Neutron Spin Echo Investigations Research Articles

Dynamics of Single-Chain Nanoparticles under Crowding: A Neutron Spin Echo Study.

We present a neutron spin echo (NSE) investigation to examine the impact of macromolecular crowding on the dynamics of single-chain nanoparticles (SCNPs), serving as synthetic models for biomacromolecules with flexibility and internal degrees of freedom, such as intrinsically disordered proteins (IDPs). In particular, we studied the dynamics of a medium-size poly(methyl methacrylate) (PMMA)-based SCNP (33 kDa) in solutions with low- (10 kDa) and high- (100 kDa) molecular weight analogous deuterated PMMA linear crowders. The dynamic structure factors of the SCNPs in dilute solution show certain degrees of freedom, yet the analysis in terms of the Zimm model reveals high internal friction that effectively stiffens the chain-a phenomenon also observed for IDPs. Under crowding conditions, the internal dynamics remains essentially unchanged, but the center-of-mass diffusion slows down. The effective viscosity felt by the SCNPs at the timescales probed by NSE is lower than the macroscopic viscosity of the crowder solution, and it does not depend significantly on the molecular weight.

Structure and Dynamics of Irreversible Single-Chain Nanoparticles in Dilute Solution. A Neutron Scattering Investigation

We present a small-angle neutron scattering (SANS) and neutron spin echo investigation on the structure and dynamics of irreversible single-chain nanoparticles (SCNPs) in dilute solution. SCNPs are...

Ground state of the geometrically frustrated compoundTb2Sn2O7

Despite the observation of magnetic neutron reflections at low temperatures, muon-spinexperiments have suggested the ground state of the geometrically frustrated magnetic materialTb2Sn2O7 to be dynamical. This unique situation requires additional characterizations of thismagnetic ground state. Here we report a neutron spin-echo investigation which supportsthe existence of a dynamical component in the ground state. This result is backed by athorough analysis of the diffraction profiles. A physical model is put forward inwhich the energy splitting between the ground and first excited states of theTb3+ ion is comparable to other energies present in this frustrated system.

Nonflexible Coils in Solution: A Neutron Spin-Echo Investigation of Alkyl-Substituted Polynorbornenes in Tetrahydrofuran

The dynamics of rigid random polymer coils in dilute solution has been investigated by neutron spin-echo (NSE) spectroscopy. The polymers used were polynorbornenes that consist of chains of the bridged norbornene rings with hexyl (PNBH) or propyl (PNBP) side groups. These polymer molecules have large internal rotation barriers that hinder conformational changes and give each chain a kind of individual shape memory. Thereby the chains exhibit a dynamical stiffness that leads to a significantly retarded relaxation as compared to flexible polyisoprene chains (PI). While the latter perfectly match the prediction of the Zimm model, the polynorbornene spectra display the signature of a dynamical stiffness due to the “frozen” coil conformation of the PNBH or PNBP chains.

Fermi Surface Topology and the Superconducting Gap Function inUPd2Al3: A Neutron Spin-Echo Study

We report on a single crystal neutron spin-echo investigation of the low-energy dynamic response in the heavy-fermion superconductor UPd2Al3 in the vicinity of the antiferromagnetic wave vector Q(0)=(0 0 0.5). Well inside the superconducting phase, antiferromagnetic quasielastic scattering, which is present in the normal state, is absent for relaxation times up to 10 ns, equivalent to an energy resolution of approximately 1 microeV. This places strong constraints on possible models for this magnetic superconductor.

Neutron Spin-Echo Investigation of Slow Spin Dynamics in Kagomé-Bilayer Frustrated Magnets as Evidence for Phonon Assisted Relaxation inSrCr9xGa12−9xO19

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr9xGa12-9xO19 (x=0.95, SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at Tg, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (approximately Tg)<T<7 K, the relaxation rate activation energy Ea=7+/-0.4 meV, equal to the energy of a phonon mode, pointing out the role of spin-lattice coupling.

Neutron spin-echo investigation of orientationally disordered crystal chloroadamantane

In the present investigation, we show that neutron-spin echo spectroscopy is perfectly suited for probing the molecular reorientations of orientationally disordered crystals. Our study confirms the existence of a dynamical crossover where molecular reorientational motions change in nature as already observed in previous NMR and Raman experiments and molecular dynamics simulation.

Molecular observation of contour length fluctuations in polymer melts

The close comparison of linear rheology data of some long-chain polymer melts with the reptation model indicates the existence of additional degrees of freedom limiting the topological chain confinement. As one candidate contour length fluctuations (CLF) were proposed and indeed account for the observed power law in the dynamic loss modulus G″( ω). In order to study these mechanisms on a microscopic scale we have performed neutron spin echo investigations of the single-chain dynamic structure factor from polyethylene (PE) melts over a large range of chain lengths. A systematic loosening of the confinement with decreasing chain length is found and can be described quantitatively by the effect of CLF on the confining tube establishing this mechanism on a molecular level in space and time.

Neutron spin-echo investigation of orientationally disordered crystal chloroadamantane

In the present investigation, we show that neutron-spin echo spectroscopy is perfectly suited for probing the molecular reorientations of orientationally disordered crystals. Our study confirms the existence of a dynamical crossover where molecular reorientational motions change in nature as already observed in previous NMR and Raman experiments and molecular dynamics simulation.

Neutron spin-echo investigation of the dynamics of block copolymer micelles

The dynamics of E92B18 (E=OCH2CH2: oxyethylene, B=OCH2CH(C2H5): oxybutylene; the subscripts are the number of repeats) block copolymer micelles is investigated by neutron spin-echo (NSE). The dynamics of chains in E92B18 micelles is investigated in the hard gel and soft gel phases of the E92B18 in D2O, as well as in the micellar solution. NSE revealed the presence of two dynamic modes, i.e., “fast” and “slow” modes, which are observed independent of the phase, i.e., micellar liquid, soft gel or hard gel. A “slow” mode that dominates at low scattering angles corresponds to the translational diffusion of micelles, the micellar size Rs being in good agreement with the apparent hydrodynamic radius from dynamic light scattering experiments on a dilute solution. An additional “fast” mode was observed, and ascribed to internal “blob scattering” in the micellar corona.

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

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

Influence of the microstructure of PPO-based glass-formers on their dynamics as investigated by neutron spin echo

We present a neutron spin echo investigation of the high temperature dynamics of two related glass-forming systems based on poly(oxypropylene): namely the molecular liquid triol and the polyurethane gel. Measurements performed at the first maximum of the static structure factor, on a rather large time window, allow one to analyse precisely the relaxation characteristics of these systems and to compare them with previous neutron time of flight and dielectric results. We discuss the differences in relaxation times and stretching exponents obtained for both samples as a function of their microstructure: molecular nature and architecture (effects of branching or/and three-dimensional network).

Intermediate length scale dynamics of polyisobutylene.

We report on a neutron spin echo investigation of the intermediate scale dynamics of polyisobutylene studying both the self-motion and the collective motion. The momentum transfer (Q) dependences of the self-correlation times are found to follow a Q(-2/beta) law in agreement with the picture of Gaussian dynamics. In the full Q range of observation, their temperature dependence is weaker than the rheological shift factor. The same is true for the stress relaxation time as seen in sound wave absorption. The collective times show both temperature dependences; at the structure factor peak, they follow the temperature dependence of the viscosity, but below the peak, one finds the stress relaxation behavior.

Neutron spin–echo investigations of membrane undulations in complex fluids involving amphiphiles

The intermediate functions I( Q, t) obtained from neutron spin–echo (NSE) experiments were well fitted to I(Q,t)=I(Q,0) exp[−(Γt) 2/3] for the bicontinuous microemulsion and the lamellar phases of the C 12E 5/ n-octane/water system and also for the lamellar phase of the DPPC/water/CaCl 2 system. The relaxation rate Γ 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 ϰ was estimated in the C 12E 5/ n-octane/water system and in the DPPC/water/CaCl 2 system using their theory.

Neutron spin echo investigation of the concentration fluctuation dynamics in melts of diblock copolymers

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⩾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.

Neutron spin echo investigations of polymer dynamics

In 1997, the instrumental basis for neutron spin echo (NSE) 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.

Neutron spin echo investigations in the α and β relaxation regime of polybutadiene

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

Dynamics of polymer brushes — What can neutron spin-echo spectroscopy contribute?

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 PEB-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.

Neutron spin echo investigations on the segmental dynamics in semidilute polymer solutions under Θ- and good solvent conditions

Semidilute polymer solutions can be considered as transient networks of lifetime, τ g, and of average meshsize, ξ. ξ depends on the polymer concentration, c, and the solvent conditions. If the segmental diffusion of such systems is studied on timescales t ⪡ τ 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 qξ( c) ≈ 1. The corresponding relaxation rates, Ω(q), vary from Ω(q) ∼ q 3 to Ω(q) q 2. In Θ-solvents, the many chain regime was not accessible. However, due to the existence of self-entanglements, which introduce an additional length scale ξ i ( c) < ξ( c), the crossover from disentangled (Ω(q) ∼ q 3) to entangled single chain relaxation (Ω(q) ∼ q) becomes visible.

Aggregating block copolymers as model systems to study polymer brush dynamics

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 «hairs» 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.