A Molecular‐Dynamics Study of Size and Chirality Effects on Glass‐Transition Temperature and Ordering in Carbon Nanotube‐Polymer Composites

Molecular‐dynamics simulations of single‐walled carbon nanotubes (CNTs) embedded in a coarse‐grained amorphous monodisperse polyethylene‐like model system have been carried out. The roles of nanotube diameter and chirality on the physical and structural properties of the composite are thoroughly discussed for several CNTs with different diameter and chirality. It is shown that the glass‐transition temperature of the polymer matrix increases with the diameter of the CNT while chirality effects are negligible. A denser and ordered layered region of polymer matrix is found in the vicinity of the nanotube surface. The density and ordering of this layer increases with the CNT diameter. All simulations indicate that chirality does not affect the atomic structure of the highly ordered layer surrounding the CNTs. Despite the simplicity of the polymer model, results of this study are qualitatively comparable with those obtained from experiments and numerical simulations that consider a chemically specific polymer matrix.

     

Nonstationary nonplanar free motions of an orbiting string with multiple internal resonances

The paper discusses the nonlinear free dynamics of an orbiting string satellite system. The focus is on the transversal oscillations, which are governed by two partial integro-differential equations in two transversal displacement components with quadratic nonlinearities. The system is weakly nonlinear but in practice works in conditions of simultaneous internal resonance. The investigation focuses on nonstationary motions arising from perturbed steady-state nonplanar oscillations. A four-mode model is used to study the problem: two modes are necessary to describe the basic oscillation and at least two other modes are involved in the resonance phenomena when the motion is perturbed. The multiple time scales method is used to obtain the equations that govern the amplitude and phase modulations. For increasing levels of system energy, fundamental and bifurcated paths of fixed points of the seven first-order differential equations are determined and their stability is investigated. The trajectories of motion of periodically modulated amplitude solutions and their stability are also studied. A model with a higher number of modes is used to evaluate the accuracy of the stability analysis of two-mode nonplanar oscillations perturbed by a two-mode disturbance.

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Sommario Nel presente lavoro si studia la dinamica libera nonlineare di un sistema filo-satellite. L'attenzione è rivolta alle oscillazioni trasversali, governate da due equazioni integro-differenziali, con nonlinearità quadratiche, nelle due componenti di spostamento. Il sistema è debolmente nonlineare ma praticamente lavora in condizioni di risonanza interna. Lo studio è concentrato sui moti nonstazionari generati da perturbazioni delle oscillazioni stazionarie spaziali. Per studiare il problema è stato sviluppato un modello con quattro modi: due sono necessari per descrivere il moto base mentre almeno altri due sono interessati dai fenomeni di risonanza interna quando il moto viene perturbato. Per ottenere le equazioni nelle ampiezze e fasi è stato utilizzato il metodo delle scale multiple. Del sistema di sette equazioni differenziali del primo ordine ottenuto, sono stati studiati i percorsi fondamentali di equilibrio e i rami biforcati, prendendo come parametro il livello di energia totale. È stata inoltre esaminata la stabilità di questi rami. Sono state studiate le traiettorie dei moti periodicamente modulati e la loro stabilità. Infine, è stato utilizzato un modello con un numero più alto di modi per valutare l'accuratezza dell' analisi di stabilità delle oscillazioni bimodali spaziali, nella quale la perturbazione è stata descritta da due soli modi.

     

Dynamical equations of constrained robotic devices

Summary Different forms of the dynamical equations for constrained chain systems are investigated. First Kane's equations with Lagrange's multipliers are deduced from the standard form of d'Alembert's principle. Next it is proved that, even for constrained robotic devices, Euler's equations are equivalent to Kane's equations provided only that relative coordinates are employed. This equivalence yields an outstanding mechanical interpretation of Kane's equations. The possibility of applying the procedure to closed-loop multibody systems is also examined.

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Sommario Vengono esaminate diverse formulazioni delle equazioni del moto per catene di corpi rigidi soggette a vincoli. Vengono dedotte le equazioni di Kane con moltiplicatori di Lagrange partendo dalla formulazione usuale del principio di d'Alembert. Viene poi provato che anche per i manipolatori vincolati le equazioni di Eulero sono equivalenti a quelle di Kane, purché vengano usate coordinate relative. Tale equivalenza fornisce una significativa interpretazione meccanica delle equazioni di Kane. Infine viene discussa la possibilità di applicare la procedura a sistemi a molti corpi con catene cinematiche chiuse.

     

Thermodynamics of apolar viscoelastic dielectrics

Summary In this paper we propose a thermodynamic theory for apolar viscoelastic dielectrics. Within this framework we discuss the possibility of accounting for the Maxwell effect, i.e., the birefringence phenomenon due to the motion of the continuum in study. To this end we show that the proposal made by Toupin, in order to account for the Maxwell effect, does not contrast with the second law of Thermodynamics.

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Sommario In questo lavoro viene formulata una teoria termodinamica dei dielettrici viscoelastici apolari. Nell'ambito di tale teoria viene esaminata la possibilità di descrivere l'effetto Maxwell, ossia il fenomeno della birifrangenza dovuta al moto del continuo. A questo proposito si mostra che il termine proposto da Toupin per tener conto dell'effetto Maxwell non è in contrasto con la seconda legge della Termodinamica.

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This work was supported by the Gruppo Nazionale per la Fisica Matematica of CNR.     

Linear wave modes for dissipative fluids with rate type constitutive equations

Summary In this paper we study in detail one-dimensional linear plane harmonic waves in dissipative fluids within the framework of extended linear irreversible thermodynamics. The results for the acoustic mode are compared with the available experimental data on the dispersion and absorption of sound in monatomic gases.

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Sommario Vengono studiate dettagliatamente le onde armoniche piane unidimensionali per i fluidi dissipativi nell'ambito della termodinamica estesa irreversibile lineare. I risultati ottenuti per il modo acustico di propagazione sono raffrontati con i risultati sperimentali sulla dispersione e l'assorbimento del suono nei gas monoatomici.

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Research supported by C.N.R., G.N.F.M.     

The contact problem for a class of orthotropic elastic solids

The contact problem between two orthotropic solids is examined. The problem is solved by using Lodge's method, which permits the transformation of the boundary-value problem of an anisotropic solid to a form identical with the corresponding problem of an isotropic medium. The proposed solution is then compared with known results of certain cases and it is observed that it producesHertz's solution when used for an isotropic case,Lodge's solution when applied to contact between an orthotropic solid and a rigid plane and, finally,Love's solution if the solid is transversely isotropic with the axis of material symmetry perpendicular to the rigid plane of contact.     

Use of liquid chromatography–high‐resolution mass spectrometry for isolation and characterization of hydrolyzed fumonisins and relevant analysis in maize‐based products

The synthesis of partially hydrolyzed fumonisins (PHFB₁ and PHFB₂) and hydrolyzed fumonisins (HFB₁ and HFB₂) by chemical hydrolysis of pure fumonisins (FB₁ and FB₂) is reported together with the isolation and characterization by liquid chromatography–high‐resolution mass spectrometry (LC–HRMS). Two structural isomers of partially hydrolyzed forms of FB₁ and FB₂ were identified, namely PHFB_1a and PHFB_1b and PHFB_2a and PHFB_2b. Reaction yields were 21% for PHFB₁ (sum of the two isomers), 52% for HFB₁, 31% for PHFB₂ (sum of the two isomers) and 30% for HFB₂. Purity of each isolated compound was >98%. An LC–HRMS method for the simultaneous determination of fumonisins and their partially and totally hydrolyzed derivatives was applied to 24 naturally contaminated samples of maize and maize‐based products. The majority of samples (18 out of 24) were contaminated with fumonisins B₁ and B₂. Fumonisins co‐occurred with both partially hydrolyzed and hydrolyzed fumonisins in four nixtamalized samples (three masa flours and one tortilla chips). Co‐occurrence of fumonisins with partially hydrolyzed fumonisins was also recorded in one sample of maize kernels and four samples of maize‐based products (i.e. maize meal, cous‐cous, corn‐cakes and cornflakes). Mycotoxins levels ranged from 60 to 5700 µg/kg for fumonisins (sum of FB₁ and FB₂), from 10 to 210 µg/kg for partially hydrolyzed fumonisins (sum of PHFB₁ and PHFB₂) and from 30 to 200 µg/kg for hydrolyzed fumonisins (sum of HFB₁ and HFB₂). This is the first report of the isolation of PHFB₂ and the co‐occurrence of FB_1, FB₂, PHFB₁, PHFB₂, HFB₁ and HFB₂ in maize products. Considering the growing use of nixtamalized and maize‐based products, the monitoring of fumonisins and their partially and totally hydrolyzed forms in these products may represent an important contributing factor in evaluating the relevant human risk exposure. Copyright © 2014 John Wiley & Sons, Ltd.     

First results from DAMA/LIBRA and the combined results with DAMA/NaI

The highly radiopure ≃ 250 kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the INFN. In this paper the first result obtained by exploiting the model independent annual modulation signature for Dark Matter (DM) particles is presented. It refers to an exposure of 0.53 ton×yr. The collected DAMA/LIBRA data satisfy all the many peculiarities of the DM annual modulation signature. Neither systematic effects nor side reactions able to account for the observed modulation amplitude and to contemporaneously satisfy all the several requirements of this DM signature are available. Considering the former DAMA/NaI and the present DAMA/LIBRA data all together (total exposure 0.82 ton×yr), the presence of Dark Matter particles in the galactic halo is supported, on the basis of the DM annual modulation signature, at 8.2 σ C.L.; in particular, in the energy interval (2–6) keV, the modulation amplitude is (0.0131±0.0016) cpd/kg/keV and the phase and the period are well compatible with June 2 ^nd and one year, respectively.     

Analytical and numerical approaches to nonlinear galloping of internally resonant suspended cables

A study is carried out on nonlinear multimodal galloping of suspended cables. A consistent model of a curved cable-beam, geometrically nonlinear and able to torque, recently formulated by the authors, is used. The model accounts for quasi-steady aerodynamic forces, including the effect of static swing of the cable and dynamic twist of the cross-section. Complementary solution methods are employed, namely, finite-difference and Galerkin spatial discretization, followed by numerical time-integration, or Galerkin spatial discretization in conjunction with Multiple Scale perturbation analysis. The different techniques are applied to a cable close to the first cross-over point, at which a number of internal resonances exist. Branches of periodic solutions and their stability are evaluated as functions of wind velocity. The existence of branches of quasi-periodic solutions, originating from narrow unstable intervals and propagating elsewhere, is also proved. Qualitative and quantitative results furnished by the different investigation tools are compared among them, and the importance of the various components of motion, accounted or neglected in the reduced models, is discussed.