Tail resonances of Fermi-Pasta-Ulam q-breathers and their impact on the pathway to equipartition

Upon initial excitation of a few normal modes the energy distribution among all modes of a nonlinear atomic chain (the Fermi-Pasta-Ulam model) exhibits exponential localization on large time scales. At the same time, resonant anomalies (peaks) are observed in its weakly excited tail for long times preceding equipartition. We observe a similar resonant tail structure also for exact time-periodic Lyapunov orbits, coined q-breathers due to their exponential localization in modal space. We give a simple explanation for this structure in terms of superharmonic resonances. The resonance analysis agrees very well with numerical results and has predictive power. We extend a previously developed perturbation method, based essentially on a Poincare-Lindstedt scheme, in order to account for these resonances, and in order to treat more general model cases, including truncated Toda potentials. Our results give a qualitative and semiquantitative account for the superharmonic resonances of q-breathers and natural packets.     

Wave propagation in a fluid flowing through a curved thin-walled elastic tube

The pulsatile flow in a curved elastic pipe of circular cross section is investigated. The unsteady flow of a viscous fluid and the wall motion equations are written in a toroidal coordinate system, superimposed and linearized over a steady state solution. Being the main application relative to the vascular system, the radius of the pipe is assumed small compared with the radius of curvature. This allows an asymptotic analysis over the curvature parameter. The model results an extension of the Womersley's model for the straight elastic tube. A numerical solution is found for the first order approximation and computational results are finally presented, demonstrating the role of curvature in the wave propagation and in the development of a secondary flow.     

Crosslinking with S2C12 of EPDM rubbers and of EPDM grafted with polyacrylic acid

EPDM rubber swollen in n-butylacetate was crosslinked with S₂Cl₂ solution in n-butylacetate. From swelling measurements in benzene and the Flory-Rehner equation, the density of elastically-active network chains was calculated as a function of time. Kinetics have been discussed; the diffusion process of S₂Cl₂ in the swollen polymer plays a very important role. Crosslinking of EPDM grafted with polyacrylic acid has been carried out on thin films using S₂Cl₂ as reagent and n-butylacetate as swelling agent, showing similar kinetic behaviour. Intermolecular bonds are due both to sulphur bridges in EPDM and to sulphur and to hydrogen bonds in EPDM grafted samples.     

Rheological Study on Polypropylene/Cycloolefin Copolymer Blends

Summary: Polypropylene, cycloolefin copolymer and their blends were characterized by means of melt flow analysis and capillary rheometry at temperatures between 190 and 230 °C in order to shed more light on COC fiber formation obtained in injection molding process. Melt viscosity and its activation energy as functions of blend composition show negative deviation from the expected additivity (Negative Deviating Blends). The COC/PP viscosity ratio increases with shear rate, but decreases with temperature. High temperature, low viscosity ratio and high shear rate seem to be favorable for fiber formation. Glass transition (from the reversible heat flow curve of modulated DSC) of dumbbell specimens produced by injection molding at 230 °C with COC minor component was 2–4 °C higher than that of grinded pellets obtained from mixing at 190 °C.     

Contact angle measurements as a tool to investigate the filler-matrix interactions in polyurethane-clay nanocomposites from blocked prepolymer

Optically transparent polyurethane-clay nanocomposite films were prepared by dispersing 5 wt% of various commercial organo-clays in a mixture of cycloaliphatic amines used as chain extender-cross-linker of a blocked prepolymer. For the first time, vibration-induced equilibrium contact angle measurements were successfully employed to rank the selected organo-clays accordingly to their hydrophobicity order. Polymer-clay intercalation degree in the nanocomposites, as assessed from X-ray diffraction, was strongly correlated to the water-clay equilibrium contact angle. Moreover, as the clay intercalation degree increased, a decrease of the cross-linking degree of the polyurethane matrix occured.Uniaxial tensile tests under quasi-static and impact conditions, and isothermal thermogravimetric analysis were performed on both unfilled polyurethane matrix and nanocomposites. Secant tensile modulus, tensile energy to break, and thermal lifetime showed a non monotonic trend with a maximum as a function of the intercalation degree. This behaviour is discussed considering that two concomitant and contrasting effects develop as the polymer-clay intercalation degree increases: a positive improvement of the filler matrix interactions, and a negative reduction of the matrix cross-linking degree.     

Effect of hydrolysis on molar mass and thermal properties of poly (ester urethanes)

Effect of hydrolysis time on molar mass, glass transition temperature, crystallinity, and resistance to thermooxidation at elevated temperatures was analyzed for Estanes 54600, 54610, and 54650. Kinetics of the hydrolysis can be plausibly described in terms of the first-order reaction with an average induction period of about 7 days. Reduction of molar mass induced by hydrolysis brings about an appreciable decrease in glass transition temperature, fraction of crystalline domains of soft segments, and thermooxidative stability. The latter effect is manifested by shortening of the lifetimes (related to 5% mass loss) the temperature dependence of which obeys the Arrhenius plot. The observed differences in hydrolysis resistance of Estanes can be related to their chemical composition.