169,941 research outputs found
“Towards the New World: Transatlantic Connections and Ideological Boundaries in Dickens’s American Notes”
No full textProbing the cooperative and local modes of a poly(methyl acrylate) sample via ESR spectroscopy
No full textElectron spin resonance spectroscopy has been employed to analyze the probe rotational dynamics in a narrow distribution poly(methyl acrylate) melt. Dynamic regions have been found in the investigated temperature range and the coupling of the probe dynamics to the main and secondary polymer relaxations has been studied. The temperature dependence of the shear viscosity has been also measured, and, in the supercooled state of the host, it has been used to reveal the effects on the probe dynamics of both the local stiffness of the polymer and the occurrence of cooperativtity and dynamic heterogeneities in the matrix. Below T, the rotational dynamics follows activated regimes. (c) 2006 Elsevier B.V. All rights reserved
Dynamic crossovers and activated regimes in a narrow distribution poly(n-butyl acrylate): an ESR study
No full textThe rotational dynamics of the spin probe cholestane dissolved in a narrow distribution poly(n-butyl acrylate) sample has been investigated via electron spin resonance (ESR) spectroscopy. The measurements were carried out in a wide temperature range: different dynamic regions have been recognized, and the coupling of the probe dynamics to the a and secondary relaxations has been revealed. In particular, the coupling with the structural relaxation is ruled by two fractionary Vogel-Fulcher laws (VF). The crossover from one VF region to the other occurs at the temperature T-C = 1.17T(g), signalling the onset of the cooperativity in the dynamics and confirming a behaviour previously observed in ESR studies carried out on polymeric glass-formers. Furthermore, in this work we discuss the activated regime at the highest temperatures and show that the activation energy does not depend on the length of the polymer main- and side-chains, while its onset temperature linearly depends on the chain length
Influence of copper-phthalocyanine on the photodegradation of polycarbonate
No full textDyes and pigments are extensively used in polymer materials to confer colour-changing properties. However, these additives can significantly affect polymer stability against degradation. While the mechanism of stabilization of polymers by some pigments, such as carbon black, has been studied and is well known, the action of chromatic colorants, mainly in the sensitization of the degradation process, remains unclear. Cu-phthalocyanine dye can stabilize polymers against degradation as well as accelerate degradation in other situations. Cu-phthalocyanine incorporated into polycarbonate resulted in an acceleration of the degradation when the material was submitted to photochemical aging. The possible mechanism to explain the photodegradative behavior of PC containing Cu-phthalocyanine is based on the hypothesis that specific interactions among excited states of PC and Cu-phthalocyanine take place and enhance the formation of reactive species in polycarbonate. Excited states of Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in polycarbonate, increasing the formation of free radicals P-., which are the starting points for the sequential photo-oxidation reactions that lead to the degradation of the polycarbonate. Electron transfer sensitization is also a possible mechanism: the excited state of Cu-Ph abstracts an electron from PC to form the Cu-Ph radical anion and the PC radical cation. These reactive species in the presence of oxygen can cause oxidation of the aromatic ring. (c) 2006 Elsevier Ltd. All rights reserved
Structural relaxation of an unentangled polymer in terms of a simple phenomenological approach
No full textThe enthalpy relaxation mechanism of a low molecular weight synthesis of polymethylmethacrylate was investigated by means of calorimetric experiments. The data were analyzed in terms of a kinetic approach treating nonlinearity in a different manner with respect to the Tool-Narayanaswamy-Moynihan model. The relaxation isotherms recorded at four different temperatures were well reproduced by this approach that, however, failed in describing the relaxation asymmetry towards the equilibrium after opposite temperature jumps. A modification of the model was proposed with an additional free parameter accounting for the stretching of the relaxation function. In this way all the experimental data were reproduced fairly well
Effects of bismuth vandate and anthraquinone dye on the photodegradation of polycarbonate
No full textBoth inorganic and organic compounds, such as oxides or salts of metals and polycyclic and azo compounds, are frequently used as colorants in polymeric systems. Bismuth vanadate pigment has been used as an environmentally friendly alternative for cadmium containing pigments and anthraquinone dyes represent a polycyclic colorant class of wide use in polymers. Besides their coloring properties, both bismuth vanadate and anthraquinone present photocatalytic activity or photochemical properties that can influence the mechanism and the kinetics of the photodegradation of the polymer into which they are incorporated. In this paper the influence of bismuth vanadate pigment and anthraquinone dye on the photodegradation of polycarbonate was evaluated. For this purpose, samples of polycarbonate containing the colorants were submitted to photochemical aging following the recommendations of ASTM G53. Bismuth vanadate accelerates the photodegradation of polycarbonate, causing a faster drop in the mechanical properties and an increase in the hydroperoxide concentration during the aging process. The coloring ability of the pigment is also affected. The photodegradation behavior of polycarbonate in presence of bismuth vanadate may be caused by a possible direct chemical reaction between pigment and polymer, promoted by UV radiation. On the other hand, the presence of anthraquinone dye seems to stabilize the polycarbonate against photodegradation through a deactivation mechanism of the excited state
Dynamic regimes and crossovers in nearly monodisperse polyacrylates: influence of the polymeric side groups as detected by Electron Spin Resonance experiments with molecular probes
No full textRisultati preliminari di uno studio multicentrico, randomizzato e controllato, con silibina+fosfatidilcolina+vitamina E vs placebo in pazienti con steatoepatite non alcolica.
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