Small-angle X-ray and nuclear-magnetic resonance study of siloxane-PMMA hybrids prepared by the sol-gel process

Transparent siloxane-polymethylmethacrylate (PMMA) hybrids were synthesized by the sol-gel process through hydrolysis of methacryloxyproyltrimethoxysilane (TMSM), tetramethoxysilane (TMOS) and polymerization of methylmethacrylate (MMA) using benzol peroxide (BPO) as catalyst. These composites have a good chemical stability due to the presence of covalent bonds between the inorganic (siloxane) and organic (PMMA) phases. The effects of siloxane content, pH of the initial sol and BPO content on the structure of the dried gels were analyzed by small-angle X-ray scattering (SAXS). SAXS results revealed the presence of an interference (or correlation) peak at medium q-range for all compositions, suggesting that siloxane groups located at the ends of PMMA chains form isolated clusters that are spatially correlated. The average intercluster distance - estimated from the q-value corresponding to the maximum in SAXS spectra - decreases for samples prepared with increasing amount of TMSM-TMOS. This effect was assigned to the expected increase in the number density of siloxane groups for progressively higher siloxane content. The increase of BPO content promotes a more efficient polymerization of MMA monomers but has no noticeable effect on the average intercluster distance. High pH favors polycondensation reactions between silicon species of both TMOS and TMSM silicon alcoxides, leading to a structure in which all siloxane clusters are bonded to PMMA chains. This effect was confirmed by Si-29 nuclear-magnetic resonance (NMR) measurements.

Pauli blocking and medium effects in nucleon knockout reactions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Suicide nucleophilic attack: Reactions of benzohydroxamate anion with bis(2,4-dinitrophenyl) phosphate

(Chemical Equation Presented) The reaction between the benzohydroxamate anion (BHO-) and bis(2,4-dinitrophenyl)phosphate (BDNPP) has been examined kinetically, and the products were characterized by mass and NMR spectroscopy. The nucleophilic attack of BHO- follows two reaction paths: (i) at phosphorus, giving an unstable intermediate that undergoes a Lossen rearrangement to phenyl isocyanate, aniline, diphenylurea, and O-phenylcarbamyl benzohydroxamate; and (ii) on the aromatic carbon, giving an intermediate that was detected but slowly decomposes to aniline and 2,4-dinitrophenol. Thus, the benzohydroxamate anion can be considered a self-destructive molecular scissor since it reacts and loses its nucleophilic ability. © 2009 American Chemical Society.

Estudo da distribuição de tamanho e composição iônica de aerossóis e seus efeitos na capacidade de nuclear gotas de nuvens

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Thermodynamics of Axial Substitution and Kinetics of Reactions with Amino Acids for the Paddlewheel Complex Tetrakis(acetato)chloridodiruthenium(II,III)

The known paddlewheel, tetrakis(acetato)chloridodiruthenium(II,III), offers a versatile synthetic route to a novel class of antitumor diruthenium(II,III) metallo drugs, where the equatorial ligands are nonsteroidal anti-inflammatory carboxylates. This complex was studied here as a soluble starting prototype model for antitumor analogues to elucidate the reactivity of the [Ru-2(CH3COO)(4)](+) framework. Thermodynamic studies on equilibration reactions for axial substitution of water by chloride and kinetic studies on reactions of the diaqua complexes with the amino acids glycine, cysteine, histidine, and tryptophan were performed. The standard thermodynamic reaction parameters Delta H degrees, Delta S degrees, and Delta V degrees were determined and showed that both of the sequential axial substitution reactions are enthalpy driven. Kinetic rate laws and rate constants were determined for the axial substitution reactions of coordinated water by the amino acids that gave the corresponding aqua(amino acid)-Ru-2 substituted species. The results revealed that the [Ru-2(CH3COO)(4)](+) paddlewheel framework remained stable during the axial ligand substitution reactions and was also mostly preserved in the presence of the amino acids.

(18O,16O) Two-neutron transfer reactions for spectroscopic studies.

A systematic study of the response of different nuclei to the (18O, 16O) two-neutron transfer reaction at 84 MeV incident energy was pursued at the INFN-LNS in Catania (Italy). The experiments were performed using several solid targets from light (9Bc, 11 B, 12,13C, 16O, 28Si) to heavier ones (58,64Ni, 120Sn, 208Pb). The 16O ejectiles were detected at forward angles by the MAGNEX magnetic spectrometer and identified without the need of time of flight measurements. Exploiting the large momentum (≈ 25%) and angular (50 msr) acceptance of the spectrometer, energy spectra were obtained with a relevant yield up to about 20 MeV excitation energy. A common feature of the light nuclei spectra is the strong population of states with well known configuration of two-particle over a core and the appearance of unknown resonant structures in the continuum. These latter can reveal the excitation of a collective mode connected with the transfer of a pair. For the heavier nuclei as 66Ni a completely different behaviour is observed indicating the presence of more dissipative processes in the reaction mechanisms that hide the spectroscopic information.

Study of reactions induced by 6He on 9Be

We present the results of experiments using a 6He beam on a 9Be target at energies 7 − 9 times the Coulomb barrier. Angular distributions of the elastic, inelastic scattering (target breakup) and the -particle production in the 6He+9Be collision have been analysed. Total reaction cross sections were obtained from the elastic scattering analyses and a considerable enhancement has been observed by comparing to stable systems.

Study of electromagnetic reactions on light nuclei with the Lorentz Integral Transform method

In dieser Arbeit aus dem Bereich der Wenig-Nukleonen-Physik wird die neu entwickelte Methode der Lorentz Integral Transformation (LIT) auf die Untersuchung von Kernphotoabsorption und Elektronenstreuung an leichten Kernen angewendet. Die LIT-Methode ermoeglicht exakte Rechnungen durchzufuehren, ohne explizite Bestimmung der Endzustaende im Kontinuum. Das Problem wird auf die Loesung einer bindungzustandsaehnlichen Gleichung reduziert, bei der die Endzustandswechselwirkung vollstaendig beruecksichtigt wird. Die Loesung der LIT-Gleichung wird mit Hilfe einer Entwicklung nach hypersphaerischen harmonischen Funktionen durchgefuehrt, deren Konvergenz durch Anwendung einer effektiven Wechselwirkung im Rahmem des hypersphaerischen Formalismus (EIHH) beschleunigt wird. In dieser Arbeit wird die erste mikroskopische Berechnung des totalen Wirkungsquerschnittes fuer Photoabsorption unterhalb der Pionproduktionsschwelle an 6Li, 6He und 7Li vorgestellt. Die Rechnungen werden mit zentralen semirealistischen NN-Wechselwirkungen durchgefuehrt, die die Tensor Kraft teilweise simulieren, da die Bindungsenergien von Deuteron und von Drei-Teilchen-Kernen richtig reproduziert werden. Der Wirkungsquerschnitt fur Photoabsorption an 6Li zeigt nur eine Dipol-Riesenresonanz, waehrend 6He zwei unterschiedliche Piks aufweist, die dem Aufbruch vom Halo und vom Alpha-Core entsprechen. Der Vergleich mit experimentellen Daten zeigt, dass die Addition einer P-Wellen-Wechselwirkung die Uebereinstimmung wesentlich verbessert. Bei 7Li wird nur eine Dipol-Riesenresonanz gefunden, die gut mit den verfuegbaren experimentellen Daten uebereinstimmt. Bezueglich der Elektronenstreuung wird die Berechnung der longitudinalen und transversalen Antwortfunktionen von 4He im quasi-elastischen Bereich fuer mittlere Werte des Impulsuebertrages dargestellt. Fuer die Ladungs- und Stromoperatoren wird ein nichtrelativistisches Modell verwendet. Die Rechnungen sind mit semirealistischen Wechselwirkungen durchgefuert und ein eichinvarianter Strom wird durch die Einfuehrung eines Mesonaustauschstroms gewonnen. Die Wirkung des Zweiteilchenstroms auf die transversalen Antwortfunktionen wird untersucht. Vorlaeufige Ergebnisse werden gezeigt und mit den verfuegbaren experimentellen Daten verglichen.

Towards a Reconstruction of Thermal Properties of Light Nuclei from Fusion - Evaporation reactions

This thesis work has been developed in the framework of a new experimental campaign, proposed by the NUCL-EX Collaboration (INFN III Group), in order to progress in the understanding of the statistical properties of light nuclei, at excitation energies above particle emission threshold, by measuring exclusive data from fusion-evaporation reactions. The determination of the nuclear level density in the A~20 region, the understanding of the statistical behavior of light nuclei with excitation energies ~3 A.MeV, and the measurement of observables linked to the presence of cluster structures of nuclear excited levels are the main physics goals of this work. On the theory side, the contribution to this project given by this work lies in the development of a dedicated Monte-Carlo Hauser-Feshbach code for the evaporation of the compound nucleus. The experimental part of this thesis has consisted in the participation to the measurement 12C+12C at 95 MeV beam energy, at Laboratori Nazionali di Legnaro - INFN, using the GARFIELD+Ring Counter(RCo) set-up, from the beam-time request to the data taking, data reduction, detector calibrations and data analysis. Different results of the data analysis are presented in this thesis, together with a theoretical study of the system, performed with the new statistical decay code. As a result of this work, constraints on the nuclear level density at high excitation energy for light systems ranging from C up to Mg are given. Moreover, pre-equilibrium effects, tentatively interpreted as alpha-clustering effects, are put in evidence, both in the entrance channel of the reaction and in the dissipative dynamics on the path towards thermalisation.

Search for K isomers in 252,254 No and 260 Sg and investigation of their nuclear structure

Study of K isomerism in the transfermium region around the deformed shells at N=152, Z=102, and N=162, Z=108 provides important information on the structure of heavy nuclei. Recent calculations suggest that the K-isomerism can enhance the stability of such nuclei against alpha emission and spontaneous fission. Nuclei showing K isomerism have neutron and proton orbitals with large spin projections on the symmetry axis which is due to multi quasiparticle states with aligned spins K. Quasi-particle states are formed by breaking pairs of nucleons and raising one or two nucleons in orbitals near the Fermi surface above the gap, forming high K (multi)quasi-particle states mainly at low excitation energies. Experimental examples are the recently studied two quasi-particle K isomers in 250,256-Fm, 254-No, and 270-Ds. Nuclei in this region, are produced with cross sections ranging from several nb up to µb, which are high enough for a detailed decay study. In this work, K isomerism in Sg and No isotopes was studied at the velocity filter SHIP of GSI, Darmstadt. The data were obtained by using a new data acquisition system which was developed and installed during this work. 252,254-No and 260-Sg were produced in fusion evaporation reactions of 48-Ca and 54-Cr projectiles with 206,208-Pb targets at beam energies close to the Coulomb barrier. A new K isomer was discovered in 252-No at excitation energy of 1.25 MeV, which decays to the ground state rotational band via gamma emission. It has a half-life of about 100 ms. The population of the isomeric state was about 20% of the ground state population. Detailed investigations were performed on 254-No in which two isomeric states (275 ms and 198 µs) were already discovered by R.-D. Herzberg, but due to the higher number of observed gamma decays more detailed information about the decay path of the isomers was obtained in the present work. In 260-Sg, we observed no statistically significant component with a half life different from that of the ground state. A comparison between experimental results and theoretical calculations of the single particle energies shows a fair agreement. The structure of the here studied nuclei is in particular important as single particle levels are involved which are relevant for the next shell closure expected to form the region of the shell stabilized superheavy elements at proton numbers 114, 120, or 126 and neutron number 184. K isomers, in particular, could be an ideal tool for the synthesis and study of these isotopes due to enhanced spontaneous fission life times which could result in higher alpha to spontaneous fission branching ratios and longer half lifes.

The Synthesis of Macrocyclic Polystyrene by Sequential Atom Transfer Radical Reactions

A method for the production of macrocyclic polystyrene via ring closing of a linear !,"-dibrominated polystyrene by an Atom Transfer Radical Coupling (ATRC) reaction is described. The dibrominated polystyrene chain was produced from two simultaneous atom transfer radical polymerizations (ATRPs) originating from a dibrominated benzal bromide initiator. To ensure the retention of the halogen end groups polymerization was allowed to proceed to less than 50% conversion. Using this precursor in an intramolecular ATRC (ring closing) reaction was found to yield in excess of 90% cyclic product based on refractive index-gel permeation chromatography (GPC) analysis. The cyclic architecture of the polymer was verified by GPC, Nuclear Magnetic Resonance (NMR), and mass spectrometry analysis. The utility of this method has been expanded by the addition of 2-methyl-2-nitrosopropane to the coupling reaction, which allows for the coupling to proceed at a faster rate and to yield macrocycles with incorporated alkoxyamine functionality. The alkoxyamine functionality allows for degradation of the cycles at high temperatures (>125° C) and we hypothesize that it may allow the macrocycles to act as a macroinitiator for a ring expansion polymerization in future studies.