Ulla Vainio

Long-living intermediates during a lamellar to a diamond-cubic lipid phase transition: a small-angle X-ray scattering investigation.

To generate nanostructured vehicles with tunable internal organization, the structural phase behavior of a self-assembled amphiphilic mixture involving poly(ethylene glycol) monooleate (MO-PEG) and glycerol monooleate (MO) is studied in excess aqueous medium by time-resolved small-angle X-ray scattering (SAXS) in the temperature range from 1 to 68 degrees C. The SAXS data indicate miscibility of the two components in lamellar and nonlamellar soft-matter nanostructures. The functionalization of the MO assemblies by a MO-PEG amphiphile, which has a flexible large hydrophilic moiety, appears to hinder the epitaxial growth of a double diamond (D) cubic lattice from the lamellar (L) bilayer structure during the thermal phase transition. The incorporated MO-PEG additive is found to facilitate the formation of structural intermediates. They exhibit greater characteristic spacings and large diffusive scattering in broad temperature and time intervals. Their features are compared with those of swollen long-living intermediates in MO/octylglucoside assemblies. A conclusion can be drawn that long-living intermediate states can be equilibrium stabilized in two- or multicomponent amphiphilic systems. Their role as cubic phase precursors is to smooth the structural distortions arising from curvature mismatch between flat and curved regions. The considered MO-PEG functionalized assemblies may be useful for preparation of sterically stabilized liquid-crystalline nanovehicles for confinement of therapeutic biomolecules.

Synthesis and characterization of copper sulfide nanocrystallites with low sintering temperatures

To study nano-inks with relatively low sintering temperatures for fabrication of functional electronics on paper by inkjet printing technology, we have successfully prepared copper sulfide nanocrystallites protected by self-assembled monolayers. Systematic characterization was performed on as-prepared nanoparticles by FTIR, NMR, thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and wide-angle X-ray scattering (WAXS) with heating. The copper sulfide nanocrystallites with crystal sizes <1.5 nm show a hexagonal Cu2S phase at low temperatures but undergo significant consolidation/crystallization from 100 to 240 °C, accompanying a transformation from the hexagonal Cu2S phase to the cubic Cu1.8S phase when heated up to ca. 150 °C. The protective ligand burnout during heating is closely associated with the nanocrystallite consolidation. Further, the copper sulfide nanoparticles were deposited on paper and sintered at 240 °C in air. The sintered particles are composed of large crystals of cubic Cu1.8S with no serious degradation due to oxidation. The resistivity of the sintered particles was of the order of 1 × 10−5 (Ω m).

Block Copolymer Hollow Fiber Membranes with Catalytic Activity and pH-Response

We fabricated block copolymer hollow fiber membranes with self-assembled, shell-side, uniform pore structures. The fibers in these membranes combined pores able to respond to pH and acting as chemical gates that opened above pH 4, and catalytic activity, achieved by the incorporation of gold nanoparticles. We used a dry/wet spinning process to produce the asymmetric hollow fibers and determined the conditions under which the hollow fibers were optimized to create the desired pore morphology and the necessary mechanical stability. To induce ordered micelle assembly in the doped solution, we identified an ideal solvent mixture as confirmed by small-angle X-ray scattering. We then reduced p-nitrophenol with a gold-loaded fiber to confirm the catalytic performance of the membranes.

Small-angle X-ray scattering and rheological characterization of aqueous lignosulfonate solutions.

Lignosulfonate is a colloidal polyelectrolyte widely used as a dispersant in various industrial applications and produced during chemical pulping of wood chips. Here we present a systematic small-angle X-ray scattering (SAXS) and rheological study of fractionated lignosulfonate (mass weighted molar mass M w 18 000 g/mol) dissolved in water and 0.2 M NaCl. The concentration range varied from semidilute to concentrated regime. SAXS intensity of all solutions followed the Porod law at all concentrations, which is a clear indication of a compact shape of the lignosulfonate particle. In water, below 10 mass % lignosulfonate, the average interparticle distance obtained from SAXS patterns relates to concentration via a power law with exponent -0.28. Deviation of the power law exponent from ideal -0.33 and a linear decrease in volume fraction normalized Porod constant as a function of concentration are taken as indications of self-association of lignosulfonate. In saline solutions at high lignosulfonate mass fractions the average distance between lignosulfonate particles was longer and the average particle size was larger than those in aqueous solutions. The intrinsic viscosity in saline solution also was larger than that in aqueous solution. Lignosulfonate solutions showed Newtonian viscosity, except at very high concentrations. The variation of the relative zero-shear viscosity eta(0),r) with concentration was interpreted with the Krieger-Dougherty equation. An oblate spheroid shape with an axial ratio of 3.5 describes the average shape of the lignosulfonate particles in saline solutions based on SAXS intensities, the size distribution obtained using gel permeation chromatography, and rheological characterization. The largest dimension of the particles was about 8 nm. SAXS and rheology studies as a function of temperature reveal indications of temperature-dependent self-association.

Synthesis and thermal stability of zirconia and yttria-stabilized zirconia microspheres

HYPOTHESIS Zirconia microparticles produced by sol-gel synthesis have great potential for photonic applications. To this end, identifying synthetic methods that yield reproducible control over size uniformity is important. Phase transformations during thermal cycling can disintegrate the particles. Therefore, understanding the parameters driving these transformations is essential for enabling high-temperature applications. Particle morphology is expected to influence particle processability and stability. Yttria-doping should improve the thermal stability of the particles, as it does in bulk zirconia. EXPERIMENTS Zirconia and YSZ particles were synthesized by improved sol-gel approaches using fatty acid stabilizers. The particles were heated to 1500 °C, and structural and morphological changes were monitored by SEM, ex situ XRD and high-energy in situ XRD. FINDINGS Zirconia particles (0.4-4.3 μm in diameter, 5-10% standard deviation) synthesized according to the modified sol-gel approaches yielded significantly improved monodispersities. As-synthesized amorphous particles transformed to the tetragonal phase at ∼450 °C with a volume decrease of up to ∼75% and then to monoclinic after heating from ∼650 to 850 °C. Submicron particles disintegrated at ∼850 °C and microparticles at ∼1200 °C due to grain growth. In situ XRD revealed that the transition from the amorphous to tetragonal phase was accompanied by relief in microstrain and the transition from tetragonal to monoclinic was correlated with the tetragonal grain size. Early crystallization and smaller initial grain sizes, which depend on the precursors used for particle synthesis, coincided with higher stability. Yttria-doping reduced grain growth, stabilized the tetragonal phase, and significantly improved the thermal stability of the particles.

Large-scale parallel alignment of platelet-shaped particles through gravitational sedimentation.

Parallel and concentric alignment of microscopic building blocks into several orders of magnitude larger structures is commonly observed in nature. However, if similarly aligned structures are artificially produced their thickness is generally limited to just about one or two orders of magnitude more than the dimensions of the smallest element. We show that sedimentation provides a promising approach to manufacture solid materials consisting of well-aligned platelet-shaped particles while being more than 30 000 times thicker than the individual particle. Such sediments contain up to 28 vol% of particles without any further treatment and can be densified to 67 vol% particle fraction by subsequent unidirectional pressing. The degree of orientation of the platelet-shaped particles within the sediments was tracked by high-energy X-ray diffraction measurements. The Hermans orientation parameter, a statistical measure of the quality of alignment, was determined to be 0.63 ± 0.03 already for as-sedimented samples while the standard deviation of the orientation distribution of particles, another measure of average misalignment, was found to be (21.5 ± 1.4)°. After pressing, these values further improved to (0.81 ± 0.01) and (14.6 ± 0.4)°, respectively. Such quality of alignment competes with, if not even exceeds, values reported in the literature.

Impact of ion valency on the assembly of vimentin studied by quantitative small angle X-ray scattering

The assembly kinetics of intermediate filament (IF) proteins from tetrameric complexes to single filaments and networks depends on the protein concentration, temperature and the ionic composition of their environment. We systematically investigate how changes in the concentration of monovalent potassium and divalent magnesium ions affect the internal organization of the resulting filaments. Small angle X-ray scattering (SAXS) is very sensitive to changes in the filament cross-section such as diameter or compactness. Our measurements reveal that filaments formed in the presence of magnesium chloride differ distinctly from filaments formed in the presence of potassium chloride. The principle multi-step assembly mechanism from tetramers via unit-length filaments (ULF) to elongated filaments is not changed by the valency of ions. However, the observed differences indicate that the magnesium ions free the head domains of tetramers from unproductive interactions to allow assembly but at the same time mediate strong inter-tetrameric interactions that impede longitudinal annealing of unit-length filaments considerably, thus slowing down filament growth.

Structural analysis of calcium reactive hydride composite for solid state hydrogen storage

Owing to a theoretical hydrogen storage capacity of 10.5 wt% H2, Ca(BH4)2+MgH2, the so-called calcium reactive hydride composite (Ca-RHC), has a great potential as a hydrogen storage material. However, its dehydrogenation temperature (∼623 K) is too high for any mobile applications. By addition of 10 mol% of NbF5 into Ca(BH4)2+MgH2, a decrease of the dehydrogenation onset temperature by ∼120 K is observed. In order to understand the reasons behind this desorption temperature decrement two sets of samples [Ca(BH4)2+MgH2 and Ca(BH4)2+MgH2+0.1NbF5] in different hydrogenation states, were prepared. The structural investigation of the above mentioned sets of samples by means of volumetric measurements, anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption spectroscopy (XAS) is reported here. The XAS results show that after the milling procedure NbB2 is formed and remains stable upon further de/rehydrogenation cycling. The results of Nb ASAXS point to nanometric spherical NbB2 particles distributed in the hydride matrix, with a mean diameter of ∼10 nm. Results from Ca ASAXS indicate Ca-containing nanostructures in the Ca-RHC+0.1NbF5 samples to be ∼50% finer compared to those without additive. Thus, a higher reaction surface area and shorter diffusion paths for the constituents are concluded to be important contributions to the catalytic effect of an NbF5 additive on the hydrogen sorption kinetics of the Ca(BH4)2+MgH2 composite system.

Structure of nickel nanoparticles in a microcrystalline cellulose matrix studied using anomalous small-angle X-ray scattering

Nickel nanoparticles were synthesized by adding aqueous nickel salt into a microcrystalline cellulose matrix. The NiII ions were reduced with either sodium borohydride, NaBH_4, or potassium hypophosphite, KH_2PO_2, in water or aqueous NH_3 medium. The mass fraction of Ni in the samples was between 3.7 and 8.9%. X-ray absorption spectra at the Ni K-edge showed that Ni was partially oxidized only in a sample reduced with NaBH_4. Wide-angle X-ray scattering results showed that nickel was in nanocrystalline or amorphous form in the samples. Upon heating fcc Ni, hcp Ni, NiO, Ni_3P and other Ni–P phases formed depending on the reduction parameters. Using anomalous small-angle X-ray scattering the nanometre-scale particle size distributions of the Ni particles were determined. A large fraction of particles less than 15 nm in size were observed in the samples reduced in aqueous ammonium compared with the samples reduced in water. Particles reduced in aqueous ammonium had a large ferromagnetic component.

Silane functionalized ethylene/diene copolymer modifiers in composites of heterophasic polypropylene and microsilica

Ethylene/1,7-octadiene copolymer was polymerised with metallocene catalyst and hydrosilylated to form silane functionalised polyethylenes (PE-co-SiX, X=Cl, OEt, Ph). The functionalised species were tested as modifiers in composites of rubber toughened polypropylene (heterophasic PP, hPP) and microsilica filler (μSi). A metallocene-based functionalised PE (PE-co-SiF) produced earlier in our laboratory and three commercial grades of functionalised polyolefins (one PE- and two PP-based) were used as reference modifiers. Major differences were seen in the toughness of the composites both above and below the glass transition temperature (Tg) of PP. In addition to increasing the stiffness, the microsilica filler enhanced the toughness of the heterophasic polypropylene by over 200% at ambient temperature. Below the Tg of PP (at −20 °C), the influence of μSi was the opposite and the impact strength of the hPP/μSi composite was below that of unfilled hPP. With the addition of just 2 wt% of functionalised polyethylene, the poor cold toughness of hPP/μSi composite was improved by nearly 100%. With the same addition, the toughness of the composites at ambient temperature was improved by 50 to 100% compared with the unfilled hPP. This behaviour was explained by significant changes in the fracture mechanism. Addition of functionalised PE increased the concentration of microsilica in the rubbery phase, allowing the crack to enter that phase. The rubbery phase was also able to absorb a large amount of impact energy below the glass transition temperature of PP.