Investigating the deformation of nanocrystalline copper with microscale kinematic metrics and molecular dynamics

Atomistic simulations are employed to investigate the deformation of nanocrystalline copper and the associated strain accommodation mechanisms at 10 K as a function of grain size. Volume-averaged kinematic metrics based on continuum mechanics theory are formulated to analyze the results of molecular dynamics simulations. The metrics rely on both reference and current configurations, along with nearest neighbor lists to estimate nanoscale behavior of atomic deformation fields in nanocrystalline copper. Various deformation mechanisms are activated in the structures, and shown to depend on average grain size of the nanocrystalline structure. Furthermore, grain boundaries, along with dislocation glide, become an important source of strain accommodation as grain size is reduced. It is demonstrated that the metrics capture the contributions of various mechanisms, and provide a sense of the history of atomic regions undergoing both elastic and plastic deformation. The significance of this research is that unique kinematic signatures of the mechanisms are uncovered using certain metrics, and we are able to resolve the contributions of the deformation mechanisms to the overall strain of the structure using Green strain.     

Correlation in the application of the triple-to-double coincidence ratio method with unequal photomultiplier tube efficiencies

Counting data from the triple-to-double coincidence ratio (TDCR) method are by their nature highly correlated. In the general case of unequal phototube detection efficiencies, the highly correlated nature of the data combined with the need to apply methods to solve a system of equations makes proper uncertainty analysis difficult. Removing the correlations from the data prior to analysis may assist in this endeavor. This paper describes the de-correlation of TDCR data by the application of a particular linear transform, the Mahalanobis transform. A programming example in MATLAB is given and a practical example of the use of this technique in the analysis of TDCR data is presented.     

Synthesis and characterization of an electroactive surface that releases gamma-aminobutyric acid (GABA)

We report the synthesis and characterization of a new electroactive surface capable of releasing the neurotransmitter gamma-aminobutyric acid (GABA) upon reduction. The GABA was anchored to an alkanethiol via electrochemically active quinone (abbreviation, TM-GABA). The quinone unit, upon reduction to the hydroquinone, cyclizes to release GABA into solution. The half-life is 99 s. The self-assembled monolayer (SAM) of TM-GABA on gold was prepared and characterized with several surface sensitive techniques. X-ray photoelectron spectroscopy (XPS) explored the SAM formation of TM-GABA on Au surfaces. Cyclic voltammograms showed the ability to electrochemically control the quinone unit at the distal end of the chain. GABA was selectively released upon electrochemical reduction at a potential of -700 mV. The functional GABA terminal group was detected by surface plasmon resonance measurements of anti-GABA antibody binding.     

Control of stereoselective protonation of enols1,2

A decalyl framework with a siloxy enolic moiety and proximate proton transferring groups was synthesized. On enolate generation with fluoride two competitive reaction modes were possible: (a) intermolecular protonation, and (b) intramolecular proton transfer by the proximate group. Control of the protonation stereochemistry proved possible by varying the proximate group and by changing the acidity of the medium. With the groups -CH2OH, -CH=O, and -CH2OCH2OCH3 as the proximate groups, only intermolecular proton transfer was observed with no dependence on acidity. In contrast, with -COO- and COOH, only intramolecular protonation resulted but again with no dependence on acidity of the medium. In contrast, with -CH2NH2 as the proximate group, intramolecular proton transfer predominated with a dependence on the effective pH of the medium. A kinetic analysis provided a linear-log relationship of the ratio of the two stereoisomers with the medium acidity. The analysis revealed that two acetic acid molecules are involved in providing the proton to the enolate moiety. A theoretical analysis was developed paralleling the experimental results. In the ketonization transition state, the hybridization was shown to be close to sp2 hybridized at the alpha-enolate carbon.     

Formation of a miscible supramolecular polymer blend through self-assembly mediated by a quadruply hydrogen-bonded heterocomplex

A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-1,8-naphthyridine (2, DAN), which form an exceptionally strong quadruply hydrogen-bonding complex, are displayed at 1-10 mol % along the main backbone of PBMA and PS, respectively. (1)H NMR studies show heterocomplexation between UG and DAN exclusively. This high-fidelity, high-affinity supramolecular connection of two different polymer coils at the molecular level produces a polymer blend. Blends containing different weight ratios of the polymers and mole percent of the recognition units were characterized by AFM and DSC experiments with no isolated domains observed and a single glass-transition temperature (T(g)). The T(g) is tunable by varying the weight ratio of the polymers in the blend. In addition, viscosity measurements, size-exclusion chromatography (SEC), and dynamic light-scattering (DLS) studies demonstrate the formation of a supramolecular network structure.     

Interplay of fidelity, binding strength, and structure in supramolecular polymers

In studies of a supramolecular network of polymers formed by self-association of UPy or UG recognition units displayed along a poly(butyl methacrylate) (PBMA) backbone, it was unexpectedly found that the more weakly dimerizing (Kdimer approximately 200 M-1) UG unit produced more assembly than did the very strongly dimerizing UPy unit (Kdimer = 2 x 107 M-1). Likewise, in examining supramolecular blends mediated by the heterocomplexation of DAN and UPy, which occurs upon the mixing of polystyrene containing the DAN unit (PS-DAN) and PBMA-UPy, increasing the mol % of UPy did not produce increased viscosity. 1H NMR showed that both observations can be explained by the intramolecular recognition of UPy. Structural studies show that the length of the chain linking the UPy unit to the backbone is critical, with longer linkers favoring intermolecular dimers. An interplay of linker chain length, polymer Mw, recognition unit mol %, and fidelity determines the extent of network growth.     

Regioselectivity of the tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry

The di-pi-methane rearrangement with two pi-groups attached to the central "methane carbon" of the reactant and which leads to a pi-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-pi-methane counterpart. The reactants with three pi groups attached to the central carbon mechanistically are capable of affording both di-pi-methane and tri-pi-methane photoproducts. In common with the di-pi-methane system, bridging of two of the pi-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring. It was found that the regioselectivity of the initial pi-pi-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-pi counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-pi-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-pi-methane) products, kinetics were employed to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route was the major one.     

Suppression of Delayed and Contact Hypersensitivity Responses in Mice Have Different UV Dose Responses

Although acute exposure to UV radiation suppresses the induction of delayed-type (DTH) and contact (CHS) hypersensitivity in mice, it is not clear whether the photo-biological mechanism(s) involved in suppressing these closely related immune reactions is the same. A careful examination of the UV dose responses and wavelength dependencies involved in suppressing CHS and DTH may provide important insights into the mechanisms involved. We compared the UV dose-response curves for suppressing four closely related immune reactions, local and systemic suppression of CHS to dinitrofluorobenzene, systemic suppression of DTH to Candida albicans and systemic suppression of DTH to alloantigen using three different UV spectra (FS40 sunlamps, Kodacel-filtered FS40 sunlamps and solar-simulated light). For each immune response studied, the amount of UVB radiation required to induce 50% immune suppression was lowest when FS40 sunlamps were used, highest with solar-simulated light and intermediate when Kodacel-filtered FS40 sunlamps were used, but the differences observed were not statistically significant. The UV dose-response curves for immune suppression differed significantly depending on the assay used, the site of antigenic sensitization and the antigen used. These findings suggest that the mechanisms by which UV radiation induces immune suppression differ for the four immunological reactions studied.