Kinetic Analysis of the Thermal Degradation of Polystyrene-Montmorillonite Nanocomposite

Nanocomposites exhibit a combination of unique properties, such as increased heat distortion temperature, reduced permeability, reduced flammability and improved mechanical properties. In this work, a polystyrene (PS) clay nanocomposite was prepared via bulk polymerization using a novel organically modified montmorillonite (MMT). The organic-modifier is the N,N-dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium chloride (VB16). The thermal stability of PS–VB16 compared to pure PS is examined in pyrolytic and thermo-oxidative conditions. It is then studied using a kinetic analysis. It is shown that the stability of PS is significantly increased in the presence of clay. The thermal behavior of PS and PS nanocomposite is modeled and simulated. A very good agreement between experimental and simulated curves both in dynamic and isothermal conditions is observed. Using kinetic analysis associated to the reaction to fire of PS nanocomposite simulated in a cone calorimeter, the peak of heat release rate is half that of virgin PS, it is suggested that the clay acts as a char promoter slowing down the degradation and providing a protective barrier to the nanocomposite. The combination of these two effects is an important factor lowering the HRR

The extent of NGC 6822 revealed by its C stars population

Using the CFH12K camera, we apply the four band photometric technique to identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C stars, outside of this area were also discovered with the Las Campanas Swope Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading to an average absolute I magnitude of -4.70 mag, a value essentially identical to the mean magnitude obtained for the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC 6822, C stars are seen at large radial distances and trace a huge slightly elliptical halo which do not coincide with the huge HI cloud surrounding NGC6822. The previously unknown stellar component of NGC 6822 has a exponential scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.Comment: accepted, to be published in A

A generalized multi-polaron expansion for the spin-boson model: Environmental entanglement and the biased two-state system

We develop a systematic variational coherent state expansion for the many-body ground state of the spin-boson model, in which a quantum two-level system is coupled to a continuum of harmonic oscillators. Energetic constraints at the heart of this technique are rationalized in terms of polarons (displacements of the bath states in agreement with classical expectations) and antipolarons (counter-displacements due to quantum tunneling effects). We present a comprehensive study of the ground state two-level system population and coherence as a function of tunneling amplitude, dissipation strength, and bias (akin to asymmetry of the double well potential defining the two-state system). The entanglement among the different environmental modes is investigated by looking at spectroscopic signatures of the bipartite entanglement entropy between a given environmental mode and all the other modes. We observe a drastic change in behavior of this entropy for increasing dissipation, indicative of the entangled nature of the environmental states. In addition, the entropy spreads over a large energy range at strong dissipation, a testimony to the wide entanglement window characterizing the underlying Kondo state. Finally, comparisons to accurate numerical renormalization group calculations and to the exact Bethe Ansatz solution of the model demonstrate the rapid convergence of our variationally-optimized multi-polaron expansion, suggesting that it should also be a useful tool for dissipative models of greater complexity, as relevant for numerous systems of interest in quantum physics and chemistry.Comment: 17 pages, 14 figure

Impact of disorder on unconventional superconductors with competing ground states

Non-magnetic impurities are known as strong pair breakers in superconductors with pure d-wave pairing symmetry. Here we discuss d-wave states under the combined influence of impurities and competing instabilities, such as pairing in a secondary channel as well as lattice symmetry breaking. Using the self-consistent T-matrix formalism, we show that disorder can strongly modify the competition between different pairing states. For a d-wave superconductor in the presence of a subdominant local attraction, Anderson's theorem implies that disorder always generates an s-wave component in the gap at sufficiently low temperature, even if a pure d_{x^2-y^2} order parameter characterizes the clean system. In contrast, disorder is always detrimental to an additional d_{xy} component. This qualitative difference suggests that disorder can be used to discriminate among different mixed-gap structures in high-temperature superconductors. We also investigate superconducting phases with lattice symmetry breaking in the form of bond order, and show that the addition of impurities quickly leads to the restoration of translation invariance. Our results highlight the importance of controlling disorder for the observation of competing order parameters in cuprates.Comment: 13 pages, 10 figure

Security of Quantum Bit-String Generation

We consider the cryptographic task of bit-string generation. This is a generalisation of coin tossing in which two mistrustful parties wish to generate a string of random bits such that an honest party can be sure that the other cannot have biased the string too much. We consider a quantum protocol for this task, originally introduced in Phys. Rev. A {\bf 69}, 022322 (2004), that is feasible with present day technology. We introduce security conditions based on the average bias of the bits and the Shannon entropy of the string. For each, we prove rigorous security bounds for this protocol in both noiseless and noisy conditions under the most general attacks allowed by quantum mechanics. Roughly speaking, in the absence of noise, a cheater can only bias significantly a vanishing fraction of the bits, whereas in the presence of noise, a cheater can bias a constant fraction, with this fraction depending quantitatively on the level of noise. We also discuss classical protocols for the same task, deriving upper bounds on how well a classical protocol can perform. This enables the determination of how much noise the quantum protocol can tolerate while still outperforming classical protocols. We raise several conjectures concerning both quantum and classical possibilities for large n cryptography. An experiment corresponding to the scheme analysed in this paper has been performed and is reported elsewhere.Comment: 16 pages. No figures. Accepted for publication in Phys. Rev. A. A corresponding experiment is reported in quant-ph/040812

Stabilizing Spin Coherence Through Environmental Entanglement in Strongly Dissipative Quantum Systems

The key feature of a quantum spin coupled to a harmonic bath---a model dissipative quantum system---is competition between oscillator potential energy and spin tunneling rate. We show that these opposing tendencies cause environmental entanglement through superpositions of adiabatic and antiadiabatic oscillator states, which then stabilizes the spin coherence against strong dissipation. This insight motivates a fast-converging variational coherent-state expansion for the many-body ground state of the spin-boson model, which we substantiate via numerical quantum tomography.Comment: 5 pages, 3 figures, supplementary file attached. This article supersedes arXiv:1301.743

Investigation of nanodispersion in polystyrene-montmorillonite nanocomposites by solid state NMR

Nanocomposites result from combinations of materials with vastly different properties in the nanometer scale. These materials exhibit many unique properties such as improved thermal stability, reduced flammability, and improved mechanical properties. Many of the properties associated with polymer–clay nanocomposites are a function of the extent of exfoliation of the individual clay sheets or the quality of the nanodispersion. This work demonstrates that solid-state NMR can be used to characterize, quantitatively, the nanodispersion of variously modified montmorillonite (MMT) clays in polystyrene (PS) matrices. The direct influence of the paramagnetic Fe3, embedded in the aluminosilicate layers of MMT, on polymer protons within about 1 nm from the clay surfaces creates relaxation sources, which, via spin diffusion, significantly shorten the overall proton longitudinal relaxation time (T1 H). Deoxygenated samples were used to avoid the particularly strong contribution to the T1 H of PS from paramagnetic molecular oxygen. We used T1 H as an indicator of the nanodispersion of the clay in PS. This approach correlated reasonably well with X-ray diffraction and transmission electron microscopy (TEM) data. A model for interpreting the saturation-recovery data is proposed such that two parameters relating to the dispersion can be extracted. The first parameter, f, is the fraction of the potentially available clay surface that has been transformed into polymer–clay interfaces. The second parameter is a relative measure of the homogeneity of the dispersion of these actual polymer–clay interfaces. Finally, a quick assay of T1 H is reported for samples equilibrated with atmospheric oxygen. Included are these samples as well as 28 PS/MMT nanocomposite samples prepared by extrusion. These measurements are related to the development of highthroughput characterization techniques. This approach gives qualitative indications about dispersion; however, the more time-consuming analysis, of a few deoxygenated samples from this latter set, offers significantly greater insight into the clay dispersion. A second, probably superior, rapid-analysis method, applicable to oxygen-containing samples, is also demonstrated that should yield a reasonable estimate of the f parameter. Thus, for PS/MMT nanocomposites, one has the choice of a less complete NMR assay of dispersion that is significantly faster than TEM analysis, versus a slower and more complete NMR analysis with sample times comparable to TEM, information rivaling that of TEM, and a substantial advantage that this is a bulk characterization method. © 2003 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys 41: 3188–3213, 200

De l'analyse des contraintes à la conception d'un système d'évaluation des performances d'un périmètre collectif irrigué : Fatnassa Nord, Kébili, Tunisie

International audienceCette communication expose la méthode suivie pour mettre en place un système d'évaluation des performances d'un périmètre collectif irrigué, appliquée à une oasis au sud tunisien. Après avoir proposé une restructuration des déterminants de dysfonctionnement et des variables identifiées en relation avec le fonctionnement actuel du périmètre irrigué, on y expose la procédure d'acquisition des données. Le diagramme de classes distingue quatre ensembles de classes relatives aux exploitations agricoles, aux systèmes de culture, aux contraintes du milieu physique et aux irrigations. Les variables ont été renseignées et validées sur la base de mesures, d'observations de terrain, d'enquêtes et d'exploitation des données disponibles (plans parcellaires, rôle et feuilles du suivi journalier du tour d'eau). L'exploitation préliminaire des données montre, en premier lieu, un dysfonctionnement systématique du tour d'eau au cours de l'année agricole 2006-2007 où sa fréquence en saison estivale reste entre 70 et 30 jours pour les trois antennes d'irrigation. Les principaux déterminants de ces dysfonctionnements sont premièrement la surface irriguée qui est 1,12 fois supérieure à la surface officielle sur la base de laquelle est calculée la durée théorique du tour d'eau ; et deuxièmement la durée d'irrigation à la parcelle où la durée théorique fixée à 10 h/ha a été respectée pour seulement 12,84 % des parcelles élémentaires irriguées sur l'oasis. L'étude montre, ensuite, que la performance des palmeraies, exprimée en rendement du palmier dattier, présente une grande diversité et que le rendement total en dattes reste inférieur à 3 t/ha pour 40,88 % des parcelles irriguées et que seulement 34,28 % des parcelles ont un rendement supérieur à 10 t/ha. La base de données sera exploitée pour analyser et évaluer les déterminants de la performance des irrigations et des dysfonctionnements du tour d'eau, d'une part, et les déterminants de la performance des systèmes de cultures oasiens, d'autre part, tout en élaborant des indicateurs de performance du fonctionnement du périmètre oasien

Diagnostic et analyse du fonctionnement d'un périmètre oasien. Cas de l'oasis de Fatnassa Nord, Kébili, sud tunisien

International audienceLes performances de l'oasis de Fatnassa sont sous la dépendance d'un ensemble de contraintes physiques et de dysfonctionnements liés aux pratiques et comportements individuels face à la gestion collective de l'aménagement. Ces contraintes et dysfonctionnements ont été abordés sous l'angle de leur perception par les agriculteurs. Le fonctionnement du périmètre a ensuite été décrit sous la forme d'un arbre des problèmes permettant de synthétiser les principales causes et les principaux effets de ces contraintes sur les performances des systèmes de culture. Malgré les investissements réalisés pour l'accroissement des capacités et la modernisation du système d'irrigation, l'allongement de la durée de retour du tour d'eau limite les possibilités d'intensification et les performances des systèmes de culture basés sur le palmier dattier. Ces dysfonctionnements sont essentiellement la conséquence de pratiques inadaptées des agriculteurs, caractérisées par : une extension continue des superficies irriguées à la périphérie de l'oasis, la mauvaise qualité de l'entretien des parcelles et de la conduite des irrigations résultant d'une implication limitée de propriétaires non résidents ou pluriactifs, et parfois l'allongement délibéré de la durée et de la dose d'irrigation en réaction aux contraintes imposées par le tour d'eau. Les effets de la dégradation de la qualité des terres ou des dysfonctionnements du système de drainage et de l'engorgement sont aussi facilement perçus par les agriculteurs qui tentent de s'y adapter par la pratique d'amendements sabloorganiques associés au travail du sol. La température élevée de l'eau d'irrigation - en partie géothermale - provoque la mortalité de certains jeunes palmiers et la disparition progressive de la strate arbustive. A l'inverse, les effets d'une salinité partout présente et depuis longtemps ne sont pas directement appréhendés, notamment en raison de la tolérance du palmier dattier. Les effets éventuels de la salinité apparaissent masqués par les autres contraintes avec lesquels elle interagit, en accentuant l'effet du stress hydrique pendant la période estivale, en favorisant la dégradation des sols, et en aggravant l'engorgement par des irrigations abondantes pendant la période hivernale