Characterization of graphs having extremal Randi? indices

The higher Randi? index R_t(G) of a simple graph G is defined as

     

Water-induced morphology changes in BaO/gamma-Al2O3 NOx storage materials

Exposure of NO(2)-saturated BaO/gamma-Al(2)O(3) NO(x) storage materials to H(2)O vapour results in the conversion of surface nitrates to Ba(NO(3))(2) crystallites, causing dramatic morphological changes in the Ba-containing phase, demonstrating a role for water in affecting the NO(x) storage/reduction properties of these materials.     

Biomimetic hydrocarbon oxidation catalyzed by nonheme iron(III) complexes with peracids: evidence for an Fe(V)=O species

Mononuclear nonheme iron(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Fe(Me(2)bpb)Cl(H(2)O)] (3 a) and [Fe(bpc)Cl(H(2)O)] (4 a), were prepared by substitution reactions involving the previously synthesized iron(III) complexes [Et(3)NH][Fe(Me(2)bpb)Cl(2)] (3) and [Et(3)NH][Fe(bpc)Cl(2)] (4). Complexes 3 a and 4 a were characterized by IR and elemental analysis, and complex 3 a also by X-ray crystallography. Nonheme iron(III) complexes 3, 3 a, 4, and 4 a catalyze olefin epoxidation and alcohol oxidation on treatment with m-chloroperbenzoic acid. Pairwise comparisons of the reactivity of these complexes revealed that the nature of the axial ligand (Cl(-) versus H(2)O) influences the yield of oxidation products, whereas an electronic change in the supporting chelate ligand has little effect. Hydrocarbon oxidation by these catalysts was proposed to involve an iron(V) oxo species which is formed on heterolytic O-O bond cleavage of an iron acylperoxo intermediate (FeOOC(O)R). Evidence for this iron(V) oxo species was derived from KIE (k(H)/k(D)) values, H(2) (18)O exchange experiments, and the use of peroxyphenylacetic acid (PPAA) as the peracid. Our results suggest that an Fe(V)=O moiety can form in a system wherein the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors. This work is relevant to the chemistry of mononuclear nonheme iron enzymes that are proposed to oxidize organic substrates via reaction pathways involving high-valent iron oxo species.     

Adsorption of hydrophobically modified poly(acrylamide)-co-(acrylic acid) on an amino-functionalized surface and its response to the external solvent environment

The adsorption of hydrophobically modified poly(acrylamide)-co-(acrylic acid), designated as PAM-C14-AA (x%) (x = 5, 10, 20, representing the mole percent of acrylic acid units), at an amino-functionalized silicon surface was studied. The effect of polymer charge density was determined by varying the acrylic acid content of the copolymer. Characteristics of the adsorbed layer were evaluated by atomic force microscopy, water contact angle measurements, and X-ray photoelectron spectroscopy. The results showed that the adsorption behavior of PAM-C14-AA (x%) is influenced by the balance among the electrostatic, hydrogen-bonding, and hydrophobic interactions. Adjusting the solution pH and polymer charge density significantly affects the morphology and thickness of the adsorbed film. Furthermore, it was found that the adsorbed PAM-C14-AA undergoes conformational rearrangements when the surface is wetted by selected organic solvents. The resultant morphology and wettability of the films indicated that the different affinities of the solvents for different segments of PAM-C14-AA (x%) can be considered to be the possible cause of the conformational rearrangements of adsorbed polymer.     

Surface-enhanced Raman spectroscopic-encoded beads for multiplex immunoassay

A new type of encoded bead, which uses surface-enhanced Raman scattering (SERS), is described for multiplex immunoassays. Silver nanoparticles were embedded in sulfonated polystyrene (PS) beads via a polyol method, and they were used as SERS-active substrates. Raman-label organic compounds such as 4-methylbenzenethiol (4-MT), 2-naphthalenethiol (2-NT), and benzenethiol (BT) were then adsorbed onto the silver nanoparticles in the sulfonated PS bead. Although only three kinds of encoding have been demonstrated here, various combinations of these Raman-label organic compounds have the potential to give a large number of tags. The Raman-label-incorporated particles were then coated with a silica shell using tetraethoxyorthosilicate (TEOS) for chemical stability and biocompatibility. The resulting beads showed unique and intense Raman signals for the labeled organic compounds. We demonstrated that SERS-encoded beads could be used for multiplex detection with a model using streptavidin and p53. In our system, the binding event of target molecules and the type of ligand can be simultaneously recognized by Raman spectroscopy using a single laser-line excitation (514.5 nm).     

Swelling of hydrophobically modified poly(acrylamide) and poly(acrylamide)-co-(acrylic acid) gels in surfactant solutions

A series of hydrophobically modified polyacrylamide and polyacrylamide-co-poly(acrylic acid) gels with systematically varying hydrophobicity were prepared by free-radical polymerization of acrylamide, n-alkylacrylamides (n = 10, 12, and 14), and acrylic acid. The swelling of these gels was examined in water and in both anionic and cationic surfactant solutions. It was found that the gels which incorporated acrylic acid showed extremely high swelling in water. Maximum swelling was observed in gels which incorporated 10 mol% acrylic acid. The swelling of these gels was much less in solutions of both anionic and cationic surfactants than in water. The gels which did not incorporate acrylic acid demonstrated little swelling in water, but showed increased swelling in both anionic and cationic surfactant solutions with increased hydrophobicity of the gel. Received: 1 February 1999 Accepted in revised form: 5 March 1999     

Near‐infrared spectroscopic studies on the cure behaviors of the CAE/DGEBA blend system initiated by a thermal latent catalyst

The latent properties and cure behaviors of an epoxy blend system based on cycloaliphatic epoxy (CAE) and diglycidyl ether of bisphenol A (DGEBA) epoxy containing N‐benzylpyrazinium hexafluoroantimonate (BPH) as a thermal latent initiator were investigated with near‐infrared (N‐IR) spectroscopy. The assignments of the latent properties and cure kinetics were performed by the measurements of the N‐IR reflectance for epoxide and hydroxyl functional groups at different temperatures and compositions. As a result, this system showed more than one type of reaction, and BPH was an excellent thermal latent catalyst without any coinitiator. The cure behaviors were identified by the changes in the absorption intensity of the hydroxyl groups at 7100 cm⁻¹ with different composition ratios. Moreover, characteristic N‐IR band assignments were used to evaluate the reactive kinetics and were shown to be an appropriate method for studying the cure behaviors of the CAE/DGEBA blend system containing a thermal latent catalyst. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 326–331, 2001     

Polyethylene/Clay Nanocomposite by In‐Situ Exfoliation of Montmorillonite During Ziegler‐Natta Polymerization of Ethylene

Complete exfoliation of montmorillonite during Ti‐based Ziegler‐Natta polymerization of ethylene has been successfully carried out by using montmorillonite (MMT‐OH) modified with intercalation agents containing hydroxyl groups. Hydroxyl groups in intercalation agents offer facile reactive sites for anchoring catalysts in between silicate layers. Comparison of exfoliation characteristics between MMT‐OH and non‐intercalated montmorillonite showed that the feasibility of exfoliation during ethylene polymerization was highly dependent on the catalyst fixation method.     

Comparison of activities of homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) for synthesis of syndiotactic polystyrene by UV/visible spectroscopic study

The catalytic activities in syndiospecific polymerization of styrene in hydrocarbon using homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) were investigated through UV/visible spectroscopic analysis. A strong UV absorption band of CpTiCl₃, itself, incipiently appeared at λ_max = 400 nm in toluene, followed by a bathochromic shift with its remarkable decrease by the addition of MAO. The absorption band intensity at λ_max = 400 nm arising from delocalization of π-electrons on the cyclopentadienyl ring decreased by methylation in the presence of MAO with regard to the mechanism for production of an active center (“cation-like”), for example, the change of the ionic nature. The intensity decrease at λ_max = 400 nm was suppressed over 2000 of the [Al]/[Ti] ratio. In the case of Ti(OC₄H₉)₄ having a σ-ligand, new and broad UV absorption bands were developed at λ_max = 340 nm and 410 nm in the presence of MAO in contrast with the CpTiCl₃/MAO system. Comparison between the relative absorption intensities at λ_max = 340 nm and 410 nm led to the determination of a maximum catalytic activity of Ti(OC₄H₉)₄ in the presence of MAO related to the polymerization yield. The maximum polymerization yield was observed with regard to the relative maximum value of the absorption intensity at λ_max = 410 nm with the [Al]/[Ti] ratio (500). From observation for polymorphism of the final products via differential scanning calorimetric analysis (DSC), the thermally unstable β-form seemed to be produced by the CpTiCl₃/MAO system independent of the MAO concentration, the Ti(OC₄H₉)₄/MAO system produced a thermally stable α-form in the low MAO concentration (up to 100 of the [Al]/[Ti] ratio), and a mixture of α- and/or β-forms over 200 of the [Al]/[Ti] ratio under our experimental conditions. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1733–1741, 1998     

Microstructure effect in the coupling reactions of polymeric organolithium compounds using chlorosilanes

The most important variable affecting the yield in the coupling reactions of polymeric organolithium compounds with chlorosilane compounds has been investigated through size-exclusion chromatographic (SEC) analysis. The coupling reaction of poly(styryl)lithium with dichlorodimethylsilane as a silane-coupling agent provided 44 wt % of the coupling yield. The coupling yield, depending on the chain end reactivity of active polymers, was not greatly affected. The addition of a Lewis base such as N,N,N′,N′-tetramethylethylenediamine (TMEDA) even after complete polymerization of the dienes in hydrocarbon seems to affect the coupling reaction, resulting in decreasing the yield. The 1,2- or 3,4-enchain contents in the polydiene backbones affected the reduction of the linking efficiency in the coupling of the poly(dienyl)lithiums with chlorosilanes as the linking agent. The linking yields of the active polymers including over 75 mol % of 1,2- or 3,4-enchainment on the polydiene segment were below 20 wt %. The linking yields exhibited a dependence not only on the steric requirement of the chain end, but also the microstructure of the polydiene segment. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1743–1753, 1998