Re-directing slow cracks in PMMA

Crack re-direction is a little explored area in the management of structural integrity. We have examined this subject by performing experiments with PMMA (polymethylmethacrylate), working mainly with quasi-static (slow) cracks, whose mean crack speed varies between 0.1 and 0.4?mm?s^?1. We observed that in this regime, secondary thermal and acoustic sources of relatively low power (of the order of 2 and 10?W, respectively) could be used to achieve significant crack re-direction. We argue that this effect is due to an interplay between macro- and micro-scale phenomena. We show that micro-photographs of quasi-static cracks present a fishbone structure and that the side lobes may be micro-cracks inside the shear bands. The respective fracture surfaces appear rippled (hackled). We also show that, in our experiments, the angle of re-direction decreases with the mean crack speed, and above a mean crack speed of 0.4?mm?s^?1—when the fracture surface becomes mirror smooth—no crack re-direction is observed. It is conceivable that the micro-cracks act as notches and alleviate crack re-direction. Therefore, one of our conclusions is that it might be possible to reproduce the effect in an entirely different speed regime, that is use relatively weak secondary sources to re-direct fast cracks that propagate at super-critical speed. This hypothesis is advanced because fast cracks are known to possess a fishbone structure and a hackled fracture surface.     

Tuning HOMO–LUMO levels: trends leading to the design of 9‐fluorenone scaffolds with predictable electronic and optoelectronic properties

Highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) tuning is an important consideration in the development of organic‐based semiconducting materials. A study of the specific effects and overall trends for the HOMO–LUMO tuning of a diverse series of 9‐fluorenones by means of extended conjugation and substituent effects is described. Trends were explored in a range of compounds, beginning with structures having highly electron‐withdrawing substituents and progressing to structures having highly electron‐donating substituents. Compounds with an incremental increase in conjugation were also examined. Electrochemical and optical measurements were used to calculate the HOMO–LUMO levels and HOMO–LUMO bandgap (HLG) for each structure. Results from both methods were compared and correlated with the differences in molecular structure. Increasing the electron‐donating character of the substituents was observed to decrease the HLG and increase the energy levels of the HOMO and the LUMO, whereas an increase in the electron‐withdrawing character produced the opposite results. Increasing conjugation decreased the HLG, increased the HOMO energy level, but decreased the LUMO energy level. Spectroscopic evidence of substituent influence on the carbonyl suggests that substituents directly impact the HLG by influencing the availability of nonbonding electrons within the carbonyl, which impacts the probability of an nπ* transition. The data presented not only elaborate on the HOMO–LUMO tuning of 9‐fluorenone systems but also enable the consideration of 9‐fluorenones as analogous models for HOMO–LUMO tuning in other more complex polyaromatic systems such as bifluorenylidenes. These trends may provide insight into developing materials with specifically tuned HLGs and HOMO–LUMO levels for a variety of applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Measurement of the parity-violating longitudinal single-spin asymmetry for W± boson production in polarized proton-proton collisions at sqrt[s] = 500 GeV

We report the first measurement of the parity-violating single-spin asymmetries for midrapidity decay positrons and electrons from W+ and W- boson production in longitudinally polarized proton-proton collisions at sqrt[s] = 500 GeV by the STAR experiment at RHIC. The measured asymmetries, A(L)(W+) = -0.27 ± 0.10(stat.) ± 0.02(syst.) ± 0.03(norm.) and A(L)(W-) = 0.14 ± 0.19(stat.) ± 0.02(syst.) ± 0.01(norm.), are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized deep-inelastic scattering measurements.     

Effect of long-chain alcohols on SDS partitioning to the oil/water interface of emulsions and on droplet size

The effect of long-chain alcohols (C(n)OH for n=8, 10, 12, 14, 16, 18) on the partitioning of sodium dodecyl sulfate (SDS) to the oil/water interface in oil-in-water macroemulsions was investigated and related to emulsion droplet size and total interfacial area (TIA) contributed by SDS. Alcohols were solubilized in hexadecane and emulsified in SDS solutions. Ultrafiltration was carried out in centrifuge tubes having nanoporous filters with a 30,000 molecular weight cutoff (MWCO), so that emulsion droplets would not pass through, and only SDS that is in the bulk water phase as monomers or micelles (i.e., not at the interface) could pass through. The results showed a chain-length compatibility effect; the maximum amount of SDS partitioned to the interface when dodecanol (C(12)OH) was added to the oil. The results also showed that partitioning of SDS is affected only when dodecanol is added. All other alcohols had no significant influence on SDS partitioning to the oil/water interface. Droplet size measurements revealed a minimum in droplet size for emulsions with added C(12)OH. In order to explain the results, it was proposed that the penetration of alcohol molecules into the interfacial film occur at the interface, resulting in more cohesive molecular packing at the interface, and the minimum droplet size and maximum partitioning of SDS at the oil/water interface for C(12)OH/SDS emulsion system. The TIA provided by the SDS molecules, as determined from our ultrafiltration method, was two orders of magnitude greater than that calculated from the droplet size measured by light scattering. Possible explanations for this disparity are discussed.     

A fluorogenic red fluorescent protein heterodimer

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Highlights? A dimerization-dependent red fluorescent protein (ddRFP) was engineered ? ddRFP fluorogenesis results from modulation of pK_a and quantum yield ? ddRFP-based biosensors exhibit reversible intensiometric responses ? ddRFP-based biosensors can detect Ca²⁺ dynamics and caspase-3 activity in live cells     

The (n, 2n) cross sections of 85Rb, 87Rb and 144Sm

The (n, 2n) cross sections at neutron energies between 14.9 and 17.0 MeV have been measured for ⁸⁵Rb, ⁸⁷Rb and ¹⁴⁴Sm by the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the ²⁷Al(n, α)²⁴Na reaction for monitoring, the measured cross sections were (in mb): ${}^{85}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>84(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Rb}\text{, 1125±141, 1177±148}\text{and}\text{1235±162}\text{at}\text{15.0±0.4}\text{MeV}\text{, 16.2±0.7}\text{MeV and}\text{17.0±0.9}\text{MeV, respectively}\text{;}{}^{85}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>84</mtext><mtext>m</mtext>}}\text{Rb}\text{, 662±83, 688±87}\text{and}\text{765±99}\text{at}\text{15.0±0.4}\text{MeV}\text{, 16.2±0.7}\text{MeV and}\text{17.0±0.9}\text{MeV}\text{,}\text{respectively}\text{;}{}^{87}\text{Rb}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>86(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Rb}\text{, 1336±168}\text{and}\text{1301±162}\text{at}\text{15.0±0.4}\text{MeV and}\text{16.2±0.7}\text{MeV respectively}\text{;}{}^{144}\text{Sm}\text{(}\text{n}\text{, 2}\text{n}\text{)}{}^{\mathrm{<mtext>143(</mtext><mtext>m+g</mtext><mtext>)</mtext>}}\text{Sm}\text{, 1202±130, 1300±141, 1516±179}\text{and}\text{1514±179}\text{at}\text{14.9±0.3}\text{MeV}\text{, 15.5±0.3}\text{MeV}\text{, 16.4±0.5}\text{MeV and}\text{16.7±0.2}\text{MeV, respectively}$. The measured values are compared with the statistical model calculations of Pearlstein.     

alpha-Sarcin catalytic activity is not required for cytotoxicity

Background  

α-Sarcin is a protein toxin produced by Aspergillus giganteus. It belongs to a family of cytotoxic ribonucleases that inactivate the ribosome and inhibit protein synthesis. α-Sarcin cleaves a single phosphodiester bond within the RNA backbone of the large ribosomal subunit, which makes the ribosome unrecognizable to elongation factors and, in turn, blocks protein synthesis. Although it is widely held that the protein synthesis inhibition caused by the toxin leads to cell death, it has not been directly shown that catalytically inactive mutants of α-sarcin are non-toxic when expressed directly within the cytoplasm of cells. This is important since recent studies have cast doubt on whether protein synthesis inhibition is sufficient to initiate apoptosis.     

Guide to enantioselective dirhodium(II)-catalyzed cyclopropanation with aryldiazoacetates

Catalytic enantioselective methods for the generation of cyclopropanes have been of long standing pharmaceutical interest. Chiral dirhodium(II) catalysts prove to be an effective means for the generation of diverse cyclopropane libraries. Rh₂(R-DOSP)₄ is generally the most effective catalyst for asymmetric intermolecular cyclopropanation of methyl aryldiazoacetates with styrene. Rh₂(S-PTAD)₄ provides high levels of enantioinduction with ortho-substituted aryldiazoacetates. The less-established Rh₂(R-BNP)₄ plays a complementary role to Rh₂(R-DOSP)₄ and Rh₂(S-PTAD)₄ in catalyzing highly enantioselective cyclopropanation of 3-methoxy substituted aryldiazoacetates. Substitution on the styrene has only moderate influence on the asymmetric induction of the cyclopropanation.     

TEM analysis of the influence of dose on damage behavior in MeV Au2+-implanted silicon

Extended lattice damage created by implantation of 3.6 MeV Au²⁺ ions has been investigated using transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). Systematic observations of damage for Au²⁺ ions implanted with varying doses into silicon are explained in terms of a model. The origin of two distinct bands of extended defects is explained in terms of annealing of the central region of implant-damage, during the course of the implantation. Two distinct bands of Au precipitates are observed in high-dose implanted samples. This observation is explained as being the result, in part, of segregation of gold in front of a recrystallizing front, and in part, of gettering of dopant-atoms to nodes in a dislocation network. The network arises as a result of dynamic annealing of damaged crystalline silicon.