Investigation into the application of chiral organotin hydrides as enantioselective free radical reducing agents

Investigates the viability of selectively preparing molecules demonstrating the property of left or right "handedness" via free radical reactions. Experimental investigations suggested that a subtle interaction exists between the substituents of an organotin hydride and a target carbon radical during hydrogen atom transfer reactions.

Compression and probing C−H···I hydrogen bonds of iodoform under high pressure by x-ray diffraction and raman scattering

High-pressure methods were applied to investigate the structural stability and hydrogen bonding of polar molecules of iodoform by synchrotron radiation X-ray diffraction and Raman spectra measurements, respectively. Up to a pressure of 40 GPa, no phase transitions were observed. The discontinuous frequency shift of the C−H stretching band is believed to be related to the enhancement of the C−H···I weak hydrogen bonds under high pressures. Ab initio calculations were performed, and the results predict the frequency shift of the C−H stretching vibration as C−H···I interacts via hydrogen bonding. The bulk modulus is 17.3 ± 0.8 GPa, with a pressure derivative of 5.2.

Miscibility, crystallization κinetics and real-time small-Angle x-ray scattering investigation of the semicrystalline morphology in thermosetting polymer blends of epoxy resin and poly(ethylene oxide)

Thermosetting polymer blends of poly(ethylene oxide) (PEO) and bisphenol-A-type epoxy resin (ER) were prepared using 4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) as curing agent. The miscibility and crystallization behavior of MCDEA-cured ER/PEO blends were investigated by differential scanning calorimetry (DSC). The existence of a single composition-dependent glass transition temperature (Tg) indicates that PEO is completely miscible with MCDEA-cured ER in the melt and in the amorphous state over the entire composition range. Fourier-transform infrared (FTIR) investigations indicated hydrogen-bonding interaction between the hydroxyl groups of MCDEA-cured ER and the ether oxygens of PEO in the blends, which is an important driving force for the miscibility of the blends. The average strength of the hydrogen bond in the cured ER/PEO blends is higher than in the pure MCDEA-cured ER. Crystallization kinetics of PEO from the melt is strongly influenced by the blend composition and the crystallization temperature. At high conversion, the time dependence of the relative degree of crystallinity deviated from the Avrami equation. The addition of a non-crystallizable ER component into PEO causes a depression of both the overall crystallization rate and the melting temperature. The surface free energy of folding σe displays a minimum with variation of composition. The spherulitic morphology of PEO in the ER/PEO blends exhibits typical characteristics of miscible crystalline/amorphous blends, and the PEO spherulites in the blends are always completely volume-filling. Real-time small-angle X-ray scattering (SAXS) experiments reveal that the long period L increases drastically with increasing ER content at the same temperatures. The amorphous cured ER component segregates interlamellarly during the crystallization process of PEO because of the low chain mobility of the cured ER. A model describing the semicrystalline morphology of MCDEA-cured ER/PEO blends is proposed based on the SAXS results. The semicrystalline morphology is a stack of crystalline lamellae; the amorphous fraction of PEO, the branched ER chains and imperfect ER network are located between PEO lamellae.

Self-assembled complexes of poly(4-vinylphenol) and poly(ε-caprolactone)-block-poly(2-vinylpyridine) via competitive hydrogen bonding

Nanostructured complexes were prepared from poly(ε-caprolactone)-block-poly(2-vinylpyridine) (PCL-b-P2VP) and poly(4-vinylphenol) (PVPh) in tetrahydrofuran (THF). The phase behavior, specific interactions, and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). In this A-b-B/C type block copolymer/homopolymer system, both blocks of the PCL-b-P2VP block copolymer have favorable intermolecular interaction toward PVPh via hydrogen bonding, but the interaction between P2VP block and PVPh is significantly stronger than that between PCL block and PVPh. It was found that the disparity in competitive intermolecular interactions, specifically PVPh and P2VP block interact strongly whereas PVPh and PCL block interact weakly, leads to the formation of a variety of nanostructures depending on PVPh concentration. Spherical micelles of 30−40 nm in diameter were obtained in the complex with 10 wt % PVPh, followed by wormlike micelles with size in the order of 40−50 nm in the complexes with 30−60 wt % PVPh. At low PVPh concentrations, PCL interacts weakly with PVPh, whereas in the complexes containing more than 20 wt % PVPh, the PCL block began to interact considerably with PVPh, leading to the formation of composition-dependent nanostructures. The complex becomes homogeneous with PVPh content beyond 60 wt %, since a sufficient amount of PVPh is available to form hydrogen bonds with both PCL and P2VP. Finally, a model was proposed to explain the self-assembly and microphase morphology of these complexes based on the experimental results obtained. The competitive hydrogen-bonding interactions cause the self-assembly and formation of different microphase morphologies.

Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(ε-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid)

Nanostructured poly(ε-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP)/poly(acrylic acid) (PAA) interpolyelectrolyte complexes (IPECs) were prepared by casting from THF/ethanol solution. The morphological behaviour of this amphiphilic block copolymer/polyelectrolyte complexes with respect to the composition was investigated in a solvent mixture. The phase behaviour, specific interactions and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelle formation occurred due to the aggregation of hydrogen bonded P2VP block and polyelectrolyte (PAA) from non-interacted PCL blocks. It was observed that the hydrodynamic diameter (D_h) of the micelles in solution decreased with increasing PAA content up to 40 wt%. After 50 wt% PAA content, D_h again increased. The micelle formation in PCL-b-P2VP/PAA IPECs was due to the strong intermolecular hydrogen bonding between PAA homopolymer units and P2VP blocks of the block copolymer. The penetration of PAA homopolymers into the shell of the PCL-b-P2VP block copolymer micelles resulted in the folding of the P2VP chains, which in turn reduced the hydrodynamic size of the micelles. After the saturation of the shell with PAA homopolymers, the size of the micelles increased due to the absorption of added PAA onto the surface of the micelles.

Methionine regulates copper/hydrogen peroxide oxidation products of Aβ

Metal-catalysed oxidation (MCO) may play a causative role in the pathogenesis of Alzheimer's disease (AD). Amyloid peptide (A), the major biomarker of AD, in the presence of copper ions reduces Cu2+ to Cu+ and catalyses the formation of H2O2 that subsequently induces radicals through Fenton chemistry. A is also subject to attack by free radicals, where the presence of Cu2+ in conjunction with H2O2 catalyses oxygenation, primarily at the methionine sulfur atom. This work investigates MCO of A, to gain further insight into the role of oxidative stress in AD. By combining a fluorescence assay with gel electrophoresis to monitor MCO reactions of A (1-28) in the presence and absence of methionine it was determined that methionine can both protect some residues against MCO and promote the oxidation of Tyr(10) specifically. Electrospray ionization mass spectrometric analysis of methionine MCO products indicated the formation of methionine sulfoxide, methionine sulfone and related hydroxylated products. Similar products could be formed from the oxidation of Met(35) of A and may relate to changes in properties of the peptide following MCO.

Qualitative and quantitative analysis of lipid classes in fish oils by thin-layer chromatography with an Iatroscan flame Ionization detector (TLC-FID) and liquid chromatography with an evaporative light scattering detector (LC-ELSD)

Combined effects of hydrogen and air flow rates on the peak response of selected neutral lipid classes (triacylglycerol, diacylglycerol, monoacylglycerol, free fatty acids, and ethyl esters) were studied to optimize and calibrate the Iatroscan Mk-6s Chromarod system for the qualitative and quantitative analysis of lipid classes by thin-layer chromatography (TLC) with flame ionization detection in fish oil during the transesterification process. Air flow rate of 2 L/min, hydrogen flow rate of 150-160 mL/min, and scan rate of 30 s/rod were found to be the optimum conditions. All samples were also analyzed by high performance liquid chromatography (HPLC) with evaporative light scattering detection. Quantitative results obtained by TLC with the flame ionization detection method were comparable to those obtained from HPLC with evaporative light scattering detection.

Competitive hydrogen bonding and self-assembly in Poly(2-vinyl pyridine)-block-Poly(methyl methacrylate)/ Poly(hydroxyether of bisphenol A) blends

Blends of poly(2-vinyl pyridine)-block-poly(methyl methacrylate) (P2VP-b-PMMA) and poly(hydroxyether of bisphenol A) (phenoxy) were prepared by solvent casting from chloroform solution. The specific interactions, phase behavior and nanostructure morphologies of these blends were investigated by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), dynamic light scattering (DLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). In this block copolymer/homopolymer blend system, it is established that competitive hydrogen bonding exists as both blocks of the P2VP-b-PMMA are capable of forming intermolecular hydrogen bonds with phenoxy. It was observed that the interaction between phenoxy and P2VP is stronger than that between phenoxy and PMMA. This imbalance in the intermolecular interactions and the repulsions between the two blocks of the diblock copolymer lead to a variety of phase morphologies. At low phenoxy concentration, spherical micelles are observed. As the concentration increases, PMMA begins to interact with phenoxy, leading to the changes of morphology from spherical to wormlike micelles and finally forms a homogenous system. A model is proposed to describe the self-assembled nanostructures of the P2VP-b-PMMA/phenoxy blends, and the competitive hydrogen bonding is responsible for the morphological changes.

Atom probe microscopy of self-assembled monolayers : preliminary results

We have achieved three-dimensional imaging of decanethiol self-assembled monolayers (SAMs) on metal surfaces by atom probe tomography (APT). The present Letter provides preliminary results on Ni [001] and Au [111], shows the analytical potential of APT analysis of SAMs, and details developments in specimen preparation and in data-treatment methodologies. Importantly, the investigation of the mass spectra from analysis of the SAMs revealed no combination of sulfur and hydrogen at the interface between the metal substrates and the organic materials, potentially providing insight about the bonding of the thiols on the substrate.

Multiple vesicular morphologies in AB/AC diblock copolymer complexes through hydrogen bonding interactions

We report for the first time multiple vesicular morphologies in block copolymer complexes formed in aqueous media via hydrogen bonding interactions. A model AB/AC diblock copolymer system consisting of polystyrene-block- poly(acrylic acid) (PS-b-PAA) and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) was examined using transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. The complexation and morphological transitions were driven by the hydrogen bonding between the complementary binding sites on PAA and PEO blocks of the two diblock copolymers. Upon the addition of PS-b-PEO, a variety of bilayer aggregates were formed in PS-b-PAA/PS-b-PEO complexes including vesicles, multilamellar vesicles (MLVs), thick-walled vesicles (TWVs), interconnected compound vesicles (ICCVs), and irregular aggregates. Among these aggregates, ICCVs were observed as a new morphology. The morphology of aggregates was correlated with respect to the molar ratio of PEO to PAA. At [EO]/[AA] = 0.5, vesicles were observed, while MLVs were obtained at [EO]/[AA] = 1. TWVs and ICCVs were formed at [EO]/[AA] = 2 and 6, respectively. When [EO]/[AA] reached 8 and above, only irregular aggregates appeared. These findings suggest that complexation between two amphiphilic diblock copolymers is a viable approach to prepare polymer vesicles in aqueous media.

Analysing ordered nanostructures in epoxy thermosets using small angle X-ray scattering techniques

Small angle X-ray scattering (SAXS) is essential for the morphological investigation of nanostructured systems as it is a bulk sampling technique and provides information about the overall distribution of the components in the system. In our study we have used SAXS to identify various ordered and disordered morphologies in block copolymer modified epoxy thermosets. We have used a reactive block copolymer and hydrogen bonding block copolymer to modify epoxy resin (ER) to see the effect of various blocks on the morphological changes.

Hydrogen bonding induced vesicular morphologies in diblock copolymer complexes

It is well-known that the self-assembly of block copolymers either in water or in organic solvents can form a wide range of morphologies in nanometer dimensions depending on its chemical nature. In the present study, the complexation and aggregate morphologies in a model AB/AC diblock copolymer system consisting of polystyrene-block-poly(acrylic acid) (PS-b-PAA) and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in water were studied using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and dynamic light scattering (DLS). By varying the relative amounts of the two block copolymers, a variety of bilayer aggregates were formed, including vesicles, multilamellar vesicles (MLVs), thick-walled vesicles (TWVs), interconnected compound vesicles (ICCVs), and irregular aggregates. The hydrophobic PS blocks were segregated as the cores while the hydrogen bonded PEO and PAA blocks formed the coronae of bilayer aggregates. We also investigate how the addition of PS-b-PEO into PS-b-PAA solutions influences the aggregate morphology of the resulting complexes. This work introduces a viable route to multicompartment vesicles in aqueous solutions. The formation of block copolymer vesicles in water is of particular interest because of their potential in various applications.

Self assembled nanostructured complexes and blends via hydrogen bonding interactions

Small angle X-ray scattering (SAXS) is useful to explain the formation of microstructures and the mechanism of microphase separation in self-assembled blends and complexes. In our study, we have used SAXA to examine the ordered and disordered nanostructures as well as morphological transitions in block copolymer/homopolymer blends and complexes [1,2].

Toward design of synergistically active carbon-based catalysts for electrocatalytic hydrogen evolution

Replacement of precious Pt catalyst with cost-effective alternatives would be significantly beneficial for hydrogen production via electrocatalytic hydrogen evolution reaction (HER). All candidates thus far are exclusively metallic catalysts, which suffer inherent corrosion and oxidation susceptibility during acidic proton-exchange membrane electrolysis. Herein, based on theoretical predictions, we designed and synthesized nitrogen (N) and phosphorus (P) dual-doped graphene as a nonmetallic electrocatalyst for sustainable and efficient hydrogen production. The N and P heteroatoms could coactivate the adjacent C atom in the graphene matrix by affecting its valence orbital energy levels to induce a synergistically enhanced reactivity toward HER. As a result, the dual-doped graphene showed higher electrocatalytic HER activity than single-doped ones and comparable performance to some of the traditional metallic catalysts.

Atom probe tomography study of the nanoscale heterostructure around an Al20Mn3Cu2 dispersoid in aluminum alloy 2024.

Atom probe tomography (APT) has been used to investigate the surface and sub-surface microstructures of aluminum alloy 2024 (AA2024) in the T3 condition (solution heat treated, cold worked, and naturally aged to a substantially stable condition). This study revealed surface Cu enrichment on the alloy matrix, local chemical structure around a dispersoid Al20Mn3Cu2 particle including a Cu-rich particle and S-phase particle on its external surface. Moreover, there was a significant level of hydrogen within the dispersoid, indicating that it is a hydrogen sink. These observations of the nanoscale structure around the dispersoid particle have considerable implications for understanding both corrosion and hydrogen embrittlement in high-strength aluminum alloys.