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Molecular-weight dependence of enthalpy relaxation of PMMA
No full textThe effects of the molecular weight on the physical aging of polymers were investigated by means of differential scanning calorimetry (DSC) experiments in five PMMA samples with mass values across the entanglement mass M,. The Tool-Narayanaswamy-Moynihan (TNM) model and a recent configurational entropy approach were systematically compared with the experiments. For the samples with the lowest molecular weights, the two approaches exhibited similar agreement with experiments; however, in the systems with higher molecular weights, the TNM model met major difficulties, and a clear improvement was obtained with the other model. The additional parameter introduced by the entropic model showed a strong molecular-weight dependence, with a sharp increase at the entanglements mass of PMMA. These results evidence the role of the chain entanglements on the physical aging of polymers. Finally, the influence of the molecular weight on the fragility was investigated and related to the recent literature debates on this topic
Crossover region and entanglement in nearly monodisperse poly(ethyl acrylates) studied with electron spin resonance spectroscopy
No full textThe topic of dynamic changes undergone by glass-forming materials in the supercooled region is addressed in this study. Crossover regions and temperatures are generally considered as key features in order for the glass transition phenomenon to be understood. The attention is here focused on the crossover region of polymers and its dependence on the polymeric entangled dynamics. To avoid the superposition of possible dependence on the polymeric polydispersity, nearly monodisperse syntheses of poly(ethyl acrylates) have been used. Rotational dynamics have been investigated with electron spin resonance spectroscopy, dissolving the cholestane molecular tracer in a poly( ethyl acrylate) (M-n = 7500 amu). Comparison is carried out with the findings obtained in the case of an almost monodisperse poly(ethyl acrylate) with M-n = 58 200 amu. Different dynamic regimes and crossover regions were recognized in the temperature dependence of the molecular rotation. The crossover temperatures T-c were found to be dependent on the molecular weight. Moreover, the dynamics in non-Arrhenius regions were satisfactorily described as a fractionary law of structural relaxation
Enthalpy relaxation in polymers: A comparison among different multiparameter approaches extending the TNM/AGV model
No full textThe enthalpy recovery mechanism of polymers is usually described in terms of the Tool-Narayanaswamy-Moynihan model (TNM). Even if it is able to qualitatively reproduce all the peculiar features of the aging process, clear discrepancies have been found in quantitative analysis. In this work, differential scanning calorimetry experiments are performed in two different polymers, and data are used in order to compare the predictions of a recently developed configurational approach with possible extensions of the TNM model. All the approaches add a free parameter with respect to the original TNM model. In this study we have found that appreciable improvements between experimental and calculated DSC traces can be observed if the configurational entropy approach is adopted. Furthermore, the long-term annealing experiments performed in one of the polymers clearly support the basic hypothesis of the entropic approach concerning the limit state of the structural relaxation
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
Probing 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
Connecting shear stress relaxation and enthalpy recovery in polymers through a modified TNM approach
No full textThe Tool-Narayanaswamy-Moynihan model (TNM) is widely accepted in order to describe the enthalpy recovery mechanism of polymers. However, some problems have been found in quantitative analysis as far as the parameters of the model are concerned. In particular, in several studies, the values of the Kauzmann temperature have been obtained 150 K or more below the glass transition, whereas other works have provided values of the activation energy of the relaxation times at the glass transition that are inconsistent with viscoelastic or dielectric measurements. In this paper, we analyze differential scanning calorimetry experiments in a low molecular weight PMMA sample in terms of a modified TNM model, where the adjustable parameters only provide the behaviour of the out-of-equilibrium relaxation times. The temperature dependence of the equilibrium relaxation times is instead set at the values determined by the shear viscosity data of the sample. A good agreement between theory and experiments is obtained suggesting that the difficulties quoted above could actually depend on the TNM handling of the nonlinear effects
Azobenzene polymers for nano-writing: structural properties, relaxation processes and dynamic heterogeneities.
No full textInfluence 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
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