Contrast Variation Method Research Articles

Advances in Small Angle Neutron Scattering on Polysaccharide Materials.

Polysaccharide materials and biomaterials gain the focus of intense research owing to their great versatility in chemical structures and modification possibilities, as well as their biocompatibility, degradability, and sustainability features. This review focuses on the recent advances in the application of SANS on polysaccharide systems covering a broad range of materials such as nanoparticulate assemblies, hydrogels, nanocomposites, and plant-originating nanostructured systems. It motivates the use of SANS in its full potential by demonstrating the features of contrast variation and contrast matching methods and by reporting the methodologies for data analysis and interpretation. As these soft matter systems may be organized in multiple length scales depending on the interactions and chemical bonds between their components, SANS offers exceptional and unique opportunities for advanced characterization and optimization of new nanostructured polysaccharide materials.

Contrast variation method applied to structural evaluation of catalysts by X-ray small-angle scattering

In the process of developing carbon-supported metal catalysts, determining the catalyst particle-size distribution is an essential step, because this parameter is directly related to the catalytic activities. The particle-size distribution is most effectively determined by small-angle X-ray scattering (SAXS). When metal catalysts are supported by high-performance mesoporous carbon materials, however, their mesopores may lead to erroneous particle-size estimation if the sizes of the catalysts and mesopores are comparable. Here we propose a novel approach to particle-size determination by introducing contrast variation-SAXS (CV-SAXS). In CV-SAXS, a multi-component sample is immersed in an inert solvent with a density equal to that of one of the components, thereby rendering that particular component invisible to X-rays. We used a mixture of tetrabromoethane and dimethyl sulfoxide as a contrast-matching solvent for carbon. As a test sample, we prepared a mixture of a small amount of platinum (Pt) catalyst and a bulk of mesoporous carbon, and subjected it to SAXS measurement in the absence and presence of the solvent. In the absence of the solvent, the estimated Pt particle size was affected by the mesopores, but in the presence of the solvent, the Pt particle size was correctly estimated in spite of the low Pt content. The results demonstrate that the CV-SAXS technique is useful for correctly determining the particle-size distribution for low-Pt-content catalysts, for which demands are increasing to reduce the use of expensive Pt.

SCOMAP-XD: atomistic deuterium contrast matching for small-angle neutron scattering in biology.

The contrast-variation method in small-angle neutron scattering (SANS) is a uniquely powerful technique for determining the structure of individual components in biomolecular systems containing regions of different neutron scattering length density ρ. By altering the ρ of the target solute and the solvent through judicious incorporation of deuterium, the scattering of desired solute features can be highlighted. Most contrast-variation methods focus on highlighting specific bulk solute elements, but not on how the scattering at specific scattering vectors q, which are associated with specific structural distances, changes with contrast. Indeed, many systems exhibit q-dependent contrast effects. Here, a method is presented for calculating both bulk contrast-match points and q-dependent contrast using 3D models with explicit solute and solvent atoms and SASSENA, an explicit-atom SANS calculator. The method calculates the bulk contrast-match points within 2.4% solvent D2O accuracy for test protein-nucleic acid and lipid nanodisc systems. The method incorporates a general model for the incorporation of deuterium at non-exchangeable sites that was derived by performing mass spectrometry on green fluorescent protein. The method also decomposes the scattering profile into its component parts and identifies structural features that change with contrast. The method is readily applicable to a variety of systems, will expand the understanding of q-dependent contrast matching and will aid in the optimization of next-generation neutron scattering experiments.

Block Copolymer Adsorption on the Surface of Multi-Walled Carbon Nanotubes for Dispersion in N,N Dimethyl Formamide

This research aims to characterize the adsorption morphology of block copolymer dispersants of the styrene-block-4-vinylpyridine family (S4VP) on the surface of multi-walled carbon nanotubes (MWCNT) in a polar organic solvent, N,N-dimethyl formamide (DMF). Good, unagglomerated dispersion is important in several applications such as fabricating CNT nanocomposites in a polymer film for electronic or optical devices. Small-angle neutron scattering (SANS) measurements, using the contrast variation (CV) method, are used to evaluate the density and extension of the polymer chains adsorbed on the nanotube surface, which can yield insight into the means of successful dispersion. The results show that the block copolymers adsorb onto the MWCNT surface as a continuous coverage of low polymer concentration. Poly(styrene) (PS) blocks adsorb more tightly, forming a 20 Å layer containing about 6 wt.% PS, whereas poly(4-vinylpyridine) (P4VP) blocks emanate into the solvent, forming a thicker shell (totaling 110 Å in radius) but of very dilute (<1 wt.%) polymer concentration. This indicates strong chain extension. Increasing the PS molecular weight increases the thickness of the adsorbed layer but decreases the overall polymer concentration within it. These results are relevant for the ability of dispersed CNTs to form a strong interface with matrix polymers in composites, due to the extension of the 4VP chains allowing for entanglement with matrix chains. The sparse polymer coverage of the CNT surface may provide sufficient space to form CNT-CNT contacts in processed films and composites, which are important for electrical or thermal conductivity.

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes.

Small-angle neutron scattering (SANS) is a powerful tool for studying biological membranes and model lipid bilayer membranes. The length scales probed by SANS, being from 1 nm to over 100 nm, are well-matched to the relevant length scales of the bilayer, particularly when it is in the form of a vesicle. However, it is the ability of SANS to differentiate between isotopes of hydrogen as well as the availability of deuterium labeled lipids that truly enable SANS to reveal details of membranes that are not accessible with the use of other techniques, such as small-angle X-ray scattering. In this work, an overview of the use of SANS for studying unilamellar lipid bilayer vesicles is presented. The technique is briefly presented, and the power of selective deuteration and contrast variation methods is discussed. Approaches to modeling SANS data from unilamellar lipid bilayer vesicles are presented. Finally, recent examples are discussed. While the emphasis is on studies of unilamellar vesicles, examples of the use of SANS to study intact cells are also presented.

Styrene-Maleic Acid Copolymer Nanodiscs to Determine the Shape of Membrane Proteins.

Lipid nanodiscs can be used to solubilize functional membrane proteins (MPs) in nativelike environments. Thus, they are promising reagents that have been proven useful to characterize MPs. Both protein and non-protein molecular belts have shown promise to maintain the structural integrity of MPs in lipid nanodiscs. Small-angle neutron scattering (SANS) can be used to determine low-resolution structures of proteins in solution, which can be enhanced through the use of contrast variation methods. We present theoretical contrast variation SANS results for protein and styrene-maleic acid copolymer (SMA) belt 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) nanodiscs with and without additional bound or transmembrane proteins. The predicted scattering properties are derived from atomistic molecular dynamics simulations to account for conformational fluctuations, and we determine deuterium-labeling conditions such that SANS intensity profiles only include contributions from the scattering of the MP of interest. We propose strategies to tune the neutron scattering length densities (SLDs) of the SMA and DMPC using selective deuterium labeling such that the SLD of the nanodisc becomes homogeneous and its scattering can essentially be eliminated in solvents containing an appropriate amount of D2O. These finely tuned labeled polymer-based nanodiscs are expected to be useful to extract the size and molecular shape information of MPs using SANS-based contrast variation experiments, and they can be used with MPs of any molecular weight.

Combined Small-Angle Neutron Scattering/Small-Angle X-ray Scattering Analysis for the Characterization of Silver Nanoparticles Prepared via Photoreduction in Water-in-Oil Microemulsions.

In this study, we used small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) to investigate the formation process of silver (Ag) nanoparticles (NPs) in water-in-oil (w/o) reverse microemulsions comprising sodium bis(2-ethylhexyl) sulfosuccinate (AOT), water, and organic solvents (such as benzene, octane, and decane) by the photoreduction of silver perchlorate (AgClO4). Combining SANS and SAXS, the structural changes in the w/o microemulsions before and after the formation of Ag NPs via photoreduction were quantitatively evaluated. From the SANS experiments performed using the contrast-variation method, the size of water cores containing Ag NPs and the thickness of the AOT shells were calculated using the core-shell hard-sphere model. The size of the Ag NPs and their aggregates was calculated via SAXS analysis based on the polydisperse sphere model with a Schulz-Zimm distribution. We found that aggregates of three or four primary Ag NPs are formed by, first, the aggregation of water droplets through the entanglement of the tails of the AOT shell, followed by the self-assembly of Ag NPs into their aggregates because of particle-particle attractive interactions.

Implications of surfactant hydrophobic chain architecture on the Surfactant-Skin lipid model interaction

Although surfactants have been widely used in skin care and other related applications, our knowledge about how surfactants interact with stratum corneum (SC) lipids remains limited. This work reports how surfactants interact with a lipid SC model by neutron diffraction and molecular dynamics (MD) simulations, focusing on examining the impact of surfactant molecular architecture. The surfactant-SC mixed membrane was constructed by an equimolar mixture of ceramide/cholesterol/fatty acids and surfactant at 1% molar ratio of total lipids. The arrangements of water and surfactant molecules in the membrane were obtained through neutron scattering length density (NSLD) profiles via contrast variation method, meanwhile, MD simulation clearly demonstrated the mechanism of hydration change in the surfactant-model SC mixed membrane. No drastic difference was detected in the repeating distance of the short periodicity phase (SPP) upon adding surfactants, however, it significantly enhanced the membrane hydration and reduced the amount of phase separated crystalline cholesterol, showing a strong dependence on surfactant chain length, branching and double bond. This work clearly demonstrates how surfactant architecture affects its interaction with the SC membrane, providing useful guidance for either choosing an existing surfactant or designing a new one for surfactant-based transdermal application.

Contrast Variation Small-Angle Neutron Scattering Study of Solubilization of Perfumes in Cationic Surfactant Micelles.

Perfume solubilization is an important process in the production of commercial products such as beverages, foods, and cosmetics. In the present study, small-angle neutron scattering (SANS) experiments were performed to investigate the solubilization behavior of perfumes in cetyltrimethylammonium bromide (CTAB) micelles. The solubilization of linalool (LL) and l-menthol (MT), which are relatively hydrophilic perfumes, did not change the size of the CTAB micelles although the perfumes were incorporated in the micelles, as indicated by a decrease in scattering length density. On the other hand, the solubilization of d-limonene (LN), a hydrophobic perfume, led to the swelling of CTAB micelles. An internal contrast variation SANS study was performed by the deuteration of CTAB molecules to directly observe the perfumes in the micelles. The radius of d-CTAB micelles solubilizing LL or MT corresponds to that of h-CTAB, which indicates that these perfumes are accommodated in the palisade layers of the micelles and are homogeneously distributed in the micelles. On the other hand, LN formed small droplets, as indicated by the SANS profile, which implies the solubilization of LN molecules in the core of the CTAB micelles. We found that the relatively hydrophilic perfumes (LL and MT) show less impact on the sizes of the cationic micelles in comparison to nonionic micelles. Thus, the internal contrast variation method of SANS allowed the direct observation of the solubilization sites of perfumes with different hydrophilicity-hydrophobicity balances. This method is a powerful tool to determine the solubilization states that affect the solubilization capacity, volatilization, or release speed of perfumes.

Heterogeneous cell performance of polymer electrolyte fuel cell at high current operation: Respiration mode as non-equilibrium phenomenon

The heterogeneous performance of polymer electrolyte fuel cell in space and time was discussed for operation at high current density. The cell voltage, detected by a segmented electrode, varies along a gas flow channel from upper to bottom stream and oscillates in time, which is referred as a respiration mode. At the higher current, the cell voltage at different positions started to be synchronized, as the current density increases. In order to investigate a role of water on the respiration, we employed a new method of contrast variation for small-angle neutron scattering (SANS) using deuterium (D2) gas as a fuel. By using D2, we introduce special scattering contrast in a polymer electrolyte film (Nafion®), when the film is originally swollen by H2O. After switching from H2 to D2 gas (humidified with H2O), we found that SANS intensity significantly decreases about 40% at the q-position of scattering maximum (qm) originating from the water-microdomains in the polymer electrolyte. After quantitative analyses of the scattering intensity, it was elucidated that 20 wt% of the total water is occupied by D2O as a steady state. At around the average intensity, SANS intensity oscillates with a time interval ∼100sec, which corresponds to the respiration mode found for voltage. The respiration behavior is considered as a non-linear &amp; non-equilibrium phenomenon in an open system, where water flooding plays a role of feedback to decelerate fuel transportation and chemical reaction of water generation.

Investigation of porous materials with large surface heterogeneity using the generalized Porod's scattering law method.

Surface heterogeneity is ubiquitous in both natural and man-made materials, and can significantly influences material properties. However, it is very challenging to noninvasively probe the variation of surface properties in porous materials. Recently, we have proposed a method, i.e., the generalized Porod's scattering law method (GPSLM), to obtain the surface heterogeneity information in bulk porous materials by extending the classic Porod's scattering method. However, it was not clear if the GPSLM can be applied to other more complex materials, such as porous materials with dead pores, i.e., pores that guest fluid molecules cannot access or porous materials whose solid matrix can adsorb small guest molecules. In this paper, we theoretically extend the GPSLM to study those more complex situations. For all five cases with different levels of complexity discussed in this work, the scattering intensity at the Porod's law region always follows a parabolic function of scattering length density (SLD) of the guest fluid. Moreover, the minimum value of the scattering intensity is all related to the surface heterogeneity of the porous materials. The SLD of the guest fluid at which the minimum intensity is reached is always related to the surface-averaged SLD of materials. We also discuss the potential limitations and possible future applications of the GPSLM. As the GPSLM is based on the contrast variation method commonly used for a wide range of materials, such as geological materials, biomaterials, and colloidal suspensions, the theoretical development here is potentially useful for researchers who would like to apply the GPSLM to more complicated materials besides porous materials.

Structural Study for Membrane Protein using Solution X-Ray Scattering Contrast Variation

Membrane protein often needs lipid bilayer membrane to stabilize its structure and to fulfill its functions. However, the presence of lipid bilayer membrane makes the structural study on the protein very difficult. Here we proposed to use X-ray scattering contrast variation method in which membrane protein (with lipid membrane) samples will be measured in a series of solutions with various sucrose concentration. X-ray scattering intensity measures the scattering length/capability difference between sample/solute and buffer solution/solvent. Since X-ray scattering lengths of protein and lipid are different and scattering length of sucrose buffer changes along with its concentration, the ratio of X-ray scattering contribution from protein part and lipid part varies along with the sucrose concentration. In such way, we can separate the scattering contribution of protein part from that of lipid. We also simultaneously fit the data sets collected from the sucrose concentration series measurements and obtain the 3-D low resolution structure for the protein and the lipid at the same time. Comparing to neutron scattering contrast variation method, this X-ray contrast method is less expensive and can provide wider q range due to the availability of high flux X-ray sources. A few membrane protein examples will be presented. Figure. SAXS data for membrane protein in various sucrose solution. In this presentation, I will also discuss the capabilities of beamline 12-ID-B of the Advanced Photon Source at Argonne National Laboratory. Acknowledgement: Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supportedby the U.S. DOE under Contract No. DE-AC02-06CH11357.

Reverse relationships of water uptake and alkaline durability with hydrophilicity of imidazolium-based grafted anion-exchange membranes.

We found unprecedented reverse relationships in anion-exchange membranes (AEMs) for Pt-free alkaline fuel cell systems, i.e., the increase in hydrophobicity increased water uptake and susceptibility to hydrolysis. AEMs with graft copolymers that composed of anion-conducting 2-methyl-N-vinylimidazolium (Im) and hydrophobic styrene (St) units were employed. We characterized two new structures in these AEMs using a small-angle neutron scattering with a contrast variation method. (1) The distribution of graft polymers in conducting (ion channel) or non-conducting (hydrophobic amorphous poly(ethylene-co-tetrafluoroethylene) (ETFE)) phase was evaluated in a quantitative manner. High fraction in conducting layer for AEMs having high grafting degrees was found using the proposed structural model of "conducting/non-conducting two-phase system". (2) Assuming a hard-sphere fluid model, we found AEMs having high St contents and low alkaline durability possessed nanophase-separated water puddles with diameters of 3-4 nm. The AEM having a low St content and the best alkaline durability did not show evident nanophase separation. The above hierarchical structures elucidate the unexpected reverse relationships that the AEM having highly hydrophobic graft polymers was subjected to the morphological transition to give water puddles at nanoscale. The imidazolium groups that were located at the boundary between graft polymers and water puddles should be susceptible to hydrolysis.

Localization of Cholesterol within Supported Lipid Bilayers Made of a Natural Extract of Tailor-Deuterated Phosphatidylcholine.

Cholesterol is an essential component of mammalian membranes and is known to induce a series of physicochemical changes in the lipid bilayer. Such changes include the formation of liquid-ordered phases with an increased thickness and a configurational order as compared to liquid-disordered phases. For saturated lipid membranes, cholesterol molecules localize close to the lipid head group-tail interface. However, the presence of polyunsaturated lipids was recently shown to promote relocation of cholesterol toward the inner interface between the two bilayer leaflets. Here, neutron reflection is used to study the location of cholesterol (both non-deuterated and per-deuterated versions are used) within supported lipid bilayers composed of a natural mixture of phosphatidylcholine (PC). The lipids were produced in a genetically modified strain of Escherichia coli and grown under specific deuterated conditions to give an overall neutron scattering length density (which depends on the level of deuteration) of the lipids matching that of D2O. The combination of solvent contrast variation method with specific deuteration shows that cholesterol is located closer to the lipid head group-tail interface in this natural PC extract rather than in the center of the core of the bilayer as seen for very thin or polyunsaturated membranes.

Evaluation of molecular volume change of block copolymer depending on temperature: A SANS study

Amphiphilic Pluronic triblock copolymers form various self-assembled structures such as sphere, cylinder, lamellae and so on, depending on temperature, leading to the increase of hydrophobicity of block copolymers. However, the effective molecular volume change of the block copolymer has not been fully exploited yet, when temperature increases. Here, we have investigated the effective molecular volume change of the block copolymer upon heating by using the contrast variation small angle neutron scattering. The scattering length densities (SLDs) of the block copolymer were experimentally obtained from the neutron scattering contrast variation method between the solvent and the block copolymer at varying temperature. Even though the SLD, which is the intrinsic property of the material, should not be changed by temperature elevation, it was dependent on temperature, indicating that the molecular volume is changed. Therefore, we obtained the increase rate of the molecular volume change of the block copolymer (the effective molecular volume change) from the comparison of the calculated SLD and the standard SLD, which is evaluated by plotting the SANS intensity at the first order Bragg peak as the function of temperature at each volume fraction of D2O and H2O that is about 25.5%–51.3% depending on temperature.

Alkaline Durable Anion-Conducting Electrolyte Membranes Prepared by Radiation Induced Grafting of 2-Methyl-4-vinylimidazole for Non-Platinum Direct Hydrazine Hydrate Fuel Cells

We have been developing highly-conductive and alkaline-durable anion-conducting electrolyte membranes (AEMs) for a platinum-free fuel cell (FC) for liquid fuel (hydrated hydrazine) type vehicles. Even though the AEM-FC system have been significantly attractive because of non-precious metals used as active catalysts, there is no commercially available AEMs due to severe alkaline operating condition.1) We applied the radiation-induced grafting technique to introduce imidazolium-based graftpolymers into thermally and mechanically tough poly(ethylene-co-tetrafluoroethylene) (ETFE) to develop new AEMs with high conductivity and alkaline durability (Scheme 1). Our strategy to overcome the alkaline unstable character of AEMs is as follows. (1 ) Basicity control of anion-conducting groups (ACG) in AEMs. The weak base is utilized for ACG to suppress the self-degradation of the membranes and high water uptakes to be stabilized because most of the AEMs possess tetraalkylammonium hydroxides as an ACG, which is well known as one of the strongest organic bases. Furthermore, the weak base AEMs should have less water uptakes due to their own stability. From the comparison of the N-alkylated forms of 5 different diaza-compounds as an ACG, the imidazolium-based AEMs (AEM(Im)) possess the high conductivity and low water uptake compared with tetraalkylammonium-type AEMs (Scheme 2). (2) Alkaline-durability of the imidazolium-based AEMs. The N-methylated imidazolium cation in AEMs are subjected to β-elimination and hydrolytic ring opening degradation to reduce the anionic conductivity in alkaline solution (1M KOH, 80°C). Therefore, for suppressing β-elimination, we introduced the graft-copolymers of vinylimidazolium with styrene because the adjacent vinylimiazolium cations led very fast elimination due to the double β-positioned hydrogen.2) Furthermore, for suppressing hydrolysis, the protecting group is introduced at 2-posion of imidazolium ring, which are subject to hydroxide attack (Scheme 2). The AEM(MIm/St) with a MIm/St ratio of 60/40 shows gradual decreases of conductivity in the alkaline solution. The alternative copolymer grafts of 2-methylimidazolium and α-methylstyrene in AEMs (AEM(MIm/aMS)), which has no adjacent imidazolium units, totally suppressed the alkaline degradation in the alkaline solution. This durability improvement strongly indicates that the alternative structure suppressed not only β-elimination but also the ring-opening degradation. As the results, the AEM(MIm/aMS) keeps their initial conductivity (50 mS/cm) in more than 1000 hours in 1M KOH at 80 °C. The high conductivity and alkaline durability of the abovementioned methylimidazolium-based AEMs can be well elucidated from the hierarchical structures of the AEMs. By contrast variation small angle neutron scattering (SANS) technique, we revealed the detailed hierarchical structures of AEM(MIm/St); the original crystalline lamellar domains of a ETFE base film and interconnected hydrated domains, composed of the entire graft chains and water, offer good mechanical properties and the conductivity, respectively.3) Using the SAXS measurement, by which the internal structure of ion channels at a length scale < 2 nm can be observed, AEM(MIm/St) has the characteristic “ion-pair” distributed in the ion-channels at RH < 80%. Namely, the ion-channels are disconnected at 80% RH but well-connected in liquid water. Judging from this RH dependence, the disconnected ion-channels even under relatively high humidified conditions (80% RH) should be the substantial factor for lower power generation efficiency of AEM-type fuel cells than those of PEM systems. We next analyzed the AEM(MIm/St) with different monomer compositions. Contrary to the above mentioned AEM(MIm/St) (MIm/St ratio of 60/40), the AEM with higher styrene ratios (MIm/St = 40/60 and 20/80) showed less durability in the alkaline solution. By a SANS contrast variation method only the AEMs with MIm/St = 40/60 and 20/80 showed new water puddle sphere with a diameter of 3-4 nm and the volume composition of spheres increases with the increases of styrene ratios. Thus, we concluded that the imidazolium groups around the water puddle are subjected to the hydrolytic attack to lead the degradation of the AEMs. Now, we are analyzing the hierarchical structures of AEM(MIm/aMS) to reveal the structural reasons for the highest alkaline durability so far. Acknowledgement This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST). References 1) J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy Environ. Sci., 7, 3135 (2014). 2) K. Yoshimura, H. Koshikawa, T. Yamaki, H. Shishitani, K. Yamamoto, S. Yamaguchi, H. Tanaka and Y. Maekawa, J. Electrochem. Soc., 161(9), F889 (2014). 3) Y. Zhao, K. Yoshimura, H. Shishitani, S. Yamaguchi, H. Tanaka, S. Koizumi, N. Szekely, A. Radulescu, D. Richter and Y. Maekawa , Soft Matter, 12, 1567 (2016). Figure 1

Investigation of various behaviors of near- and far-field diffractions from multiplicatively separable structures in the x and y directions, and a detailed study of the near-field diffraction patterns of 2D multiplicatively separable periodic structures using the contrast variation method

In this paper, first a detailed investigation of the various behaviors of the near- and far-field diffractions from multiplicatively separable (MS) structures in the x and y directions is presented. It is shown that in near-field propagation, the diffraction pattern of a 2D MS structure is same as the product of the corresponding near-field diffractions of the 1D individual components of the structure. For the far-field diffraction, although the resulting diffraction pattern is not equal to the product of the individual 1D structures’ diffraction patterns, we show that it is still a 2D MS pattern. Second, a detailed theoretical investigation of the contrast enhancement effect for the multiplication of two 1D orthogonal intensity patterns (not necessarily periodic) is presented. By merging the above-mentioned facts, we deduced that in near-field diffraction, the contrast of the diffraction pattern of a 2D MS structure is always larger than the contrast of each of the diffraction patterns of the corresponding individual 1D structures. In the second part of this paper, we implement the intensity contrast as a quantity to describe self-images of the 2D MS periodic structures. For the first time, two very important results are obtained based on the contrast enhancement effect. We show that the depth of focus of the self-images increases for the 2D periodic structures in comparing to their corresponding 1D structures. We also predict the existence of additional self-images in addition to the Talbot images located at the least common multiple of each of the individual 1D Talbot distances for the 2D MS periodic structures. In addition, in this work we present a very straightforward manner for the prediction of the 1D or 2D forms of the diffraction pattern and the direction of the 1D pattern strips at given propagation distances from the 2D structure by introducing another intensity-contrast-based parameter. Furthermore, we show that the diffraction pattern of a 2D MS periodic structure depends strongly on the square of the ratio of the 1D structure periods in the x and y directions. These theoretical achievements are verified by some experimental works.

Elucidation of the Morphology of Imidazolium-Based Anion Exchange Membranes for Alkaline Anion Fuel Cells

Due to the growing concerns on the depletion of petroleum based energy resources and the climate change, polymer electrolyte membrane fuel cells (FCs) technologies have received much attention in recent years owing to their high efficiencies and low emissions. Among them, alkaline anion exchange membrane fuel cell (AEM-FC) is a good choice, because it avoids the consumption of acid-resistant precious metal catalysts and costs up. However, the biggest challenge in developing AEM-FCs is to fabricate AEM with high ion conductivity and mechanical stability without chemical deterioration at elevated pH and temperatures. So far, most strategies were focused on synthesizing new thermally and chemically durable fluorinated and aromatic polymers. For the first time, our group tried to develop the new type of 2Me-AEMs by radiation-induced grafting of 2-methyl-1-vinylimidazole and styrene into poly(ethylene-co-tetrafluoroethylene) (ETFE) films and a subsequent N-alkylation with methyliodide. The resultant AEMs exhibit high ion conductivity (> 100 mS/cm) and longer alkaline durability, owing to the fact that the methyl protecting group at 2-imidazole position prevented the ring-opening degradation. 2Me-AEM with an IEC(ion exchange capacity) of 1.82 mmol/g shows the best well-balanced properties required for fuel cell applications. All these findings on one hand, are the result of sample preparation procedure of the radiation grafting method and the introduction of alkylimidazolium cations as an anion conducting group, and on the other hand, are believed to be controlled by the microphase separated structures of the membranes in the hydrated state. In this work, we aim to elucidate the morphology of these 2Me-AEMs and understand the structure related unique properties such as the mechanical property and the anion conductivity. We investigated the morphology and swelling behavior of these new graft-type AEMs by using contrast variation small angle neutron scattering (SANS) technique, performed mainly on KWS-2 SANS diffractometer operated by Juelich Centre for Neutron Science at the neutron source Heinz Maier-Leibnitz (FRM II reactor) in Garching, Germany. Our results showed that the scattering intensity and the shape of the profiles vary significantly upon grafting, but change little upon alkylation; while the incorporation of water in the hydrophilic domains upon swelling, not only increases the intensity and domain size, but also leads to the excess scattering at high-q region with q being the scattering vector. From these results, we concluded that the crystalline lamellar and crystallite structures originating from the pristine ETFE films were more or less conserved in these AEMs, but the lamellar d-spacing in both dry and wet membranes were enlarged, indicating an expansion of the amorphous lamellae due to the graft chains introduced in the grafting process and the water incorporated in the swelling process. We further quantitatively studied the swelling behavior of the AEMs in various water mixtures of water and deuterated water with different volume ratios (contrast variation method), and the morphology of these membranes was elucidated by three phases: phase 1) crystalline ETFE domains, which offer good mechanical properties; phase 2) hydrophobic amorphous domains, which are made up of amorphous ETFE chains and offer a matrix to create conducting regions; phase 3) interconnected hydrated domains, which are composed of the entire graft chains and water and play a key role to promote the conductivity.

Small-Angle Neutron Scattering Investigation of Core-Shell Carbon-Coated Silicon Nanoparticles for Lithium-Ion Anodes

Due to their great success for portable electronic devices, lithium-based batteries are becoming increasingly important for powering (hybrid) electric vehicles. Yet, in order to achieve driving ranges comparable to those provided by combustion engines, substantial improvements regarding their energy density are required. Concerning the anode side, most commercial lithium-ion batteries use carbonaceous materials and silicon material seems promising due to its high theoretical specific capacity (up to 3580 mAh.g-1) and its relatively low operational potential. However, the huge volume expansion of silicon alloys upon lithiation (up to 300%) leads to electrical contact loss and pulverization of the material, as well as a continuous consumption of lithium due to the decomposition of the electrolyte on the renewed fresh surface of Si exposed by the cracks of the particles volume expansion. Thus, it leads to poor cycling stability and rapid capacity fading. These effects can be partially counteracted by using nanometer sized Si particles (Si-NPs) that can better sustain the mechanical strains induced by the heterogeneous volume changes. In addition, the particle expansion could be limited by carbon shell coating onto the nanoparticles, which can enhance the electronic conductivity and stabilize the initially formed solid-electrolyte interphase (SEI). In an attempt to combine both approaches, we have synthetized using a laser pyrolysis technique carbon-coated (Si@C-NPs) or non-coated Si nanoparticles (Si-NPs) [1] [2]. As it was shown that both surface state and crystallinity of the Si-NPs govern the electrochemical behavior of the materials [1] [2], both amorphous and crystalline cores were prepared. Transmission electron microscopy (TEM) was performed on native NPs (see figure 2c and d for illustration on crystalline Si NPs) and revealed the core-shell structure. Electrodes with the various NPs samples (see table 1) were processed and tested in coin cell configuration versus lithium foil. The electrochemical behavior was monitored and it was shown that carbon coating leads to improved capacity upon cycling (see figure 1). To characterize the morphology of the various electrodes upon cycling and its impact on performances, we have carried out in-depth microstructural investigation of NPs (shape, size, core-shell structure) using ex situ small-angle neutron scattering (SANS) at the Institut Laue-Langevin (Neutron reactor-Grenoble). The SANS technique is particularly suited to investigate such complex systems, because neutrons are highly sensitive to lithium (by contrast to X-rays) and can provide detailed chemical/microstructural information hardly attainable using TEM on cycled electrodes. Taking advantage of the contrast variation method (i.e. use of specific isotopic mixtures of solvent to selectively match a compound in multicomponent systems) we were able to unravel the effect of carbon coating on the NPS morphology at different (de-)lithiation stages (see figure 2a and b for results on electrodes with crystalline Si NPs). In particular, we show that Si@C NPs exhibit a different mechanism of Li insertion compared to non-coated NPs. This work elucidates the microstructural evolution of NPs used in battery electrodes and therefore helps in designing innovative nanomaterials for high density Si-based anodes.

SAXS studies of RNA: structures, dynamics, and interactions with partners.

Small-angle X-ray scattering, SAXS, is a powerful and easily employed experimental technique that provides solution structures of macromolecules. The size and shape parameters derived from SAXS provide global structural information about these molecules in solution and essentially complement data acquired by other biophysical methods. As applied to protein systems, SAXS is a relatively mature technology: sophisticated tools exist to acquire and analyze data, and to create structural models that include dynamically flexible ensembles. Given the expanding appreciation of RNA's biological roles, there is a need to develop comparable tools to characterize solution structures of RNA, including its interactions with important biological partners. We review the progress toward achieving this goal, focusing on experimental and computational innovations. The use of multiphase modeling, absolute calibration and contrast variation methods, among others, provides new and often unique ways of visualizing this important biological molecule and its essential partners: ions, other RNAs, or proteins. WIREs RNA 2016, 7:512-526. doi: 10.1002/wrna.1349 For further resources related to this article, please visit the WIREs website.