Measurements Of Small-angle Neutron Research Articles

Flooding and performance of polymer electrolyte fuel cell, investigated by small-angle neutron scattering, neutron radiography and segmented electrode

Abstract In order to elucidate the cell performance related to swelling or flooding of the polymer electrolyte membrane (Nafion®), we performed in-situ measurements of small-angle neutron scattering (SANS) and neutron radiography (NR) for a polymer electrolyte fuel cell (PEFC) during electricity generation. SANS determined the swelling behavior by observing inter-particle scattering of water-clusters and mean inter-distance of nm order (Lw). With increased current density, Lw heterogeneously increases along the serpentine-shaped gas channel. NR revealed that flooding occurs along the channel after reaching Lw to the maximum. Lw determined by SANS was converted to the number of water molecules (λw) hydrating around a sulfonic group. Using this key value, we evaluated the membrane resistance Rm. To determine the local cell performance, corresponding to SANS & NR observations, we developed a segmented electrode cell combining electrochemical impedance spectroscopy. The segmented electrode independently determined the membrane resistance and I-V curve were determined for the local area, which is sensitive to the heterogeneous swelling along a film thickness. As a result of comparison with SANS and the results by segmented electrode, we found heterogeneous swelling along the gas flow path or along a thickness direction. The heterogeneity changes as the current increases. By comparing with NR, we found that flooding appears after swelling of the membrane is saturated.

Design and implementation of a differential scanning calorimeter for the simultaneous measurement of small angle neutron scattering

For almost 30 years, at synchrotron facilities, it has been possible to perform small-angle x-ray scattering experiments whilst simultaneously measuring differential scanning calorimetry (DSC). However, a range of challenges exist to enable the collection of simultaneous small-angle neutron scattering (SANS) and DSC data associated not only with intrinsic flux limitations but also scattering geometry and thermal control. The development of a DSC (temperature range ca. −150 °C to 500 °C) suitable for SANS is detailed here and its successful use is illustrated with combined measurements on a binary blend of normal alkanes in which one component has been deuterium labelled.

Isothermal Evolution of Long Range Order in Bulk Metallic Glass Ni15Pt60P25 near Glass Transition: In-Situ SANS Measurement

In-situ measurements of small angle neutron scattering from a bulk metallic glass Ni15Pt60P25 were carried out in the momentum transfer range of 0.008 < Q < 0.08Å−1 as a function of the holding time during isothermal annealing at the temperature Ta = 483 K just below the glass transition temperature Tg = 488K. The small angle neutron scattering intensities initially increase with holding time and reach saturation values. The small angle neutron scattering intensity of the glass cooled down to room temperature after the annealing increases at the every momentum transfer value observed. The small angle neutron scattering intensities follow a Q−4 dependence and have no peak structure in the momentum transfer range observed. There is no evidence for any phase separations in nanometer length scale to occur in this glass.

Multiple small-angle neutron scattering for an arbitrary value of the born parameter

Computer calculations are made of the intensity of multiple small-angle neutron scattering using the general Moliere formula [8] over a wide range of variation of the Born parameter, embracing the diffraction and refraction regimes, and a transition region between diffraction and reflection. A comparison is made with approximate formulas obtained earlier by Maleev et al. [9, 10] in the limiting cases of the Born parameter α ≪ 1 and α ≫ 1 for the diffraction and refraction regimes, respectively. It is shown that over a wide range of values of α the results of the calculations using the approximate and general formulas are the same. The theoretical conclusions were checked experimentally using data from measurements of small-angle neutron scattering for the domain structure of ferromagnets. Measurements were made of the neutron beam broadening for samples of different thickness and these were used to determine the effective domain sizes in pure iron and nickel exposed to thermal treatment and plastic deformation, and also in the Invar alloys Fe65Ni35and Fe3Pt. An analysis is made of the angular dependence of magnetic small-angle neutron scattering at the asymptote.

The polymerization of actin: Study by small angle neutron scattering

We report measurements of small angle neutron scattering from solutions of rabbit muscle G-actin at 3.00 mg/mL in D2O buffer solution, with [Ca2+]=0.52 mM and with [KCl]=15, 8.9, and 5.4 mM. We observe the onset of the polymerization of G-actin to F-actin as the temperature is increased. The polymerization takes place on a time scale of 30–45 min for each temperature jump of 2 °C–3 °C. As the temperature is increased further, the average size of the polymers increases, and the characteristic length scale (or correlation length), ξ, of the F-actin in the dilute solution grows: ξ is about 10 Å below Tp, and about 70 Å a few degrees above Tp. The transition is sharper for lower concentrations of KCl. For the sample with [KCl]=8.9 mM, we observe a peak in ξ at about 2 °C above Tp, which indicates a crossover into the semidilute regime. The transition is essentially reversible, but shows evidence of incomplete depolymerization on cycling. We are unable to apply the available theoretical model for reversible polymerization to rabbit muscle actin because of a lack of information on the enthalpy and entropy of polymerization. However, our observations for rabbit muscle actin are quite analogous to observations on the equilibrium polymerization of α-methylstyrene [A. P. Andrews, K. P. Andrews, S. C. Greer, F. Boué, and P. Pfeuty, Macromolecules 27, 3902 (1994)].

Small Angle Neutron Scattering from Nanodroplet Aerosols

We report the first measurements of small angle neutron scattering from an aerosol. The aerosol was produced by expanding a D{sub 2}O -N{sub 2} vapor mixture in a supersonic Laval nozzle. The neutron wavelength (0.5 nm) is less than the typical particle size, and we can therefore derive the average particle size (5{endash}8 nm), number density ({approximately}10{sup 12} cm{sup {minus}3}), and polydispersity of the size distribution directly from the experimental data rather than by inferring them from complex models of particle formation and growth. We also predict and observe a Doppler shift-induced anisotropy in the scattering pattern due to the directed motion of the aerosol in the nozzle. Further applications of this new technique are discussed. {copyright} {ital 1997} {ital The American Physical Society}

Role of Spacer Chain Length in Dimeric Micellar Organization. Small Angle Neutron Scattering and Fluorescence Studies

Micelles of different dimeric amphiphiles Br-, n-C(16)H(33)NMe(2)(+) -(CH)(m)-N(+)Me(2)-n-C16H33, Br- (where m = 3, 4, 5, 6, 8, 10, and 12) adapt different morphologies and internal packing arrangements in aqueous media depending on their spacer chain length (m). Detailed measurements of small angle neutron scattering (SANS) cross sections from different bis-cationic, dimeric surfactant micelles in aqueous media (D2O) are reported. The data have been analyzed using the Hayter and Penfold model for macro ion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the dimeric micelles. The SANS analysis clearly indicated that the extent of aggregate growth and the variations of shapes of the dimeric micelles depend primarily on the spacer chain length. With spacer chain length, m less than or equal to 4, the propensity of micellar growth was particularly pronounced. The effects of the variation of the concentration of dimeric surfactants with m = 5 and 10 on the SANS spectra and the effects of the temperature variation for the micellar system with m = 10 were also examined. The critical micelle concentrations (cmc) and their microenvironmental feature, namely, the microviscosities that the dimeric micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were also determined. The changes of cmcs and microviscosities as a function of spacer chain length have been explained in terms of conformational variations and progressive looping of the spacer in micellar core upon increasing m values.

Interactions in a Miscible Polymer Blend

Abstract Characterization of blends of atactic poly(vinylethylene) with cis-1,4-poly-isoprene provides unique insights into the factors governing the properties of polymer mixtures. While studies of polymer blends have heretofore focussed on systems in which chemical reaction between the components enabled miscibility, any properties arising from the blending of PVE with PIP are not a result of greater cohesive energy density or densification. It is also interesting to consider the different crosslinking reactivity of the main-chain unsaturation in PIP versus that of the pendant vinyl group of PVE, which can give rise to interesting network properties. These will be the subject of a subsequent communication. While it is clear that the Flory interaction parameter for these blends must be non-negative, direct determination of its magnitude is best accomplished through measurement of small-angle neutron scattering. Currently deuterated polybutadienes of varying microstructure are being synthesized to enable such experiments to be carried out, and thereby allow the effect of vinyl level on the miscibility and phase behavior to be quantified.