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In this work, we compare the single chain dynamic structure factors for five different polymers: polyolefins (PE and PEP), poly-dienes (PB and PI), and a polyether (PEO). For this purpose, we have extended the De Gennes approximation for the dynamic structure factor. We describe the single chain dynamic structure factor in multiplying the coherent scattering functions for local reptation and Rouse motion within the Rouse blob. Important results are (i) the simple De Gennes structure factor S(Q, t)DG approximates within a few Å the outcome for the tube diameter of the more elaborate structure factor (exception PI); (ii) the extended De Gennes structure factor together with the Rouse blob describes the neutron spin echo spectra from the different polymers over the complete momentum transfer range and the full time regime from early Rouse motion to local reptation; and (iii) the representation of the scattering functions could significantly be improved by introducing non-Gaussianity corrections to the Rouse-blob dynamics. (iv) The microscopic tube step length in all cases is significantly larger than the rheological one; further tweaking the relation between tube length and entanglement blob size may indicate a possible trend toward an anisotropic lean tube with a step-length larger than the lateral extension. (v) All considered polymer data coincide after proper (Q, t) scaling to a universal behavior according to the length scale of the tube, while the relevant time scale is the entanglement time τe. (vi) In terms of the packing model, the required number of chains spanning the entanglement volume consistently is about 40% larger than that obtained from rheology. [ABSTRACT FROM AUTHOR]

We study the dynamics of pure oleic acid and grafted oleic acid synthesized by decomposing iron oleate into oleic acid grafted iron oxide nanoparticles. Our quasielastic neutron scattering study shows that oleic acid dominantly performs translational diffusion at room temperature. On the other hand, in nanocomposites, constraints imposed by grafting and crowding of neighboring chains restrict the grafted oleic acid to uniaxial rotation. Interestingly, it also manifests mobility in grafted oleic acid below the crystallization temperature of pure oleic acid. The data from grafted oleic acid could be effectively described using a uniaxial rotational diffusion model with an additional elastic scattering contribution. This kind of elastic scattering arises due to the restricted bond mobility and increases with decreasing temperature. The radius of rotation obtained from the fitted data agrees very well with the geometry of the molecule and grafting density. These results open possibilities of research on the confined surfactant systems, which could be analyzed using the approach described here. [ABSTRACT FROM AUTHOR]

This article discusses the development of an analytic dynamic structure factor to describe neutron spin-echo (NSE) spectra of polymers in the reptation regime. The structure factor includes local reptation, Rouse motion, non-Gaussian dynamics, and contour length fluctuations. The new approach improves the matching of experimental data and model predictions, and allows for the extraction of tube diameter and Rouse rate parameters. The article also discusses the scaling of data and the potential applications of this fitting tool in analyzing the dynamics of entangled long chain polymers. [Extracted from the article]

The performance of fuel cells depends largely on the proton diffusion in the proton conducting membrane, the core of a fuel cell. High temperature polymer electrolyte fuel cells are based on a polymer membrane swollen with phosphoric acid as the electrolyte, where proton conduction takes place. We studied the proton diffusion in such membranes with neutron scattering techniques which are especially sensitive to the proton contribution. Time of flight spectroscopy and backscattering spectroscopy have been combined to cover a broad dynamic range. In order to selectively observe the diffusion of protons potentially contributing to the ion conductivity, two samples were prepared, where in one of the samples the phosphoric acid was used with hydrogen replaced by deuterium. The scattering data from the two samples were subtracted in a suitable way after measurement. Thereby subdiffusive behavior of the proton diffusion has been observed and interpreted in terms of a model of fractal diffusion. For this purpose, a scattering function for fractal diffusion has been developed. The fractal diffusion dimension d w and the Hausdorff dimension df have been determined on the length scales covered in the neutron scattering experiments. [ABSTRACT FROM AUTHOR]

A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macromol. Theory Simul. 21, 565 (2012)] is scrutinized by small-angle neutron scattering (SANS) with respect to its static prediction, the single-chain structure factor. We find a remarkable agreement although the model simplifies the effect of the confinement to a harmonic potential. The effective confinement size from fits of SANS data with the model agrees well with the actual pore size. Starting from this result we discuss the possibility of an experiment on the dynamic structure factor predicted by the model. It turns out that such an experiment would need a large ratio polymer dimension/pore size which is difficult but not impossible to achieve. [ABSTRACT FROM AUTHOR]

Long-range conformational changes in proteins are ubiquitous in biology for the transmission and amplification of signals; such conformational changes can be triggered by small-amplitude, nanosecond protein domain motion. Understanding how conformational changes ate initiated requires the characterization of protein domain motion on these timescales and on length scales comparable to protein dimensions. Using neutron spin-echo spectroscopy (NSE), normal mode analysis, and a statistical-mechanical framework, we reveal overdamped, coupled domain motion within DNA polymerase I from Thermus aquaticus (Taq polymerase). This protein utilizes correlated domain dynamics over 70 [Angstrom] to coordinate nucleotide synthesis and cleavage during DNA synthesis and repair. We show that NSE spectroscopy can determine the domain mobility tensor, which determines the degree of dynamical coupling between domains. The mobility tensor defines the domain velocity response to a force applied to it or to another domain, just as the sails of a sailboat determine its velocity given the applied wind force. The NSE results provide insights into the nature of protein domain motion that are not appreciated by conventional biophysical techniques. normal mode analysis | statistical mechanics | protein dynamics quasielastic neutron scattering

We report a comprehensive study on the molecular conformation and dynamics of very large poly(ethylene oxide) rings in the melt: (i) for all rings, independent of the ring size, by small angle neutron scattering we observe a crossover from a strong Q -dependence at intermediate Q to a Q − 2 dependence at higher Q. Constructing a generic model including a crossover from Gaussian statistics at short distances to more compact structures at larger distances, we find the crossover at a distance along the ring of N e , 0 = 45 ± 2.5 monomers close to the entanglement distance in the linear counterpart. This finding is clear evidence for the predicted elementary loops building the ring conformation. (ii) The radius of gyration R g (N) follows quantitatively the result of numerous simulations. However, other than claimed, the crossover to mass fractal statistics does occur around N ≅ 10 N e , 0 , but up to N ≅ 44 N e , 0 , the relation R g (N) ∼ N 0.39 holds. The self-similar ring dynamics was accessed by pulsed field gradient-NMR and neutron spin echo spectroscopy: we find three dynamic regimes for center of mass diffusion starting (i) with a strong subdiffusive domain ⟨ r c o m 2 (t) ⟩ ∼ t α (0.4 ≤ α ≤ 0.65) , (ii) a second subdiffusive region ⟨ r c o m 2 (t) ⟩ ∼ t 0.75 that (iii) finally, crosses over to Fickian diffusion. The internal dynamics at scales below the elementary loop size is well described by the ring Rouse motion. At larger scales, the dynamics is self-similar and follows very well the predictions of scaling models with a preference for the fractal loopy globule model. Finally, we note that the key results were previously published in the form of two letters [Kruteva et al., ACS Macro Lett. 9, 507–511 (2020) and Kruteva et al., Phys. Rev. Lett. 125, 238004 (2020)]. [ABSTRACT FROM AUTHOR]

Poly(ethylene oxide) confined in an anodic aluminum oxide solid matrix has been studied by different neutron scattering techniques in the momentum transfer (Qvector ) range 0.2≤Q=|Qvector |≤1.9 Å-1. The cylindrical pores of the matrix present a diameter (40 nm) much smaller than their length (150 μm) and are parallel and hexagonally ordered. In particular, we investigated the neutron intensity scattered for two orientations of the sample with respect to the incident beam, for which the Qvector direction was either parallel or perpendicular to the pores for a scattering angle of 90°. Diffuse neutron scattering at room temperature has shown that the aluminum oxide has amorphous structure and the polymer in the nanoporous matrix is partially crystallized. Concerning the dynamical behavior, for Q<1 Å-1, the spectra show Rouse-like motions indistinguishable from those in the bulk within the uncertainties. In the high-Q limit we observe a slowing down of the dynamics with respect to the bulk behavior that evidences an effect of confinement. This effect is more pronounced for molecular displacements perpendicular to the pore axis than for parallel displacements. Our results clearly rule out the strong corset effect proposed for this polymer from nuclear magnetic resonance (NMR) studies and can be rationalized by assuming that the interactions with the pore walls affect one to two adjacent monomer monolayers. [ABSTRACT FROM AUTHOR]

We have studied the influence of plasticization on the microscopic dynamics of a glass-forming polymer. For this purpose we studied polyvinylchloride (PVC) with and without the commercially used plasticizer dioctylphthalate (DOP). We used dielectric spectroscopy and inelastic neutron scattering employing the neutron spin echo (NSE) technique. For both kinds of spectra the α relaxation could be consistently described by a model involving a distribution of individual relaxations of the Kohlrausch type. In contrast to earlier studies it turned out that an asymmetric distribution is necessary to fit the data at the lower temperatures investigated here. The shape parameters of the distribution (width, skewness) for PVC and PVC/DOP turned out to coincide when the characteristic relaxation times were the same. This means that the plasticizer only induces a remapping of the temperature dependence of the α relaxation. Comparison of NSE spectra S(Q,t)/S(Q) at different scattering vectors Q gave the result that the slowing down at the structure factor peak Qmax is surprisingly small for PVC while it is in the normal range for PVC/DOP. [ABSTRACT FROM AUTHOR]

The properties of bicontinuous microemulsions, consisting of water, oil, and a surfactant, can be modified by the addition of diblock copolymers (boosting effect) and homopolymers (inverse boosting effect) or a combination of both. Here, the influence of the addition of homopolymers (PEPX and PEOX, X=5k or 10k molecular weight) on the dynamics of the surfactant layer is studied with neutron spin echo spectroscopy (NSE). Combining the results with the previous findings for diblock copolymers allows for a better separation of viscosity and bending modulus effects. With the addition of homopolymers, a significant increase of the relaxation rate compared to the pure microemulsion has been observed. The influence on the bending rigidity κ is measured with NSE experiments. Homopolymer addition reduces κ by up to Δκ≃-0.5kBT, whereas the diblock copolymer yields an increase of κ by ∼0.3kBT. Comparison of the bending moduli that are obtained by analysis of the dynamics to those obtained from small angle neutron scattering (SANS) sheds light on the different renormalization length scales for NSE and SANS. Variation of the surfactant concentration at otherwise constant conditions of homopolymer or diblock-copolymer concentration shows that NSE results are leading to the pure bending rigidity, while the renormalized one is measured with SANS. [ABSTRACT FROM AUTHOR]

By using small-angle neutron scattering (SANS) and neutron spin echo (NSE), we have quantitatively investigated the static inhomogeneity in poly (N-isopropyl acrylamide) gel (PNIPA) in microscopic length scales of 0.015-1, where q is a wave number of scattered neutrons. NSE revealed that at lower q(≅0.015 A-1), the concentration fluctuations in the PNIPA gel decays more slowly as compared to the PNIPA solution without crosslinks. According to our scenario that the slower decay found for the PNIPA gel is due to the static inhomogeneity coexisting in the swollen gel, small-angle scattering S(q) obtained by SANS has been quantitatively decomposed into thermal and static scattering components, respectively, Sth(q) and Sst(q). It was further revealed that (i) the q-region where Sst(q) becomes dominant is closely related to that for the abnormal butterfly scattering under stretching, and (ii) as the temperature increases toward the temperature for volume phase transition, Sst(q) of a squared Lorentzian shape increases more drastically than Sth(q) of a Lorentzian shape. These findings were quantitatively understood in the theoretical framework by Panyukov and Rabin [Macromolecules 29, 7960 (1996)] or by Onuki [J. Phys. II. France 2, 45 (1992)], taking into account stress-fluctuation coupling under coexistence of the inherent structural heterogeneity in the real gel. We further found that the static inhomogeneity showing Sst(q) seems to relate to the necklacelike microstructure, appearing after a shallow quench into the collapsed phase. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]