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Structural dynamics 7(3), 034302 (2020). doi:10.1063/4.0000014, Published by AIP Publishing LLC, Melville, NY

We report a neutron-scattering investigation on the structure and dynamics of a single-component nanocomposite based on SiO2 particles that were grafted with polyisoprene chains at the entanglement limit. By skillful labeling, we access both the monomer density in the corona as well as the conformation of the grafted chains. While the corona profile follows a r−1 power law, the conformation of a grafted chain is identical to that of a chain in a reference melt, implying a high mutual penetration of the coronas from different particles. The brush crowding leads to topological confinement of the chain dynamics: (i) At local scales, the segmental dynamics is unchanged compared to the reference melt, while (ii) at the scale of the chain, the dynamics appears to be slowed down; (iii) by performing a mode analysis in terms of end-fixed Rouse chains, the slower dynamics is tracked to topological confinement within the cone spanned by the adjacent grafts; (iv) by adding 50% matrix chains, the topological confinement sensed by the grafted chain is lifted partially and the apparent chain motion is accelerated. We observe a crossover from pure Rouse motion at short times to topological confined motion beyond the time when the segmental mean squared displacement has reached the distance to the next graft.

Using grazing incidence scattering methods allows for depth profiling near surface structures very efficiently Dosch (1986). In parallel, layered structures have been used as resonators to enhance the wave field Kozhevnikov et al. (2007), Khaydukov et al. (2011), Kozhevnikov et al. (2011) and Nesnidal and Walker (1996) that directly increases the scattered intensity too. Third, the combination of these methods with neutron spin echo spectroscopy allows for near surface studies of dynamics Jaksch et al. (2015) and Frielinghaus et al. (2012) that can be correlated to tribological effects on the molecular level. This field of science, the tribology, - so far - has been driven mainly by the surface force balance that measures the macroscopic response of the system (latest research employs also AFM) Raviv et al. (2003) [1], Chung et al. (2016) [2] and Mocny and Klok (2016) [3]. The progress of this method was to reach the nano-scale distances that were necessary to obtain information about the friction of the nano-structures. The proposed method of grazing incidence neutron spin echo spectroscopy gives access to much more detailed information of molecular response to confinement by one or two hard walls, and therefore would pave the way for very rich and precise tribological comparisons of theory with experiments.

10 págs.; 7 figs.; 1 tab. ; PACS number s : 75.25. z, 75.30.Ds, 75.40.Gb, 75.47.Gk, The spin dynamics of substituted colossal magnetoresistive (CMR) manganites of general formula La0.7 Pb0.3 (Mn1-x Fex) O3, 0≤x≤0.2 is investigated by means of neutron spin-echo measurements. Substitution of Mn by Fe leads to a strong decrease of the temperature of macroscopic magnetic long-range ordering with a concomitant enhancement of the CMR effect. For x=0.2, a long-range-ordered state is not achieved as a result of the increase in antiferromagnetic interactions brought forward by Fe+3 -Mn couplings. The results display two relaxations having well separated decay constants. A fast process with a relaxation time of about 10 ps within the paramagnetic phase is found for all compositions. It shows a remarkably strong dependence with temperature and sample composition as the apparent activation energy for spin diffusion as well as the preexponential term exemplify. The physical origin of such a fast relaxation is assigned to heavily damped or overdamped spin waves (spin diffusion) on the basis of some signatures of excitations having finite frequencies found for the parent compound La0.7 Pb0.3 Mn O3 at temperatures just below Tc, together with preliminary data on the effect of Fe doping on the stiffness constant. A slower relaxation is present for all compositions. Its temperature dependence follows the behavior of the macroscopic magnetization, and its intensity grows within the ordered ferromagnetic state. Its physical origin is ascribed to collective reorientation of nanoscale ferromagnetic domains on the basis of the wave-vector dependence of its relaxation rate and amplitude. © 2007 The American Physical Society., J.G. and J.M.B. thank the Spanish Ministerio de Educacion y Ciencia for financial support under research Grant No. MAT2005-0686-C04-03. F.J.B. and P.R. acknowledge financial support from the European Commission through NMI3 to carry out preliminary measurements at the FZJ facilities.

The spin dynamics of substituted colossal magnetoresistive (CMR) manganites of general formula La0.7Pb0.3(Mn1-xFex)O-3, 0

In order to study the mechanisms limiting the topological chain confinement in polymer melts, we have performed neutron-spin-echo investigations of the single-chain dynamic-structure factor from polyethylene melts over a large range of chain lengths. While at high molecular weight the reptation model is corroborated, a systematic loosening of the confinement with decreasing chain length is found. The dynamic-structure factors are quantitatively described by the effect of contour-length fluctuations on the confining tube, establishing this mechanism on a molecular level in space and time. \ud \ud \ud \ud

The quality of life partially depends on the ability to maintain the transportation of goods and people in an efficient and reliable fashion, even under difficult climatic conditions. A well-known problem at low temperatures is the filter blockage of diesel fuel. Fuel oils contain alkanes that precipitate at low temperature as large crystals or spherulites of wax in such a way as to form gels. Thereupon the fuel loses its ability to flow and the transportation system falters or is stopped. The lowest temperature at which the fuel will still flow is known as the pour point. As the temperature approaches that of the pour point, difficulties emerge in transporting the fuel through lines and pump. Furthermore, wax crystals can plug screens and filters at temperatures above the pour point; the so-called cold filter plugging point. To combat this behavior various additives have been developed to depress the pour point or to decrease the size and alter the shape of the wax crystals; e.g., smaller sized crystals are less likely to clog either screens or filters. This paper reports on the use of a crystalline-amorphous diblock copolymer that performs, in its self-assembled state, as an efficient nucleator for the wax in middle distillate fuels.

State of the art diode rf-sputtering is used to fabricated high quality polarizing monochromators for neutrons. Optimization of the deposition parameters is achieved using in- situ kinetic ellipsometry and a great number of ex-situ characterization techniques such as grazing x-ray reflection, x-ray diffusion, alternating field gradient magnetometry. Mossbauer spectrometry, and electron microscopy. A precise picture of the structural characteristics of the system is deduced and related to the neutron performance as measured by polarized neutron reflectometry. We show that the structural behavior is controlled by crystallization of the iron layers and by the occurrence of amorphous interdiffused layers at each interface. As a consequence, the polarizing efficiency of these mirrors depends directly on the amount of iron involved in the interdiffusion. Using optimized deposition conditions, the flipping ratio is found to be around 40 for Fe/Ge mirrors with a medium period value of 120 angstroms. Positive spin-state reflectivity at the first Bragg peak is close to 100% when 150 bilayers are included and a controlled graded layer thickness allows the angular acceptance to be enhanced up to 39%.