16E7 (=C16H33(OC2H4)(7)OH) are known to form "wormlike micelles", whose static properties are analogously understood as chain- like polymers. However, their dynamical properties are different from those of polymers because the micelles not only entangle but also fuse each other. In order to clarify collective motions of the wormlike micelles, neutron spin echo measurements were carried out. Observed correlation functions were explained by a stretched exponential function with an index of 3/4, The obtained relaxation rates obeyed Q(8/3) as Zilman and Granek (Phys. Rev. Lett. 77 (1996) 4788) expected for the one-dimensional polymer-like model. (C) 1999 Elsevier Science Ltd. All rights reserved.

[SM97]

R. Schatzler and M. Monkenbusch. The julich spin-echo instrument nse: Commencement of operations. Physica B, 234:1128-1129, 1997.

Technical aspects and first measurements of the new neutron spin-echo spectrometer (NSE-FRJ2) in Julich are reported.

[SMM85]

S. M. Shapiro, H. Maletta, and F. Mezei. Neutron-spin-echo study of the reentrant spin-glass euxsr1-xs. Journal of Applied Physics, 57(8):3485-3487, 1985.

[SOA+94]

D. W. Schaefer, B. J. Olivier, C. Ashley, G. Beaucage, D. Richter, B. Farago, B. Frick, and D. A. Fischer. Structure and topology of silica aerogels during densification. Journal of Non-crystalline Solids, 172:647-655, 1994.

A combination of scattering techniques, neutron powder diffraction, small-angle X-ray scattering (SAXS), neutron spin echo (NSE) and neutron time-of-flight. (TOF), is used to follow the structure and topology of silica aerogels during densification. NSE and TOF show that short-scale branching increases at the expense of large-scale connectivity leading to micron-size fluctuations, the existence of which is confirmed by SAXS. On shorter length scales, heat treatment leads to a transition from a spaghetti-like polymeric network to a colloidal microstructure. With increasing exposure to temperatures of the order of 1000 degrees C, the chord length of the colloidal solid phase increases exponentially with bulk density, even though the microscopic skeletal density remains at half that of amorphous silica.

[SOA+92]

D. W. Schaefer, B. J. Olivier, C. S. Ashley, D. Richter, B. Farago, B. Frick, L. Hrubesh, M. J. Vanbommel, G. Long, and S. Krueger. Structure and topology of silica aerogels. Journal of Non-crystalline Solids, 145(1-3):105-112, 1992.

Neutron spin-echo spectroscopy is used to study the topology of aerogels. Topology or connectivity is varied through precursor chemistry and thermal annealing. Topology is characterized using the concept of fractons (the vibrational excitations of a fractal network). A qualitative difference is observed in the spectrum of polymeric vs. colloidal aerogels, the latter showing a peak in the density of vibrational states. For colloidal aerogels whose structure appears to arise from phase separation in the solution precursor, low-energy excitations were only observed in the lowest density material studied. Finally. a transition from fractal to colloidal microstructure was observed during the sintering of polymeric aerogels. This transformation revealed itself as a transition from a fracton- like to a peaked density of states function.

[SP98]

G. D. Smith and W. Paul. United atom force field for molecular dynamics simulations of 1,4-polybutadine based on quantum chemistry calculations on model molecules. Journal of Physical Chemistry A, 102(7):1200-1208, 1998.

We present a united atom force field for simulations of 1,4- polybutadiene based on ab initio quantum chemistry calculations on model molecules. The geometries and energies of conformers and rotational energy barriers in model alkenes and dienes have been determined from high-level quantum chemistry calculations. A rotational isomeric state (RIS) model for 1,4-polybutadiene based on the conformer geometries and energies of the model molecules has been derived. The characteristic ratio and its temperature dependence for cis-1,4-polybutadiene and trans-1,4- polybutadiene, and the characteristic ratio of a random copolymer of cis and trans units, as predicted by tile RIS model, are in good agreement with experimental values, thereby supporting the accuracy of the quantum chemistry calculations. Torsional potentials fur the united atom force field have been parametrized to reproduce the quantum chemistry conformer energies and rotational energy barriers for rotations about the C(sp(2))-C(sp(2)), C(sp(2))-C(sp(3)), and C(sp(3))-C(sp(3)) dihedrals for the model compounds. The CH2-CH2 united atom nonbonded potential has been taken from previous work on polyethylene melts, while the CH-CH united atom nonbonded potential has been parametrized so as to reproduce the energies of chose conformers of the model molecules involving conformation-dependent second-order interactions. Finally, NPT molecular dynamics simulations have been performed on a melt of 1,4-poly(cis(0.5)-r-trans(0.5)butadiene). and the CH2-CH nonbonded potential has been adjusted so that the experimental melt density of the polymer as a function of temperature is accurately reproduced.

[SPM+99]

G. D. Smith, W. Paul, M. Monkenbusch, L. Willner, D. Richter, X. H. Qiu, and M. D. Ediger. Molecular dynamics of a 1,4-polybutadiene melt. comparison of experiment and simulation. Macromolecules, 32(26):8857-8865, 1999.

We have made detailed comparison of the local and chain dynamics of a melt of 1,4-polybutadiene (PBD) as determined from experiment and molecular dynamics simulation at 353 K. The PBD was found to have a random microstructure consisting of 40% cis, 50% trans, and 10% 1,2-vinyl units with a number-average degree of polymerization (X-n) = 25.4. Local (conformational) dynamics were studied via measurements of the C-13 NMR spin- lattice relaxation time T-1 and the nuclear Overhauser enhancement (NOE) at a proton resonance of 300 MHz for 12 distinguishable nuclei. Chain dynamics were studied on time scales up to 22 ns via neutron spin-echo (NSE) spectroscopy with momentum transfers ranging from q = 0.05 to 0.30 Angstrom(-1). Molecular dynamics simulations of a 100 carbon (X-n = 25) PBD random copolymer of 50% trans and 50% cis units employing a quantum chemistry-based united atom potential function were performed at 353 K. The T-1 and NOE values obtained from simulation, as well as the center of mass diffusion coefficient and dynamic structure factor, were found to be in qualitative agreement with experiment. However, comparison of T1 and NOE values for the various distinguishable resonances revealed that the local dynamics of the simulated chains were systematically too fast, whereas comparison with the center of mass diffusion coefficient revealed a similar trend in the chain dynamics. To improve agreement with experiment, (1) the chain length was increased to match the experimental M-z, (2) vinyl units groups were included in the chain microstructure, and (3) rotational energy barriers were increased by 0.4 kcal/mol in order to reduce the rate of conformational transitions. With these changes, dynamic properties from simulation were found to differ 20-30% or less from experiment, comparable to the agreement seen in previous simulations of polyethylene using a quantum chemistry-based united atom potential.

[SR91]

B. V. B. Sarkissian and B. D. Rainford. Neutron spin-echo studies in pd-mn alloys. Solid State Communications, 78(3):185-189, 1991.

Neutron Spin Echo (NSE) experiments were performed on Pd-2%Mn and Pd-5%Mn alloys. For Pd-2%Mn specimen the critical dynamics in the vicinity of the Curie temperature T(c) and for 0.01 less-than-or-equal-to Q --> less-than-or-equal-to 0.039 angstrom-1 were obtained. Results are reminiscent of those of spin conserved pure magnetic systems and support the general dynamical scaling hypothesis. For the Pd-5%Mn alloy, the spin dynamics associated with the freezing process above and below the freezing temperature T(f) = 2.75K were obtained for the Q - -> value of 0.04angstrom-1. Results show that the spin relaxation can be described in terms of Kohlrausch stretched exponential relation exp(-(t/tau)beta); similar to the cluster dynamics observed in the Au-15% Fe alloy reported previously.

[SS95]

K. S. Schweizer and G. Szamel. Crossover to entangled dynamics in polymer-solutions and melts. Journal of Chemical Physics, 103(5):1934-1945, 1995.

A statistical dynamical theory of the crossover from unentangled Rouse dynamics to entangled behavior is constructed for chain polymer solutions and melts. Both time and spatial crossovers in long chain fluids, and the degree of polymerization crossover for short polymers, are treated. The analysis is based on a microscopic theory of the perturbative dynamical corrections to Rouse theory arising from chain connectivity and intermolecular excluded volume forces. The dependence of crossover properties such as the plateau shear modulus and entanglement time and length scale on solution density, solvent quality, and chain statistical segment length are derived by combining the dynamical theory with equilibrium liquid state integral equation methods. Scaling relations are obtained which appear to be in general accord with most experiments on both solutions and melts. The physical origin of the predicted scaling behaviors is the fractional power law temporal decay of the entanglement friction memory function on intermediate time scales, and power law reduced density dependence of the equilibrium force correlations. The theory is also applied to compute the dependence of the chain normal mode relaxation times on polymer density and chain length. Favorable qualitative comparisons with recent neutron spin echo experiments are made. (C) 1995 American Institute of Physics.

[TEH99a]

S. Tasaki, T. Ebisawa, and M. Hino. Development of a neutron spin echo spectrometer using four sequential reflection of multilayer spin splitters. Journal of Physics and Chemistry of Solids, 60(8-9):1607-1609, 1999.

We propose two types of modified neutron spin echo (NSE) spectrometer using the quantum precession caused by multilayer spin splitters. The first spectrometer is constructed by replacing the precession magnet with a pair of two spin splitters. The new NSE spectrometer requires much less volume and magnetic field compared to the conventional NSE spectrometer. The second spectrometer is designed to measure the small angle scattering from a plane sample. The principle and performances of the two types of the modified methods are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.

[TEH99b]

S. Tasaki, T. Ebisawa, and M. Hino. Development of a neutron spin echo spectrometer with multilayer spin splitters. Physica B, 268:299-303, 1999.

A new neutron spin echo (NSE) spectrometer using multilayer spin splitter (MSS) is proposed. Two MSSs are arranged in (+ +) configuration for the quantum precession of neutrons. The NSE spectrometer requires only a small magnet and is compact. Detailed discussion on the requirements for MSS is presented using the results of Zygo interferometer. (C) 1999 Elsevier Science B.V. All rights reserved.

[TKS+99]

T. Takeda, Y. Kawabata, H. Seto, S. Komura, S. K. Ghosh, M. Nagao, and D. Okuhara. Neutron spin-echo investigations of membrane undulations in complex fluids involving amphiphiles. Journal of Physics and Chemistry of Solids, 60(8-9):1375-1377, 1999.

The intermediate functions I(Q, t) obtained from neutron spin- echo (NSE) experiments were well fitted to I(Q, t) = I(Q, 0), exp[-(Gamma t)(2/3)] for the bicontinuous microemulsion and the lamellar phases of the C12E5/n-octane/water system and also for the lamellar phase of the DPPC/water/CaCl2 system. The relaxation rate Gamma increased as Q(3). These results support the theory presented by Zilman and Granek [Phys. Rev. Lett, 77 (1996) 4788]. Bending modulus of the membrane x was estimated in the C12E5/n-octane/water system and in the DPPC/water/CaCl2 system using their theory. (C) 1999 Elsevier Science Ltd. All rights reserved.

[TKS+95a]

T. Takeda, S. Komura, H. Seto, M. Nagai, H. Kobayashi, E. Yokoi, T. Ebisawa, S. Tasaki, C. M. E. Zeyen, Y. Ito, S. Takahashi, and H. Yoshizawa. Neutron spin-echo spectrometer at jrr-3m. Physica B, 213:863-865, 1995.

We have designed and have been constructing at the C-2-2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE). It is equipped with two optimal-field-shape coils for neutron spin precession, a position-sensitive detector a converging polarizer and a wide-area analyzer, The dynamic range of scattering vector Q covers from b 0.005 Angstrom(-1) to 0.2 Angstrom(-1) and that of energy E from 10 neV to 30 mu eV. In a performance test of the precession coils, the NSE signal amplitude P-NSE decreased slightly for the neutron beam cross-section of 30(phi) mm as the Fourier time t increases up to 25 ns. We have carried out NSE experiments on complex fluid systems and obtained their diffusion coefficient D-eff.

[TKS+95b]

T. Takeda, S. Komura, H. Seto, M. Nagai, H. Kobayashi, E. Yokoi, C. M. E. Zeyen, T. Ebisawa, S. Tasaki, Y. J. Ito, S. Takahashi, and H. Yoshizawa. A neutron spin-echo spectrometer with 2 optimal field shape coils for neutron spin precession. Nuclear Instruments & Methods in Physics Research Section A-, 364(1):186-192, 1995.

We have designed and have been constructing at the C-2-2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE) which is equipped with two optimal field shape (OFS) coils for neutron spin precession with the maximum field integral of 0.22 T m, an assembly of position sensitive detectors (PSD), a converging polarizer and a wide area analyzer. The dynamic range of scattering vector Q covers from 0.005 Angstrom(-1) to 0.2 Angstrom(-1) and that of energy ($) over bar h omega from 10 neV to 30 mu eV. Performance tests of the OFS coils show that the inhomogeneity of the magnetic field integral in the OFS coils with the spiral coils is so small that the NSE signal amplitude decreases little even for the neutron cross section of 30 mm diameter as the Fourier time t increases up to 25 ns, though the precession coils are close to iron covers of the neighboring neutron guide. This verifies that the OFS precession coils are appropriate for this NSE spectrometer. Another test experiment shows that the homogeneity condition of the precession magnet is loosened by use of PSD.

[TLC92]

N. J. Tao, G. Li, and H. Z. Cummins. Brillouin-scattering study of the liquid-glass transition in cakno3 - mode-coupling analysis. Physical Review B-condensed Matter, 45(2):686-696, 1992.

Polarized and depolarized Brillouin scattering from 0.4 Ca(NO3)2-0.6KNO3 was investigated in the temperature range of 320 to 640 K using a six-pass Sandercock tandem Fabry-Perot interferometer. The polarized spectra were analyzed with a simplified version of mode-coupling theory, in which the small- q density fluctuation modes probed by light scattering couple only to pairs of modes at q0 corresponding to the first peak in the structure factor S(q). The density correlation function phi(q0,t) determined by neutron spin-echo experiments by Mezei et al. was used as input in the analysis, producing excellent fits to the experimental data. The depolarized spectra were analyzed with a phenomenological viscoelastic model including NO3- orientational relaxation.

[TLC91a]

N. J. Tao, G. Li, and H. Z. Cummins. Brillouin-scattering study of the liquid-glass transition in supercooled aqueous lithium-chloride solutions - generalized hydrodynamics and mode-coupling analyses. Physical Review B-condensed Matter, 43(7):5815-5830, 1991.

Brillouin scattering in aqueous LiCl solutions was investigated in the concentration range 5-36 mol% at temperatures between 375 and 78 K. The polarized spectra were found to be accurately described by generalized hydrodynamics with a single-relaxation-time structural-relaxation process. The relaxation times obey the Arrhenius relation above approximately 200 K with activation energies between 2.5 kcal/mol at 36% and 6 kcal/mol at 5%. The transverse-acoustic mode that was observed in the depolarized spectra was qualitatively analyzed with a simple viscoelastic form for the transverse-current correlation function. The polarized spectra were also compared to preliminary predictions of the mode- coupling theory as formulated by Bengtzelius et al., and showed qualitative agreement but significant quantitative disagreement. Both the LA and TA sound velocities exhibit discontinuous slope changes in their temperature dependences at the glass transition.

[TLC91b]

N. J. Tao, G. Li, and H. Z. Cummins. Self-similar light-scattering spectra of beta relaxation near the liquid-glass transition. Physical Review Letters, 66(10):1334-1337, 1991.

We report a light-scattering observation of beta-relaxation in [Ca(NO3)2]0.4-(KNO3)0.6 in the vicinity of the liquid-glass transition. The spectra exhibit self-similarity with I(omega) = A/omega-1-a over a spectral range of nearly 3 orders of magnitude in both the liquid and glass phases. This result agrees with predictions of mode-coupling theories of the glass transition. However, the value of alpha is strongly temperature dependent, a result not predicted by the theory. The spectra are strongly depolarized with a depolarization ratio of approximately 0.75.

[TSK+99]

T. Takeda, H. Seto, Y. Kawabata, D. Okuhara, T. Krist, C. M. E. Zeyen, I. S. Anderson, P. Hoghoj, M. Nagao, H. Yoshizawa, S. Komura, T. Ebisawa, S. Tasaki, and M. Monkenbusch. Improvement of neutron spin echo spectrometer at c2-2 of jrr3m. Journal of Physics and Chemistry of Solids, 60(8-9):1599-1601, 1999.

We have improved a neutron spin echo spectrometer at the C2-2 port of JRR-3M, JAERI. The new double reflection supermirror monochromators with the wavelength lambda = 5.9 Angstrom (Delta lambda/lambda = 15%) and lambda = 7.14 Angstrom (Delta lambda/lambda = 18%) increased the polarized neutron flux at the sample position about three times more than the old monochromator. The deflection angle 2 theta(M) at C2-2 port increased up to 7.00 degrees using double reflection, which reduced the disturbance from the iron cover of C2 guide for C2- 3 port. In order to gain more neutron intensity, we replaced the spiral coils with a wider one and installed a new detector system with an assembly of eight one-dimensional PSDs, (C) 1999 Elsevier Science Ltd. All rights reserved.

[TTY99]

S. Takasaka, Y. Tsujimi, and T. Yagi. Dispersion relation of the relaxation time of the dimer mode in kdco₃. Journal of the Korean Physical Society, 35:S1343-S1345, 1999.

Low frequency light scattering spectra of a single crystal KDCO3 have been observed near the phase transition temperature T-N = 353 K. A bilinear coupling between the (DCO3-)(2) dimer mode and the transverse acoustic c(66) mode has been employed to analyze the observed spectra. It gives a good description for the experimental result with the dispersion relation of the coupled modes system. The relaxation time tau of the dimer mode is investigated consistently with the result reported by the neutron spin-echo study. The dynamics of the coupled modes system is discussed in terms of the dispersion relation.

[UGM+98]

Y. J. Uemura, B. D. Gaulin, M. Mekata, I. Mirebeau, S. Pouget, and J. L. Tholence. Neutron spin echo studies of srcr8ga4o19. Journal of Magnetism and Magnetic Materials, 177:701-702, 1998.

We performed neutron spin echo (NSE) measurements of the geometrically frustrated Kagome lattice spin system SrCrxGa12- xO19(SCGO)(x = 8), with a momentum transfer at Q = 1.4 Angstrom(-1). At high temperature (T = 20 K), the echo intensity quickly drops to zero, reflecting fast spin fluctuations. Below T congruent to 3.5 K, we found a finite intensity persisting to long times, up to t congruent to 1 ns. The persisting intensity increases with decreasing temperature, corresponding to the build-up of a static order parameter. (C) 1998 Elsevier Science B.V. All rights reserved.

[VBFM91]

P. Verkerk, U. Bafile, B. Farago, and F. Mezei. Neutron spin-echo measurements on dense hydrogen gas. Physica B, 168(1):1-8, 1991.

We measured the coefficient of self-diffusion D in high-density hydrogen gas at 120 K and 75 MPa by means of the neutron spin- echo technique. Because of the relatively large value of D the energy window of the spectrometer IN11 of the ILL had to be enlarged by means of additional Larmor coils with a small field integral. We find D = 0.0543 nm2ps-1 with an accuracy of 4%. This result is not inconsistent with time-of-flight incoherent neutron scattering measurements performed previously in Delft, but disagrees with NMR spin-echo data. An explanation might be found in the different time and space scales of the various methods.

[VM92]

O. T. Valls and G. F. Mazenko. Nonlinear hydrodynamics and the liquid-glass transition - a numerical study. Physical Review A, 46(12):7756-7764, 1992.

We present a numerical study of a Langevin-equation model for a simple fluid. We study the relaxation kinetics of the density- time-correlation function. We find a stretched exponential regime with an exponent beta depending on wave vector in agreement with results obtained from mode-coupling theory and correlated with the wave-number dependence of the static- structure factor. At larger wave vectors, we find a transition to power-law behavior after a rapid decay.

[VM91]

O. T. Valls and G. F. Mazenko. Numerical study of a simple langevin model for the kinetics of dense fluids. Physical Review A, 44(4):2596-2601, 1991.

We use numerical methods to study the simplest Langevin model applicable to the description of dense fluids. We find in qualitative agreement with theory and experiment that there is a "stretching" of the dynamics for low temperatures and short wavelengths.

[VP91]

W. Vanmegen and P. N. Pusey. Dynamic light-scattering study of the glass-transition in a colloidal suspension. Physical Review A, 43(10):5429-5441, 1991.

This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard- sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.

[vZdL99]

A. van Zon and S. W. de Leeuw. Self-motion in glass-forming polymers: a molecular dynamics study. Physical Review E, 60(6):6942-6950, 1999.

We present results of molecular dynamics simulations of an undercooled polymer melt, performed to study the validity of mode-coupling theory (MCT) for realistic polymer melts in general. The mean square displacements of the chain segments are computed to study the diffusion constant of the Rouse-like motion. It is shown that this diffusion constant follows a power law behavior as a function of the temperature, as predicted by the MCT. In addition, we studied the incoherent part of the intermediate scattering function and show that these functions obey the second scaling law of the MCT. We also calculated the relaxation times of the alpha-relaxation and found that they follow the same power law (gamma = 2.9) as the diffusion constant. Using gamma, and the relationships given by MCT, we obtain values for a (0.27) and b (0.46) and use these exponents to describe the beta-relaxation regime. We find that the long time part of the beta-relaxation can be described accurately by the Von Schweidler relaxation over a wide range of wave numbers. In the short time regime of the beta- relaxation, no critical decay is observed. [S1063- 651X(99)08111-8].

[WCR+96]

J. Wuttke, I. Chang, O. G. Randl, F. Fujara, and W. Petry. Tagged-particle motion in viscous glycerol: Diffusion- relaxation crossover. Physical Review E, 54(5):5364-5369, 1996.

Incoherent neutron-backscattering and field gradient NMR are combined to measure the self-correlation function of protons in glycerol. Extensive simulations reveal and delimit systematic effects of multiple scattering. At microscopic wave numbers, tagged-particle motion couples strongly to structural relaxation. Between 1 and 0.1 Angstrom(-1) a crossover to diffusional motion is observed: quasielastic stretching diminishes and the wave-number dependence of the mean relaxation time approaches q(-2).

[WL95]

J. D. Wilson and R. F. Loring. Dynamic structure factor in a bidisperse polymer melt. Journal of Chemical Physics, 103(4):1641-1649, 1995.

We present calculations of the single-chain dynamic structure factor for a polymer melt composed of linear molecules of the same chemical identity but of two different chain lengths. The fluid is treated within a dynamical mean-held approach, in which each molecule is represented as a freely jointed chain moving among stochastic obstacles. The obstacles are of two types, each representing the obstruction of local conformational changes by one of the species present. The obstacle dynamics are determined self-consistently by equating the relaxation rate of an obstacle of a given type to the smallest conformational relaxation rate of the species that it represents. Calculation of the dynamic structure factor is mapped onto the solution of a random walk with dynamical disorder, in which a walker moves on a one-dimensional lattice with hopping rates that randomly fluctuate among three states. The relevant random walk problem is solved within the effective medium approximation, and the results are employed to examine the dependence of the dynamic structure factor on time, wave vector, chain lengths, and fluid composition. (C) 1995 American Institute of Physics.

[WPB94]

J. Wittmer, W. Paul, and K. Binder. The intermediate coherent scattering function of entangled polymer melts - a monte-carlo test of descloizeaux theory. Journal De Physique Ii, 4(5):873-879, 1994.

Using the bond fluctuation model for flexible polymer chains in a dense melt the intermediate coherent scattering function for chains containing N = 200 monomers is calculated and interpreted in terms of a recent theory of des Cloizeaux. The theory yields an explicit description for the crossover from the Rouse model to the regime where reptation prevails, for the limit N --> infinity. While the Monte Carlo data are qualitatively compatible with this description, an accurate estimation of the tube diameter is prevented due to the onset of a diffusive decay of the scattering function, not included in the theory. For a full quantitative analysis of the Monte Carlo data (as well as of experiments on chains with not extremely large molecular weight) an extension of the theory for finite N would be required.

[WPB92]

J. Wittmer, W. Paul, and K. Binder. Rouse and reptation dynamics at finite temperatures - a monte- carlo simulation. Macromolecules, 25(26):7211-7219, 1992.

The temperature dependence of the dynamics of polymer chains in the melt is studied in a Monte Carlo simulation of the bond fluctuation model. Temperature enters via a bond angle potential determining the stiffness of the chains. The model is simulated at a melt volume fraction of PHI = 0.5 and for chains of lengths N = 20, 50, 100, and 200. For the short chains we are determining the temperature range over which a random coil description of these semiflexible chains is possible, i.e. the temperature region for which the chains show the behavior of random coils in the melt. We then examine to what extent the Rouse model is able to describe the dynamics of these chains. We will look especially at the Rouse scaling for the dynamic structure factor. For the longest chains it has been shown that our model in the athermal limit can be described by the reptation picture of DeGennes and Doi and Edwards and the density dependence of the tube diameter has been established. Here we now measure the temperature dependence of this dynamic length scale by analyzing the dynamic structure factor. It is shown to decrease slightly with decreasing temperature in accord with recent findings of neutron spin echo experiments.

[WPCF95]

J. Wuttke, W. Petry, G. Coddens, and F. Fujara. Fast dynamics of glass-forming glycerol. Physical Review E, 52(4):4026-4034, 1995.

The vibrational and relaxational dynamics of glycerol has been measured by incoherent neutron scattering for energy transfers from some 10 mu eV to several 10 meV, and for temperatures from 4 to 413 K. On heating towards and above the glass transition, scattering from low-frequency vibrations shows an unspectacular increase that is consistent with ultrasonic data. We propose using the vibrational density of states for calculating the Fourier transformed time correlation function S(q,t) without the cutoff usually imposed by the kinematics of neutron scattering. For high temperatures and low frequencies, structural relaxation is well described by the empirical Kohlrausch law. There is no extended crossover region between vibrations and structural relaxation.

[YHN+97]

J. Y. Ye, T. Hattori, H. Nakatsuka, Y. Maruyama, and M. Ishikawa. Microscopic dynamics of the glass transition investigated by time-resolve fluorescence measurements of doped chromophores. Physical Review B-condensed Matter, 56(9):5286-5296, 1997.

The microscopic dynamics of several monomeric and polymeric glass-forming materials has been investigated by time-resolved fluorescence measurements of doped malachite green molecules in a wide temperature region. For monomers, 1-prapanol, propylene glycol, and glycerol, and a polymer without side chains, polybutadiene, the temperature dependence of nonradiative decay time of doped malachite green molecules behaves in a similar way through the glass-transition region. Besides a kink around the calorimetric glass-transition temperature T-g, another crossover at a critical temperature T-c about 30-50 K above T-g has been clearly observed. This experimental finding is in agreement with the prediction of the mode-coupling theory that a dynamical transition exists well above T-g. On the other hand, for the complex polymers with side chains, poly(vinyl acetate), poly(methyl acrylate), and poly(ethyl methacrylate), the crossover at T-g is less pronounced than those for the monomers and the polymer without side chains. Moreover, although we could not distinguish any singularities above T-g for these complex polymers, we observed another kink below T-g, which may be attributed to the side-chain motions.

[YO98]

R. Yamamoto and A. Onuki. Heterogeneous diffusion in highly supercooled liquids. Physical Review Letters, 81(22):4915-4918, 1998.

The diffusivity of tagged particles is demonstrated to be heterogeneous on time scales comparable to or less than the stress relaxation time congruent to tau(alpha) in a highly supercooled model liquid. The particle motions in the relatively active regions dominantly contribute to the mean square displacement, giving rise to a diffusion constant larger than the Stokes-Einstein value. The van Hove self-correlation function G(s)(r, t) is shown to have a large r tail which can be scaled in terms of r/t(1/2) for t less than or similar to 3 tau(alpha). Its presence indicates heterogeneous diffusion in the active regions. However, the diffusion process becomes homogeneous on time scales longer than the life time of the heterogeneity structure (similar to 3 tau(alpha)). [S0031- 9007(98)07758-8].

[YWI99]

X. P. Yang, S. Q. Wang, and H. Ishida. A solution approach to component dynamics of a/b miscible blends. 1. tube dilation, reptation, and segmental friction of polymer a. Macromolecules, 32(8):2638-2645, 1999.

A model solution system of 1,4-polybutadiene in oligomeric butadiene of high vinyl content is studied to show how polymer friction dynamics depend explicitly on the solvent dynamics and are approximately determined by the overall glass transition temperatures of the solutions as a function of concentration (i.e., weight fraction phi) and temperature T. Among the most striking findings are the invariance of the overall molecular relaxation time with phi from phi = 1.0 to 0.3 at a special temperature To Below To, the solutions possess longer reptation times than that of the pure PBD melt. Moreover, through oscillatory shear measurements of linear viscoelasticity, the scaling of the tube dilation with phi is found rheologically as a proportional to phi(-2/3), which agrees with the previous neutron spin echo studies. Since viscoelastic properties of the entangled solutions reveal dynamics associated with the polymeric component, a general methodology is established here based on the present model system for characterization of component dynamics in miscible polymer blends.

[Zey99]

C. M. E. Zeyen. Spin echo three-axes spectrometers for improved energy and momentum resolution. Journal of Physics and Chemistry of Solids, 60(8-9):1573-1578, 1999.

Optimised Field Shape [1] spin precession coils can be made sufficiently short, so as to be added to existing Three-Axis Spectrometers (TAS) without increasing the neutron flight paths significantly. Furthermore, homogeneity corrections can be applied in such a way as to feature uniform magnetic induction line integrals even for very divergent beams. This way, intense TAS with Spin Echo combinations can be built, the loss of luminosity with respect to the normal polarised mode being negligible. Based on the experience with resistive TAS and Spin Echo spectrometers (TASSE) on PONTA (ISSP) and a superconducting version on IN20 (ILL) used mainly for high momentum transfer quasielastic scattering experiments (mu eV- neV resolution), a variety of other applications will be made possible: inelastic scattering from excitations with mu eV resolution and very high momentum transfer resolution (10(-3) Angstrom(-1)), a domain until now reserved for synchrotron radiation. The fields of application will include the extension of the accessible momentum transfer region with respect to cold neutron Spin Echo, for example in the study of glass and liquid dynamics phase transitions and slow dynamics in general. The advantage of thermal neutrons as well as of the TAS background spectrometer with a well-defined transmission function will allow addressing new subjects such as excitation lifetimes across the entire Brillouin zone. Dispersive excitations can be 'focused' by the use of gradient coils that allow restoring the echo polarisation blurred by the dispersion. Another application of these gradient coils is improved momentum resolution for elastic scattering that allows TASSE to compete with synchrotron radiation facilities. (C) 1999 Elsevier Science Ltd. All rights reserved.

[Zey83]

C. M. E. Zeyen. A neutron spin-echo triple axis spectrometer. Physica B & C, 120(1-3):70-70, 1983.

[ZR96]

C. M. E. Zeyen and P. C. Rem. Optimal larmor precession magnetic field shapes: Application to neutron spin echo three-axis spectrometry. Measurement Science & Technology, 7(5):782-791, 1996.

Resolution characteristics of neutron spectrometers using Larmor precession of the neutron spin are limited by magnetic field homogeneities of a special type. The line integral of the modulus of the magnetic induction B integral(L) |B| dl along a neutron trajectory of length L is a measure of the amount of precession performed by the neutron. Hence it should be precisely the same for all neutron trajectories in a diverging beam. We present an analytical solution to the variational problem integral L |B| dl = constant, for the case of cylindrical magnets (better than any lower symmetry geometry) coaxial to the beam axis. This solution describes the best irrotational (rot B = 0) field shape along the beam axis z. It can be obtained in practice by superposing a number of solenoids of different lengths. The optimal homogeneity is significantly better than for a simple solenoid of comparable dimensions, the only magnets used until now. For realistic lengths L and beam radii r, it is however not good enough for very high-resolution measurements. We therefore introduce a technique to correct both for the residual inhomogeneities of optimized cylinder magnets and the line integral variations due to path length differences resulting from finite angular beam divergence. Such corrections can only be done by introducing current distributions in the beam. Their optimal distributions can also be calculated analytically. Until now only the residual inhomogeneities have been corrected by in beam currents. With the two concepts of optimal field shape (OFS) and path length corrections described here, the resolution properties of Larmor precession techniques can be pushed to their intrinsic limits. As a further result of the correction technique introduced here, wider angular divergences can be used, for example using multi-detectors, resulting in substantially improved neutron economy. Several neutron spin echo (NSE) spectrometers based on above ideas have in the meantime been constructed by different working patties. The experimental results confirm the calculations reported here; no significant polarization drop is observed at maximum field, For the investigation of dispersive elementary excitations in solids where the neutron energy change depends on the momentum transfer, a special type of gradient coils is needed. We describe their design for OFS precession magnets.

[ZRF+92]

R. Zorn, D. Richter, B. Farago, B. Frick, F. Kremer, U. Kirst, and L. J. Fetters. Comparative-study of the segmental relaxation in polyisoprene by quasi-elastic neutron-scattering and dielectric-spectroscopy. Physica B, 180:534-536, 1992.

We have used neutron spin echo and dielectric spectroscopy to investigate the relaxational properties of polyisoprene. NSE data reveal a relaxation similar to what was found previously in polybutadiene. The comparison to the dielectric relaxation and rheological data exhibits that all time scales shift with the same temperature dependent factor.

[ZRFF93]

R. Zorn, D. Richter, B. Frick, and B. Farago. Neutron-scattering experiments on the glass-transition of polymers. Physica A, 201(1-3):52-66, 1993.

Neutron scattering experiments have been performed to explore the dynamics of polymers near the glass transition. The experimental data show three distinct types of relaxations: (1) Susceptibility spectra exhibit a contribution in addition to phonons around a temperature independent frequency. (2) Neutron-spin-echo (NSE) experiments show a slow relaxation of the stretched exponential type. Above 220 K, the characteristic time of this process strictly follows the Vogel-Fulcher dependence of viscosity data indicating the direct connection between microscopic and macroscopic relaxation. (3) Below 220 K, the temperature dependence of the relaxation observed by NSE changes to an Arrhenius form while the viscosity still follows the Vogel-Fulcher law. Analysis of the Q dependence from IN13 backscattering data reveals a change of mechanism at the crossover. The experimental findings will be discussed emphasizing the comparison with the mode coupling theory. Aspects of agreement as well as deviations will be pointed out in the discussion.