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The author was supported by a NSF grant and by an American Mathematical Society Centennial Research Fellowship     

Rheological properties of diblock copolymer/layered‐silicate nanocomposites

The melt‐state viscoelastic properties of nanocomposites prepared with a symmetrical polystyrene–polyisoprene block copolymer and organically modified layered silicates are examined. Nanocomposites based on three thermodynamically equivalent organically modified layered silicates, primarily differing in lateral disk diameter (d), are studied with small‐amplitude oscillatory shear. The effects of the domain structure of the ordered block copolymer and the mesoscale dispersion of the layered silicates on the rheological properties are examined via a comparison of data for the nanocomposites in the ordered and disordered states of the block copolymer. Hybrids prepared with 5 wt % organically modified fluorohectorite (d ～ 10 μm) and montmorillonite (d ～ 1 μm) demonstrate a notable decrease in the frequency dependence of the moduli at low frequencies and a significant enhancement in the complex viscosity at low frequencies in the disordered state. This behavior is understood in terms of the development of a percolated layered‐silicate network structure. However, the viscoelastic properties in the disordered state with 5 wt % organically modified laponite (d ～ 30 nm) and in the ordered state of the block copolymer for all layered silicates demonstrate only minor changes from those observed for the unfilled polymer. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1434–1443, 2002     

Synthesis of ThCr2Si2-type arsenides from Bi flux

LaCo(2)As(2) can be synthesized as pure crystalline material by annealing a mixture of elements in Bi flux. The reaction, however, is accompanied by the incorporation of a small quantity of Bi into the structure and the formation of vacancies in the Co sublattice, which lead to substantial changes in structural and magnetic properties of the material.     

Connecting theory with experiment to understand the initial nucleation steps of heteropolyoxometalate clusters

A complimentary combination of Density Functional Theory (DFT) methodology and Electrospray Ionization-Mass Spectrometry (ESI-MS) has been utilized to increase our limited understanding of the first nucleation steps in the formation of the [XM(12)O(40)](n-) Keggin polyoxometalates (POMs) (where addenda metal atom M = W or Mo, and the heteroatom X = P or As). We postulate that the first key steps of nucleation into discrete, high nuclearity heteropolyanions proceed via the formation of isodinuclear species (e.g. [M(2)O(7)](2-)), which undergo successive steps of protonation and water condensation to form a heterotrinuclear fragment, which acts as a template for the constituent parts required for subsequent aggregation and formation of the plenary Keggin heteropolyanion. The stability of calculated structures of the numerous postulated intermediates has been analysed and discussed in detail, and these results complemented using experimental mass spectrometry, using an assembly (reaction solution analysis) and disassembly (fragmentation of single crystals) approach. Overall, no significant differences between the Keggin POMs were found when changing the addenda metal atom (W or Mo) or the heteroatom (P or As); although small differences among the lowest-energy structures were detected.     

Selective formation of 1,5-substituted sulfonyl triazoles using acetylides and sulfonyl azides

The reaction of acetylides with sulfonyl azides was found to selectively form 1,5-substituted sulfonyl triazoles. This reaction thus provides access to the regioisomeric product as compared to the popular copper-catalyzed azide-alkyne cycloaddition. The reaction is efficient and selective with a variety of alkyne sources and sulfonyl azides and can incorporate an additional electrophile to yield 1,4,5-trisubstituted sulfonyl triazoles.     

Inhibitors of SARS-3CLpro: virtual screening, biological evaluation, and molecular dynamics simulation studies

SARS-CoV from the coronaviridae family has been identified as the etiological agent of Severe Acute Respiratory Syndrome (SARS), a highly contagious upper respiratory disease that reached epidemic status in 2002. SARS-3CL(pro), a cysteine protease indispensible to the viral life cycle, has been identified as one of the key therapeutic targets against SARS. A combined ligand and structure-based virtual screening was carried out against the Asinex Platinum collection. Multiple low micromolar inhibitors of the enzyme were identified through this search, one of which also showed activity against SARS-CoV in a whole cell CPE assay. Furthermore, multinanosecond explicit solvent simulations were carried out using the docking poses of the identified hits to study the overall stability of the binding site interactions as well as identify important changes in the interaction profile that were not apparent from the docking study. Cumulative analysis of the evaluated compounds and the simulation studies led to the identification of certain protein-ligand interaction patterns which would be useful in further structure based design efforts.     

Probing the structure of the crystalline core of field-aligned, monodisperse, cylindrical polyisoprene-block-polyferrocenylsilane micelles in solution using synchrotron small- and wide-angle X-ray scattering

The self-assembly of block copolymers in selective solvents represents a powerful approach to functional core-shell nanoparticles. Crystallization of the core can play a critical role in directing self-assembly toward desirable, nonspherical morphologies with low mean interfacial curvature. Moreover, epitaxial growth processes have been implicated in recent advances that permit access to monodisperse cylinders, cylindrical block comicelles with segmented cores and/or coronas, and complex hierarchical architectures. However, how the core-forming block crystallizes in an inherently curved nanoscopic environment has not been resolved. Herein we report the results of synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) studies of well-defined, monodisperse crystalline-coil polyisoprene-block-polyferrocenylsilane cylindrical micelles aligned in an electric field. WAXS studies of the aligned cylinders have provided key structural information on the nature of the PFS micelle core together with insight into the role of polymer crystallinity in the self-assembly of these and potentially related crystalline-coil block copolymers.     

Smart polymer nanoparticles designed for environmentally compliant coatings

We describe the synthesis, characterization, and film-forming properties of two-component nanoparticles that undergo a reversible morphology transformation in water as a function of pH. The particles consist of a high molecular weight acrylate copolymer and an acid-rich oligomer designed to be miscible with the polymer when its -COOH groups are protonated. Attaching a fluorescence resonance energy transfer (FRET) pair to components inside the nanoparticles enabled us to assess morphology at the molecular level. By inspecting changes in the donor fluorescence decay profile at different pH values, we established miscibility of the components in acidic solution but with charge-induced phase separation when the oligomers were neutralized to their carboxylate form. Complementary titration experiments revealed that the nanoparticles adopt a core-shell structure when the acid groups are deprotonated. We studied the effect of the acid-rich oligomer on the diffusion rate of the high molecular weight polymers following film formation. Our results show that the carboxylated oligomer enhanced the rate of diffusive mixing between high molecular weight molecules by more than 2 orders of magnitude. FRET measurements carried out on partially dried films using a low-resolution microscope showed that the carboxylate oligomer shell can delay coalescence for ca. 30 min after passage of the drying front. This delay is expected to help with increasing the 'open time' of latex paints, a desirable property of solvent-based paints that remains difficult to achieve with (environmentally compliant) waterborne paints. Use of ammonia as a volatile base resulted in synergistic effects: initial retardation of coalescence followed by acceleration of diffusive mixing as the ammonium salts dissociated and ammonia evaporated from the film.