Elasticity and dynamics of particle gels in non-Newtonian melts

We investigate the relation between the structure and the viscoelastic behavior of a model polymer nanocomposite system based on a mixture of titanium dioxide (TiO₂) nanoparticles and polypropylene. Above a critical volume fraction, Φ _c, the elasticity of the hybrids dramatically increases, and the frequency dependence of the elastic and viscous moduli reflects the superposition of the independent responses of the suspending polymer melt and of an elastic particle network. In addition, the elasticity of the hybrids shows critical behavior around Φ _c. We interpret these observations by hypothesizing the formation of a transient network, which forms due to crowding of particle clusters. Consistent with this interpretation, we find a long-time, Φ-dependent, structural relaxation, which emphasizes the transient character of the structure formed by the particle clusters. For times below this characteristic relaxation time, the elasticity of the network is Φ-independent and reminiscent of glassy behavior, with the elastic modulus, G^′, scaling with frequency, ω, as G^′∼ω ^0.3. We expect that our analysis will be useful for understanding the behavior of other complex fluids where the elasticity of the components could be superimposed.     

Thermal decomposition processes in aromatic-aliphatic polyamides investigated by mass spectrometry

The thermal decomposition processes of some aromatic-aliphatic polymides, derived from terephthalic acid and aliphatic diamines, were studied by flash Pyrolysis-GCMS and by direct Pyrolysis-Mass Spectrometry, using both Chemical lonization and Electron Impact modes. The results indicate that the primary thermal decomposition proceeds via a β-CH hydrogen transfer process, with formation of pyrolysis products containing amide and olefin end-groups. Nitrile end-groups are also formed by dehydration of the amide groups formed in the primary decompostion process.     

Multiwalled carbon nanotubes in donor-acceptor nanohybrids-towards long-lived electron transfer products

Novel multiwalled carbon nanotube/metalloporphyrin nanohybrids are devised and probed as versatile donor-acceptor hybrids.     

Recognition of quaternary ammonium cations by calix[4]arene derivatives supported on gold nanoparticles

Calix[4]arene derivatives supported on monolayer protected gold clusters show a remarkable enhanced binding efficiency towards quaternary ammonium salts in homogeneous solution of apolar solvents with respect to the corresponding unsupported hosts.     

Non-covalent synthesis in aqueous solution of discrete multi-porphyrin aggregates with programmable stoichiometry and sequence

This unprecedented calixarene templated, noncovalent synthesis of multi-porphyrin complexes in aqueous solution makes it possible to plan the nature, stoichiometry, and sequence of porphyrins. The proposed method presents the same accuracy of the covalent approach and a 100% yield, but it is not as time-consuming as the latter method.     

Compactly Supported Wavelets Through the Classical Umbral Calculus

We explore compactly supported scaling functions of wavelet theory by means of classical umbral calculus as reformulated by Rota and Taylor. We set a theory of orthonormal scaling umbra which leads to a very simple and elementary proof of Lawton's theorem for umbrae. When umbrae come from a wavelet setting, we recover the usual Lawton condition for the orthonormality of the integer translates of a scaling function.     

On the selection rules for spin-Lorentz cobordisms

A recent result of Gibbons and Hawking on the existence of spin-Lorentz cobordisms is applied to the class of 3-manifolds that can support a non-negative scalar curvature metric. We call such manifolds admissible. It is shown how in general the existence of a spin-Lorentz cobordism restricts the number-mod-2 of certain prime manifolds to occur in the prime decomposition, which are explicitly listed in the case of admissible manifolds.     

Global monotonicity and oscillation for second order differential equation

Oscillatory properties of the second order nonlinear equation

The Dirichlet energy of mappings from ${\bf B^3}$ into a manifold: density results and gap phenomenon

Weak limits of graphs of smooth maps with equibounded Dirichlet integral give rise to elements of the space . We assume that the 2-homology group of has no torsion and that the Hurewicz homomorphism is injective. Then, in dimension n = 3, we prove that every element T in , which has no singular vertical part, can be approximated weakly in the sense of currents by a sequence of smooth graphs {u _k } with Dirichlet energies converging to the energy of T. We also show that the injectivity hypothesis on the Hurewicz map cannot be removed. We finally show that a similar topological obstruction on the target manifold holds for the approximation problem of the area functional.Received: 9 May 2003, Accepted: 5 June 2003, Published online: 25 February 2004