Degradation of polymer mixtures—Part 10: The thermal degradation of blends of polyacrylonitrile and poly(methyl methacrylate)

Although the thermal degradation of polyacrylonitrile (PAN) is unchanged by blending with poly(methyl methacrylate) (PMMA), the degradation of PMMA is profoundly altered in the presence of PAN. The low temperature phase of the reaction is hindered although monomer is still the predominating product. At higher temperatures the monomer production gives way to the appearance of methanol, carbon dioxide, carbon monoxide and chain fragments which incorporate a variety of carbonyl structures.These results are interpreted in terms of initial reaction of methyl methacrylate units with the ammonia formed by degradation of the PAN. The amide-ester copolymer thus formed undergoes a complex degradation process at higher temperatures which includes inter unit cyclisations, chain fragmentation and the formation of methanol and oxides of carbon. Mechanisms are proposed and discussed.     

Reduction Mechanism of Tellurium Species from Copper Electrowinning Solutions

The presence of tellurium in copper–nickel sulfide ores results in downstream contamination of leach solutions and electrolytes. Tellurium must be removed from leach solutions before electrowinning to prevent contamination of copper cathodes. Cuprous ion is industrially used as a reductant to remove tellurium ions. However, the reduction kinetics of tellurium ions with cuprous is not well known. In this study, the slow Te^VI reduction step with cuprous was identified and a plausible Te^VI reduction mechanism was suggested. The rate‐determining step was investigated by studying the kinetics of the coupled reaction of Te^VI and Te^IV with cuprous. The rate law suggested for the reduction of Te^IV with cuprous and the coupled kinetics equation for consecutive reduction reactions of Te^IV and Te^VI were derived. It was shown that the rate constant of the coupled reaction is very close to the rate constant calculated based on the assumption that Te^VI reduction is a rate‐determining step.     

An International Laboratory Comparison Study of Volumetric and Gravimetric Hydrogen Adsorption Measurements

In order to determine a material's hydrogen storage potential, capacity measurements must be robust, reproducible, and accurate. Commonly, research reports focus on the gravimetric capacity, and often times the volumetric capacity is not reported. Determining volumetric capacities is not as straight-forward, especially for amorphous materials. This is the first study to compare measurement reproducibility across laboratories for excess and total volumetric hydrogen sorption capacities based on the packing volume. The use of consistent measurement protocols, common analysis, and figure of merits for reporting data in this study, enable the comparison of the results for two different materials. Importantly, the results show good agreement for excess gravimetric capacities amongst the laboratories. Irreproducibility for excess and total volumetric capacities is attributed to real differences in the measured packing volume of the material.     

A carbazole–oxadiazole diad molecule for single-emitting-component white organic light-emitting devices (WOLEDs)

The synthesis is reported of a new molecule consisting of N-phenylcarbazole linked through a butadiyne bridge to 2,5-diphenyl-1,3,4-oxadiazole. The compound shows blue photoluminescence with positive solvatochromism: e.g., λ_max 401 nm in cyclohexane, 461 nm in chloroform solution (quantum yield Φ_f 0.55) and 428 nm in thin film, with a low intensity band extending to ca. 700 nm in films. OLEDs with a device configuration of [ITO/PEDOT:PSS (50 nm)/molecule:PVK:OXD-7 (80 nm)/Bphen (20 nm)/LiF (0.8 nm)/Al (120 nm)] were fabricated. Pure white-light emission with Commission Internationale De L'Eclairage (CIE) coordinates of (0.313, 0.330) at 13 V and good colour stability with a very small offset of coordinates (0.034, 0.033) under biases of 9–15 V has been achieved.     

Solid‐state 11B and 13C NMR,IR, and X‐ray crystallographic characterization of selected arylboronic acids and their catechol cyclic esters

Nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (boroxines) are investigated using ¹¹B solid‐state NMR spectroscopy at three different magnetic field strengths (9.4, 11.7, and 21.1 T). Through the analysis of spectra of static and magic‐angle spinning samples, the ¹¹B electric field gradient and chemical shift tensors are determined. The effects of relaxation anisotropy and nutation field strength on the ¹¹B NMR line shapes are investigated. Infrared spectroscopy was also used to help identify peaks in the NMR spectra as being due to the anhydride form in some of the arylboronic acid samples. Seven new X‐ray crystallographic structures are reported. Calculations of the ¹¹B NMR parameters are performed using cluster model and periodic gauge‐including projector‐augmented wave (GIPAW) density functional theory (DFT) approaches, and the results are compared with the experimental values. Carbon‐13 solid‐state NMR experiments and spectral simulations are applied to determine the chemical shifts of the ipso carbons of the samples. One bond indirect ¹³C‐¹¹B spin‐spin (J) coupling constants are also measured experimentally and compared with calculated values. The ¹¹B/¹⁰B isotope effect on the ¹³C chemical shift of the ipso carbons of arylboronic acids and their catechol esters, as well as residual dipolar coupling, is discussed. Overall, this combined X‐ray, NMR, IR, and computational study provides valuable new insights into the relationship between NMR parameters and the structure of boronic acids and esters. Copyright © 2012 John Wiley & Sons, Ltd.     

Solid-state 23Na NMR study of sodium lariat ether receptors exhibiting cation-pi interactions

Noncovalent cation-pi interactions are important in a variety of supramolecular and biochemical systems. We present a 23Na solid-state nuclear magnetic resonance (SSNMR) study of two sodium lariat ether complexes, 1 and 2, in which a sodium cation interacts with an indolyl group that models the side chain of tryptophan. Sodium-23 SSNMR spectra of magic-angle spinning (MAS) and stationary powdered samples have been acquired at three magnetic field strengths (9.4, 11.75, 21.1 T) and analyzed to provide key information on the sodium electric field gradient and chemical shift (CS) tensors which are representative of the cation-pi binding environment. Triple-quantum MAS NMR spectra acquired at 21.1 T clearly reveal two crystallographically distinct sites in both 1 and 2. The quadrupolar coupling constants, CQ(23Na), range from 2.92 +/- 0.05 MHz for site A of 1 to 3.33 +/- 0.05 MHz for site B of 2; these values are somewhat larger than those reported previously by Wong et al. (Wong, A.; Whitehead, R. D.; Gan, Z.; Wu, G. J. Phys. Chem. A 2004, 108, 10551) for NaBPh4, but very similar to the values obtained for sodium metallocenes by Willans and Schurko (Willans, M. J.; Schurko, R. W. J. Phys. Chem. B 2003, 107, 5144). We conclude from the 21.1 T data that the spans of the sodium CS tensors are less than 20 ppm for 1 and 2 and that the largest components of the EFG and CS tensors are non-coincident. Quantum chemical calculations of the NMR parameters substantiate the experimental findings and provide additional insight into the dependence of CQ(23Na) on the proximity of the indole ring to Na+. Taken together, this work has provided novel information on the NMR interaction tensors characteristic of a sodium cation interacting with a biologically important arene.     

Convergent synthesis of 10 nm aryleneethynylene molecular wires by an iterative regioselective deprotection/Sonogashira coupling protocol

[structure: see text] The synthesis of a new series of rigid-rod aryleneethynylene derivatives of up to ca. 10 nm molecular length (compounds 16 and 17) is reported using iterative Pd-mediated Sonogashira coupling methodology combined with regioselective removal of the different protecting groups (namely, trimethylsilyl and 2-hydroxyprop-2-yl groups) from the terminal alkyne units. Additionally, the TMS-acetylene unit has been cleanly deprotected to afford a terminal alkyne in the presence of a cyanoethylsulfanyl group. Some of these molecular wires are functionalized with terminal protected thiophenol units for attachment to metal surfaces (compounds 16 and 17). Internal electron-acceptor units have been incorporated into their structures, namely, 9-[di(4-pyridyl)methylene]fluorene (compound 17) or fluorenone (compounds 19-22). Optical absorption and photoluminescence spectra reveal a red shift in the value of lambda(max) with increasing molecular length, which approaches saturation at an effective conjugation length of ca. 15-20 pi-units in the molecules, where each phenyl ring or a triple bond is counted as one pi-unit.