Fabrication of Reversibly Crosslinkable, 3‐Dimensionally Conformal Polymeric Microstructures

Multifaceted porous materials were prepared through careful design of star polymer functionality and properties. Functionalized core crosslinked star (CCS) polymers with a low glass transition temperature (T_g) based on poly(methyl acrylate) were prepared having a multitude of hydroxyl groups at the chain ends. Modification of these chain ends with 9‐anthracene carbonyl chloride introduces the ability to reversibly photocrosslink these systems after the star polymers were self‐assembled by the breath figure technique to create porous, micro‐structured films. The properties of the low T_g CCS polymer allow for the formation of porous films on non‐planar substrates without cracking and photo‐crosslinking allows the creation of stabilized honeycomb films while also permitting a secondary level of patterning on the film, using photo‐lithographic techniques. These multifaceted porous polymer films represent a new generation of well‐defined, 3D microstructures.     

The effect of microstructural representation on simulations of microplastic ratcheting

This paper assesses the sensitivity of cyclic plasticity to microstructure morphology by examining and comparing the microplastic ratcheting behavior of different idealized microstructures (square, hexagonal, tessellated, and digitized from experimental data). This analysis demonstrates the sensitivity of computational accuracy to the various approximations in microstructural representation. The methodology used to perform this study relies on a coupling between microstructural characterization, mechanical testing and numerical simulations to investigate the influence of the microstructure on the purely tensile uniaxial microplastic ratcheting behavior of pure nickel polycrystals. The morphology and deformation behavior of polycrystals were characterized using electron back-scatter diffraction (EBSD), while a finite element model (FEM) of crystal plasticity was used in a computational framework. The predicted cyclic behavior is compared to experimental results both at the macroscopic and microstructural scales. The stress–strain response is less sensitive to the details of the microstructural representation than might be expected with all representations displaying similar macroscopic constitutive response. However, the details of the plastic strain distribution at the microstructural scale and the related estimations of damage mechanics vary substantially from one microstructural representation to another.     

A Pyridine–Pyridine Cross‐Coupling Reaction via Dearomatized Radical Intermediates

A pyridine–pyridine coupling reaction has been developed between pyridyl phosphonium salts and cyanopyridines using B₂pin₂ as an electron‐transfer reagent. Complete regio‐ and cross‐selectivity are observed when forming a range of valuable 2,4′‐bipyridines. Phosphonium salts were found to be the only viable radical precursors in this process, and mechanistic studies indicate that the process does not proceed through a Minisci‐type coupling involving a pyridyl radical. Instead, a radical–radical coupling process between a boryl phosphonium pyridyl radical and a boryl‐stabilized cyanopyridine radical explains the C?C bond‐forming step.     

Formation and hydrolysis of gas‐phase [UO2(R)]+: R═CH3, CH2CH3, CH═CH2, and C6H5

The goals of the present study were (a) to create positively charged organo‐uranyl complexes with general formula [UO₂(R)]⁺ (eg, R═CH₃ and CH₂CH₃) by decarboxylation of [UO₂(O₂C─R)]⁺ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas‐phase H₂O. Collision‐induced dissociation (CID) of both [UO₂(O₂C─CH₃)]⁺ and [UO₂(O₂C─CH₂CH₃)]⁺ causes H⁺ transfer and elimination of a ketene to leave [UO₂(OH)]⁺. However, CID of the alkoxides [UO₂(OCH₂CH₃)]⁺ and [UO₂(OCH₂CH₂CH₃)]⁺ produced [UO₂(CH₃)]⁺ and [UO₂(CH₂CH₃)]⁺, respectively. Isolation of [UO₂(CH₃)]⁺ and [UO₂(CH₂CH₃)]⁺ for reaction with H₂O caused formation of [UO₂(H₂O)]⁺ by elimination of ·CH₃ and ·CH₂CH₃: Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO₂(O₂C─CH═CH₂)]⁺ and [UO₂(O₂C─C₆H₅)]⁺, caused decarboxylation to leave [UO₂(CH═CH₂)]⁺ and [UO₂(C₆H₅)]⁺, respectively. These organometallic species do react with H₂O to produce [UO₂(OH)]⁺, and loss of the respective radicals to leave [UO₂(H₂O)]⁺ was not detected. Density functional theory calculations suggest that formation of [UO₂(OH)]⁺, rather than the hydrated U^VO₂⁺, cation is energetically favored regardless of the precursor ion. However, for the [UO₂(CH₃)]⁺ and [UO₂(CH₂CH₃)]⁺ precursors, the transition state energy for proton transfer to generate [UO₂(OH)]⁺ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO₂(H₂O)]⁺. The situation is reversed for the [UO₂(CH═CH₂)]⁺ and [UO₂(C₆H₅)]⁺ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO₂(H₂O)]⁺ by elimination of CH═CH₂ or C₆H₅ radical.     

Ultrahigh-pressure supercritical fluid extraction and chromatography of Moringa oleifera and Moringa peregrina seed lipids

An ultrahigh-pressure supercritical fluid extraction method was optimized and applied to extract seed oil lipids from two moringa species, namely Moringa oleifera (MO) and Moringa peregrina (MP). A full-factorial design was used to investigate the direct and interaction influence of pressure and temperature in the range of 40 to 80 MPa and 40 to 70 °C, respectively, on the extracted amount of oil from crushed seeds. The results revealed that pressure has a significant positive influence on the extracted amount of oil. The best extraction condition using neat CO₂ was found at 80 MPa and 57 °C, yielding 396 ± 23 and 529 ± 26 mg oil per gram of seeds for MO and MP, respectively. An extraction kinetics study revealed a mainly solubility-controlled extraction of oil, and 28 g of CO₂ was required to extract 400 mg of oil per gram of seeds of MO using the developed method. Addition of ethanol to the sample prior to the extraction increased the proportion of extractable polar lipids as well as the total amount of extracted oil. The developed method increased the extracted amount of oil twofold compared to a reference method based on solvent sonication. The obtained oil consisted mainly of glycerolipids, sterol esters, and phospholipids. Phospholipids, campesterol, and stigmasterol ester concentrations were found to be higher in MO while cholesterol ester was more abundant in MP.

     

On the theory of the temperature dependence of the refractive index of insulators and semi-conductors

A theory of the temperature dependence of the refractive index of insulators and semi-conductors was elaborated on the basis of the Lorentz theory of absorption and dispersion of insulators, which holds in a wide region of the spectrum including the region of intrinsic absorption. On Ge, ZnSand SiO₂ it is shown that the theoretical results agree with the experimental results at least qualitatively, and with Gealso quantitatively to a satisfactory degree.     

Characterisation by X-ray photoelectron spectroscopy of surface layers of poly(vinyl alcohol) on particles formed by dispersion polymerisation of divinylbenzene

Well-defined dispersions of polydivinylbenzene in methanol and a binary liquid mixture of methanol and water were stabilised by samples of partially hydrolysed poly(vinyl alcohol) (PVOH). Mean particle diameters in the range 0.3–1.0 m were produced in dispersion polymerisations. In order to assess surface coverage by PVOH, alcohol groups in the stabilising surface layer were derivatised with trifluoroacetic anhydride. Characterisation of trifluoroacetate groups by X-ray photoelectron spectroscopy permits the quantitative determination of atomic fluorine. The relative concentrations of surface fluorine are in agreement with expectation based on the degrees of hydrolysis of 35 mol% and 72.5 mol% of the PVOH stabilisers. Treatments based on broadscan and high-resolution spectra permit reasonable estimates of the layer thickness for PVOH chains collapsed onto the surface of dry particles. Calculations of the area occupied per PVOH chain are consistent with estimates for other stabilising chains in hydrocarbon media.     

Silicon-based nanowires from silicon wafers catalyzed by cobalt nanoparticles in a hydrogen environment

We present here the synthesis of silicon-based nanowires directly from silicon wafers at high temperatures and in the presence of cobalt nanoparticles and hydrogen gas. All three ingredients were critical to the growth of Si-based nanowires, which were between 5-60 nm in diameter and microm-mm long. Both heavily coiled and straight Si-based nanowires were made. Experimental evidence suggested that the sources of silicon for the nanowires growth were in the gas phase.     

Synthesis of the beta-hydroxydopa-gamma-hydroxy-delta-sulfinylnorvaline component of ustiloxins A and B

The dopa-sulfinylnorvaline component of ustiloxins A and B has been prepared using an Evans salen-Al-catalysed aldol reaction with a 6-mercaptoisovanillin derivative, followed by incorporation of the norvaline group and asymmetric oxidation of the resulting sulfide to give the corresponding sulfoxide.