Regenerated Bombyx mori silk fibroin (RSF) is a widely recognized protein for biomedical applications; however, its hierarchical gel structure is poorly understood. In this paper, the hierarchical structure of photocrosslinked RSF and RSF-based hybrid hydrogel systems: (i) RSF/Rec1-resilin and (ii) RSF/poly(N-vinylcaprolactam (PVCL) is reported for the first time using small-angle scattering (SAS) techniques. The structure of RSF in dilute to concentrated solution to fabricated hydrogels were characterized using small angle X-ray scattering (SAXS), small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) techniques. The RSF hydrogel exhibited three distinctive structural characteristics: (i) a Porod region in the length scale of 2 to 3nm due to hydrophobic domains (containing β-sheets) which exhibits sharp interfaces with the amorphous matrix of the hydrogel and the solvent, (ii) a Guinier region in the length scale of 4 to 20nm due to hydrophilic domains (containing turns and random coil), and (iii) a Porod-like region in the length scale of few micrometers due to water pores/channels exhibiting fractal-like characteristics. Addition of Rec1-resilin or PVCL to RSF and subsequent crosslinking systematically increased the nanoscale size of hydrophobic and hydrophilic domains, whereas decreased the homogeneity of pore size distribution in the microscale. The presented results have implications on the fundamental understanding of the structure-property relationship of RSF-based hydrogels.

We show in this Letter that the magnetic coupling of a ͓26 Å Fe͞15 Å Nb͔ 3 18 multilayer is changed in a continuous and reversible way by introducing hydrogen into the sample. The magnetic structure and its change during hydrogen charging (and the following decharging) is measured in situ by neutron reflectivity. The alteration of the magnetic coupling upon hydrogenation is confirmed by SQUID magnetization measurements. We attribute the change of the coupling to a change of the effective Fermi wave vector in Nb due to hydrogen uptake. [S0031-9007(97)02308-9] PACS numbers: 75.70.Cn, 68.55.Ln, 75.50.Bb In the ten years since the discovery of antiferromagnetic (AFM) exchange coupling in magnetic thin film structures [1] a large amount of experimental and theoretical work has appeared on this subject [2]. This research field receives its impetus not only from the challenging possibility to study magnetic properties on a length scale of atomic layers but also from the fact that technical applications of these AFM coupled layered structures are in sight. The technical applications make use of the "giant" magnetoresistance (GMR) effect [3] to construct miniaturized magnetic field sensors.After the experimental discovery that the exchange coupling is oscillating in sign, i.e. that with increasing spacer layer thickness alternately ferro/antiferromagnetic coupling is observed, several theories were developed to describe the phenomenon. In Ruderman-Kittel-Kasuya-Yoshida-like models [4] as well as in the quantum interference model [5] the coupling energy J oscillates in crude approximation aswhere k F and t S are the Fermi wave vector and the thickness of the nonmagnetic layer, respectively. Up to now, the dependence of the exchange energy on the spacer layer thickness has been extensively studied in many systems. But, surprisingly, only very few papers exist in the literature dealing with a manipulation of the Fermi wave vector [6]. An example for the change of the Fermi vector is the Fe͞V x Cr 12x system in which V and Cr form a solid solution over the entire composition range. It has been shown experimentally and theoretically that by changing the electron concentration of the spacer layer by alloying, the period as well as the amplitude of the coupling energy can be altered.In this Letter, we present evidence for a continuous and reversible change of the magnetic coupling in one and the same multilayer sample. In the present case the sample is charged with hydrogen from the gas phase at 473 K. At this temperature hydrogen enters or leaves the sample according to the concentration in equilibrium with the ap-plied external hydrogen atmosphere [7]. As argued above, one expects a manipulation of the exchange coupling by hydrogen charging [8] via the change of the electronic structure in the spacer layer. Fe͞Nb multilayers were chosen for these experiments since oscillating FM͞AFM coupling had been positively identified in recent experiments [9,10] and Nb, as the spacer layer, has a high solubility for hydrogen [8].The sa...

The electrocatalyst layer (ECL) of the proton-exchange membrane fuel cell (PEMFC) is commonly fabricated from colloidal catalyst ink containing carbon-supported catalyst nanoparticles (NPs), ionomer stabilizer, and dispersion medium (DM). The structure, stability, and aggregate size distribution of fuel cell catalyst ink are critically dependent on the quality of DM. However, understanding of the influence of the quality of DM on the hierarchical structure of the ECL is lacking. This work presents a systematic investigation of the effects of reducing alcohol content in isopropyl alcohol/water (IPA/H2O) binary mixtures as DM on the structural evolution of water-rich (green) catalyst ink using contrast-variation small-angle and ultrasmall-angle neutron scattering techniques. Both qualitative and quantitative information are extracted from the data to obtain information about the size, structure, and organization of the catalyst ink using different model functions fit to the experimental data. The catalyst ink prepared using 70% IPA (commonly employed in industry and extensively reported in the literature) is shown to consist of randomly distributed globular carbon aggregates (mean radius of gyration of ∼178.9 nm) stabilized by an ionomer mass fractal shell (thickness of ∼13.0 nm), which is dispersed in the matrix of rodlike (∼1.3 nm radius and ∼35.0 nm length) negatively surface-charged ionomer NPs. These well characterized baseline data are then compared and contrasted with DM formulations of lower IPA content. A sequential reduction in IPA content of DM shows a progressive increase in the ionomer NP radius and electrostatic repulsion, concomitantly with the decrease in the carbon aggregate size and ionomer shell thickness of the catalyst ink. Therefore, the changes in the interfacial structure via adjustments of the DM composition can be used as a controlling parameter to tailor the hierarchical structure of the colloidal fuel cell catalyst ink and to further optimize the performance of the ECL.