Narrow (n,m)-distribution of single-walled carbon nanotubes grown using a solid supported catalyst

Unusually structure-selective growth of single-walled carbon nanotubes (SWNTs) has been attained using a CVD method with a solid supported catalyst. In this method, CO feedstock disproportionates on silica-supported catalytic nanoclusters of Co that are formed in situ from mixed salts of Co and Mo. The nanotube products are analyzed by spectrofluorimetry to reveal distributions resolved at the level of individual (n,m) structures. Two structures, (6,5) and (7,5), together dominate the semiconducting nanotube distribution and comprise more than one-half of that population. The average diameter of produced SWNTs is only 0.81 nm, and a strong propensity is found favoring chiral angles near the armchair limit.     

Determination of naturally radioactive elements in construction materials by means of gammaspectroscopy,track registration techniques and neutron activation analysis

Five types of Cuban concretes and their main components (mineral aggregates and cements) were analyzed for uranium, thorium, radium, potassium and radon (²²⁰Rn,²²²Rn) using -spectroscopy, track registration methods and neutron activation analysis. Comparative evaluation of different concretes, aggregates and two types of cements concerning their natural radioactivity is presented.     

Development and implementation of the k0-standardization and other parametric methods of INAA in Cuba. Part I

This paper explains chronologically the implementation of thek ₀-standardization and other parametric methods in Cuba. Fundamental and applied results in the determination of the most important parameters are obtained, using the IBR-2 at Dubna and Triga Mark III at Mexico reactor facilities. Some modifications in the Stoughton-Halpering convention, and its comparison with the modified Westcott and Høgdahl method are presented. The application of the described methods at the CS-ISCTN low flux facility, using the big samples techniques, is presented.     

HAV-1: A multipurpose multimonitor for reactor neutron flux characterization

A simple non-solid multimonitor HAV-1 for the systematic evaluation of reactor neutron flux parameters fork ₀ neutron activation analysis is presented. Solutions of Au, Zr, Co, Zn, Sn, U and Th (deposited in filter paper) are used to study the parameters andf. Dissolved Lu is used to neutron temperature (T _n) determination, according to the Wescott's formalism. A multipurpose multimonitor HAV-1 preparation, certification and evaluation is presented.     

Characterization and degradation behavior of poly(butylene adipate‐co‐terephthalate)s

Random copolyesters based on 1,4‐butanediol and different ratios between adipic and terephthalic units were synthesized from thermal polycondensation of the appropriate mixture of monomers or by melt transesterification of the mixture of homopolymers. ¹H NMR spectroscopy makes feasible the study of the average block lengths of polymers once synthesized and after degradation in different media. Calorimetric data are reported, including those referred to the study of isothermal and nonisothermal crystallizations. Degradability of samples was evaluated by different methods including NMR and thermal analysis, evaluation of molecular weight by gel permeation chromatography or from intrinsic viscosity measures, scanning electron micrographs, and changes in mechanical properties. Distilled water at 70 °C acidic conditions provided by a pH 2.3 aqueous medium and enzymatic media containing lipases from Pseudomonas cepacia or Candida cylindracea were considered in this study. The degradability of the studied copolyesters strongly depends on the terephthalate content and the degradation media. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4141–4157, 2002     

In situ control of surface molecular order in liquid crystals using a localised polymer network and its application to electro-optical devices

A liquid crystal (LC) alignment technique has been developed that allows local control of the polar pretilt angle over the range of 0–90°. This was achieved through the formation of a polymer network localised in the vicinity of the LC cell substrates. The network was formed as a result of in situ UV-induced polymerisation of a photo-reactive monomer added at concentrations of 0.5–1%. Localisation of the polymer network at the LC–substrate boundary was achieved by the application of a high voltage before polymerisation. The resultant pretilt angle was determined by the voltage applied during the polymerisation and/or the duration of the voltage application before the polymerisation step. The desired pretilt angle could be set over a small area of the sample, which allows the fabrication of LC devices with spatially variable optical retardation. Using this method we fabricated a converging lens, a bi-prism, and a phase diffraction grating with resolution greater than 50 lines mm^?1.     

Studying the Chemistry of Cationized Triacylglycerols Using Electrospray Ionization Mass Spectrometry and Density Functional Theory Computations

Analysis of triacylglycerols (TAGs), found as complex mixtures in living organisms, is typically accomplished using liquid chromatography, often coupled to mass spectrometry. TAGs, weak bases not protonated using electrospray ionization, are usually ionized by adduct formation with a cation, including those present in the solvent (e.g., Na⁺). There are relatively few reports on the binding of TAGs with cations or on the mechanisms by which cationized TAGs fragment. This work examines binding efficiencies, determined by mass spectrometry and computations, for the complexation of TAGs to a range of cations (Na⁺, Li⁺, K⁺, Ag⁺, NH₄ ⁺). While most cations bind to oxygen, Ag⁺ binding to unsaturation in the acid side chains is significant. The importance of dimer formation, [2TAG + M]⁺ was demonstrated using several different types of mass spectrometers. From breakdown curves, it became apparent that two or three acid side chains must be attached to glycerol for strong cationization. Possible mechanisms for fragmentation of lithiated TAGs were modeled by computations on tripropionylglycerol. Viable pathways were found for losses of neutral acids and lithium salts of acids from different positions on the glycerol moiety. Novel lactone structures were proposed for the loss of a neutral acid from one position of the glycerol moiety. These were studied further using triple-stage mass spectrometry (MS³). These lactones can account for all the major product ions in the MS³ spectra in both this work and the literature, which should allow for new insights into the challenging analytical methods needed for naturally occurring TAGs.