Thermoplastic Xylan Derivatives with Propylene Oxide

Polymeric xylan can be reacted with propylene oxide (PO) in aqueous alkali homogeneously. Since xylan is isolated from biomass in aqueous alkaline solution, an in-line modification with PO as part of the isolation protocol, is most practical. Hydroxypropyl xylan (HPX) is a low molecular weight, branched, water-soluble polysaccharide with low intrinsic viscosity and thermoplasticity. Following peracetylation of HPX in formamide solution, water-insoluble acetoxypropyl xylan (APX) is formed that is also thermoplastic but no longer water soluble. The glass transition temperature (T _g) of APX varies in relation to degree of substitution with hydroxypropyl groups (DS_PO), and this is found to decline from 160 to 70°C as DS_PO rises from 0.2 to 2.0. At a temperature above the T _g of HPX a molecular reorganization is noted, and a faint transition due to melting (T _m) is observed at 205°C. HPX thermally degrades with a weight loss maximum at 317°C, or approximately 60°C below that of a corresponding cellulose derivative. HPX forms clear films when solvent cast from aqueous solution. Films are higher in ultimate tensile strength and lower in toughness than corresponding cellulose derivative films. The properties of HPX and APX derivatives qualify this material as a potential biodegradable and thermoplastic additive to melt-processed plastics. Blend characteristics with polystyrene reveal a shear-thinning effect in melt and a plasticization effect in solid state.     

A low noise 0.9 GHz FBAR clock

A low noise 0.9 GHz FBAR clock consisting of an oscillator and divider circuit for single-sided-to-differential conversion for a high-speed interleaved pipeline A/D-converter was designed, realized with an in-house FBAR and a commercial 0.35 μm CMOS process, and tested. The circuit showed very good jitter and phase noise performance. A temperature coefficient of –47 ppm/K was measured.     

Accretion Product Formation from Self‐ and Cross‐Reactions of RO2 Radicals in the Atmosphere

Hydrocarbons are emitted into the Earth's atmosphere in very large quantities by human and biogenic activities. Their atmospheric oxidation processes almost exclusively yield RO₂ radicals as reactive intermediates whose atmospheric fate is not yet fully unraveled. Herein, we show that gas‐phase reactions of two RO₂ radicals produce accretion products composed of the carbon backbone of both reactants. The rates for accretion product formation are very high for RO₂ radicals bearing functional groups, competing with those of the corresponding reactions with NO and HO₂. This pathway, which has not yet been considered in the modelling of atmospheric processes, can be important, or even dominant, for the fate of RO₂ radicals in all areas of the atmosphere. Moreover, the vapor pressure of the formed accretion products can be remarkably low, characterizing them as an effective source for the secondary organic aerosol.     

Atomic layer deposition of crystalline molybdenum oxide thin films and phase control by post-deposition annealing

Molybdenum forms a range of oxides with different stoichiometries and crystal structures, which lead to different properties and performance in diverse applications. Herein, crystalline molybdenum oxide thin films with controlled phase composition are deposited by atomic layer deposition. The MoO₂(thd)₂ and O₃ as precursors enable well-controlled growth of uniform and conformal films at 200–275 °C. The as-deposited films are rough and, in most cases, consist of a mixture of α- and β-MoO₃ as well as an unidentified suboxide MoO_x (2.75 ≤ x ≤ 2.89) phase. The phase composition can be tuned by changing deposition conditions. The film stoichiometry is close to MoO₃ and the films are relatively pure, the main impurity being hydrogen (2–7 at-%), with ≤1 at-% of carbon and nitrogen. Post-deposition annealing is studied in situ by high-temperature X-ray diffraction in air, O₂, N₂, and forming gas (10% H₂/90% N₂) atmospheres. Phase-pure films of MoO₂ and α-MoO₃ are obtained by annealing at 450 °C in forming gas and O₂, respectively. The ability to tailor the phase composition of MoO_x films deposited by scalable atomic layer deposition method represents an important step towards various applications of molybdenum oxides.     

Bounds for binary multiple covering codes

We study codes that are multiple coverings of the Hamming space and discuss lower and upper bounds onK(n, r, ), the minimum cardinality of a binary code of lengthn such that the Hamming spheres of radiusr centered at the codewords cover at least times. We also study the similar problem of multiple coverings containing repeated words. A table of bounds forn16,r4, 4 is given.     

Chemical compounds formed from diacetylene and rare-gas atoms: HKrC4H and HXeC4H

New organic rare-gas compounds, HRgC4H (Rg = Kr or Xe), are identified in matrix-isolation experiments supported by ab initio calculations. These compounds are the largest molecules among the known rare-gas hydrides. They are prepared in low-temperature rare-gas matrixes via UV photolysis of diacetylene and subsequent thermal mobilization of H atoms at approximately 30 and 45 K for Kr and Xe, respectively. The strongest IR absorption bands of the HRgC4H molecules are the H-Rg stretches with the most intense components at 1290 cm(-1) for HKrC4H and at 1532 cm(-1) for HXeC4H, and a number of weaker absorptions (C-H stretching, C-C-C bending, and C-C-H bending modes) are also found in agreement with the theoretical predictions. As probably the most important result, the IR absorption spectra indicate some further stabilization of the HRgC4H molecules as compared with the corresponding HRgC2H species identified recently (Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 4696 and Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 6876). The computational energetic results support this trend. HXeC4H is predicted to be 2.5 eV lower in energy than H + Xe + C4H, which is approximately 1 eV larger than the corresponding value for HXeC2H. We expect that the larger molecules HRgC6H and HRgC8H are even more stable and the HRgC2nH species are good candidates for bulk organic rare-gas material.     

A neutral xenon-containing radical,HXeO

We report an open-shell species containing xenon, HXeO ((2)Sigma), prepared by UV photolysis of H(2)O/Xe or N(2)O/HBr/Xe solid mixtures at 7 K and subsequent thermal mobilization of oxygen atoms at >/=30 K. The H-Xe stretching absorption of HXeO in solid Xe is at 1466.1 cm(-1), and it shifts to 1070.3 cm(-1) upon deuteration. The extensive ab initio calculations indicate that HXeO is intrinsically stable, owing to significant ionic and covalent contributions to its bonding. The formation of HXeO ((2)Sigma) radicals in these experiments suggests extensive stabilization and thermal mobility of singlet ((1)D) oxygen atoms in solid Xe and holds promises for the stability of the HKrO and HArO species.     

Atomic layer deposition chemistry: recent developments and future challenges

New materials, namely high-k (high-permittivity) dielectrics to replace SiO(2), Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combinations, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required.     

NMR studies of interactions between periplasmic chaperones from uropathogenic E. coli and pilicides that interfere with chaperone function and pilus assembly

Adherence of uropathogenic Escherichia coli to host tissue is mediated by pili, which are hair-like protein structures extending from the outer cell membrane of the bacterium. The chaperones FimC and PapD are key components in pilus assembly since they catalyse folding of subunits that are incorporated in type 1 and P pili, respectively, and also transport the subunits across the periplasmic space. Recently, compounds that inhibit pilus biogenesis and interfere with chaperone-subunit interactions have been discovered and termed pilicides. In this paper NMR spectroscopy was used to study the interaction of different pilicides with PapD and FimC in order to gain structural knowledge that would explain the effect that some pilicides have on pilus assembly. First relaxation-edited NMR experiments revealed that the pilicides bound to the PapD chaperone with mM affinity. Then the pilicide-chaperone interaction surface was investigated through chemical shift mapping using 15N-labelled FimC. Principal component analysis performed on the chemical shift perturbation data revealed the presence of three binding sites on the surface of FimC, which interacted with three different classes of pilicides. Analysis of structure-activity relationships suggested that pilicides reduce pilus assembly in E. coli either by binding in the cleft of the chaperone, or by influencing the orientation of the flexible F1-G1 loop, both of which are part of the surface by which the chaperone forms complexes with pilus subunits. It is suggested that binding to either of these sites interferes with folding of the pilus subunits, which occurs during formation of the chaperone-subunit complexes. In addition, pilicides that influence the F1-G1 loop also appear to reduce pilus formation by their ability to dissociate chaperone-subunit complexes.     

Preparation of hindered piperidine and its copolymerization with propylene over a supported high activity ziegler–natta catalyst

A new polymerizable stabilizer 4-(hex-5-enyl)-2,2,6,6-tetramethylpiperidine is prepared. This sterically hindered piperidine was copolymerized with propylene over a fourth generation TiCl₄/MgCl₂ Ziegler–Natta catalyst, using Al(C₂H₅)₃ as cocatalyst and diphenyldimethoxysilane, DMS, as external electron donor. The copolymer exhibited high thermo-oxidative stability even after exhaustive extraction with n-heptane.