Age determination of a 75Se gamma radiography source for nuclear forensics

Journal of Radioanalytical and Nuclear Chemistry - Two methods were used to evaluate the age of 75Se sealed source material. Both methods utilized gamma spectroscopy to determine the quantity of...     

Homopolymer and small‐molecule tracer diffusion in a gyroid matrix

Forced Rayleigh scattering was used to measure the tracer diffusion coefficients of the photochromic dye tetrathioindigo (TTI) and a 1,4‐polyisoprene (PI) homopolymer (8000 g/mol) in a poly(styrene‐b‐isoprene) (SI) diblock copolymer matrix that formed a bicontinuous gyroid microstructure. The diblock copolymer contained 63% polystyrene (PS) by volume and had a total molecular weight of 21,300 g/mol. Rheology and small‐angle X‐ray scattering confirmed that the diblock copolymer microphase‐separated into the bicontinuous gyroid over the temperature range 60–230 °C, where the sample disordered. For both the TTI and PI tracers, two distinct modes of transport were observed. The faster mode displayed a temperature dependence consistent with diffusion within a PI matrix, whereas the slower mode had a temperature dependence more similar to diffusion within PS. The fast diffusivities were both over an order of magnitude lower than in a corresponding PI homopolymer matrix. For TTI, this was attributed to the preferential selectivity of the dye for PS and, therefore, an averaging of the mobility between the PS and PI domains. The slow mode was consistent with a small fraction of the TTI dye molecules becoming trapped within the much slower PS domains. For the PI tracer, the reduction in the diffusion coefficient for the fast mode was attributed to a combination of the tortuosity of the struts, the suppression of constraint release within the diblock matrix, and additional friction due to the presence of some styrene segments within the PI domains. The inevitable presence of grain boundaries or defects within the matrix interrupted the percolation of the PI struts, thereby forcing some of the PI tracers to diffuse through PS. Consequently, the slow mode was attributed to the diffusion through these defects, where the PI diffusion was retarded by both the increased segmental friction and the thermodynamic barrier to entering the PS domains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 843–859, 2001     

Recrystallization inhibition assessed by splat cooling and optical recrystallometry

Levels of recrystallization have been measured by two distinct techniques; a splat cooling assay and a new device, an optical recrystallometer, which measures the change in light transmittance through a frozen sample. Both techniques indicate the presence of recrystallization inhibitors in a grass extract and in other samples. The advantages of each method of measuring recrystallization are discussed.     

Comparing a selenium accumulator plant (Brassica juncea) to a nonaccumulator plant (Helianthus annuus) to investigate selenium-containing proteins

Selenoproteins have been identified in a diverse range of organisms, including bacteria and animals. Their occurrence and role in the plant kingdom are, however, less well-understood. This work investigated the water-soluble selenium-containing proteins extracted from a selenium-accumulating plant species (Brassica juncea) and a nonaccumulator species (Helianthus annuus) exposed to varying forms and concentrations of selenium. Firstly, protein extracts were analyzed by size exclusion chromatography coupled to inductively coupled plasma mass spectrometry; specific detection was achieved by monitoring characteristic isotopes. Then, proteolytic digests of the plant extracts were analyzed by reversed phase chromatography coupled to ICP–MS in order to investigate selenoamino acid and selenopeptide content. Selenomethionine was observed to be the primary constituent of the proteins of the nonaccumulator plant, while selenocystine and selenomethionine were found in the same proportion in the accumulator extract. One main selenium-containing species was present at higher levels in the root digests than in the leaf digests; levels were greater in the nonaccumulator than in the accumulator plant.     

The role of problem representation and feature knowledge in algebraic equation-solving

Domain experts have two major advantages over novices with regard to problem solving: experts more accurately encode deep problem features (feature encoding) and demonstrate better conceptual understanding of critical problem features (feature knowledge). In the current study, we explore the relative contributions of encoding and knowledge of problem features (e.g., negative signs, the equals sign, variables) when beginning algebra students solve simple algebraic equations. Thirty-two students completed problems designed to measure feature encoding, feature knowledge and equation solving. Results indicate that though both feature encoding and feature knowledge were correlated with equation-solving success, only feature knowledge independently predicted success. These results have implications for the design of instruction in algebra, and suggest that helping students to develop feature knowledge within a meaningful conceptual context may improve both encoding and problem-solving performance.     

Solution-phase structural characterization of supramolecular assemblies by molecular diffraction

Structures of four molecular squares based on rhenium coordination chemistry have been characterized in the solution phase using pair distribution function (PDF) analyses of wide-angle X-ray scattering measured to better than 1 A spatial resolution. In this report we have focused, in particular, on a comparison of structures for pyrazine- and bipyridine-edged squares measured in solution with structures determined for these molecules in the solid state using X-ray crystallography and models derived from geometry optimization and molecular dynamics simulations using a classical force field. The wide-angle scattering for these assemblies is dominated by pair correlations involving one or more rhenium atoms, with both edge and diagonal Re-Re interactions appearing prominently in PDF plots. The pyrazine square is characterized by a relatively rigid structure in solution, with PDF peak positions and linewidths corresponding closely to those calculated from crystal structure data. For the bipyridine-edged square, the experimental PDF peaks measured along the molecular sides match the positions and linewidths of the PDF peaks calculated from static models. In contrast, PDF peaks measured across the diagonal distances of the molecular square deviate significantly from those calculated from the static crystallographic and energy minimized models. The experimental data are instead indicative of configurational broadening of the diagonal distances. In this respect, molecular dynamics simulations point to the importance of butterfly type motions that modulate the Re-Re diagonal distance. Indeed, the experimental data are reasonably well fit by assuming a bimodal distribution of butterfly conformers differing by approximately 25 degrees in the Re-Re-Re-Re torsion angle. Additionally, the measurements provide evidence for solvent ordering by the supramolecular assemblies detected as regions of solvent association and exclusion.     

Synthesis of Polystyrene‐block‐poly(butyl acrylate) Copolymers Using Nitroxide‐Mediated Living Radical Polymerization in Miniemulsion

Living free‐radical butyl acrylate polymerization in miniemulsion was initiated by polystyrene bearing a nitroxyl end group to yield polystyrene‐block‐poly(butyl acrylate) block copolymers. Polystyrene macroinitiator was obtained using different initiating systems (potassium persulfate or benzoyl peroxide) in the presence of 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) or the more water‐soluble 4‐hydroxy‐2,2,6,6‐tetramethylpiperidin‐N‐oxyl (OH‐TEMPO). The nitroxide water‐solubility has an important influence in determining molecular weight distribution and controlling the growth of the second block.     

Styrene and isoprene friction factors in styrene–isoprene matrices

Monomeric friction factors, Ξ, for polystyrene (PS), polyisoprene (PI), and a polystyrene–polyisoprene (SI) diblock copolymer have been determined as a function of temperature in four poly(styrene-b-isoprene-b-styrene-b-isoprene) tetrablock copolymer matrices. The Rouse model has been used to calculate the friction factors from tracer diffusion coefficients measured by forced Rayleigh scattering. Within the experimental temperature range the tetrablock copolymers are disordered, allowing for measurement of the diffusion coefficient in matrices with average compositions determined by the tetrablock copolymers (23, 42, 60, and 80% styrene by volume). Remarkably, for a given matrix composition the styrene and isoprene friction factors are essentially equivalent. Furthermore, at a constant interval from the system glass transition temperature, T_g, all of the friction factors (obtained from homopolymer, diblock copolymer, and tetrablock copolymer dynamics) agree to within an order of magnitude. This is in marked contrast to results for miscible polymer blends, where the individual components generally have distinct composition dependences and magnitudes at constant T − T_g. The homopolymer friction factors in the tetrablock matrices were systematically slightly higher than those of the diblock, which in turn were slightly higher than those of the homopolymers in their respective melts, when all compared at constant T − T_g. This is attributed to the local spatial distribution of styrene and isoprene segments in the tetrablocks, which presents a nonuniform free energy surface to the tracer molecules. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3079–3086, 1998