Confinement-induced vitrification in polyethylene terephthalate

9 págs.; 5 figs.; 1 app; PACS number s : 61.82.Pv, 61.82.Rx, 64.70.Pf, 05.20. y; Erratum Phys. Rev. B 79, 019903 (2009)Dynamic mechanical thermal analysis performed on cold-drawn polyethylene terephthalate (PET), cold crystallized (annealed) in the temperature interval 100-140°C, reveals the presence of marginally glassy domains above the annealing temperature Ta. This suggests that the thermodynamic force driving crystallization causes the structural arrest of some noncrystalline domains. The latter thus need a temperature higher than Ta to completely defreeze. Differential scanning calorimetry supports this point of view. Analogous investigations on unoriented PET, cold crystallized in the same conditions, do not show the same peculiarities; thus, chain orientation is relevant to vitrification. This phenomenology is first cast in the language of thermodynamics by introducing an excess chemical potential δμ describing the presence of structural constraints in the amorphous domains and the effect of chain orientation. For a first test of this picture, the orientation contribution to δμ is calculated by means of the Gaussian chain model (this implicitly assumes that δμ is related to the density fluctuations). The resulting expression is then used to discuss the structural differences between cold-drawn and unoriented PET samples reported in the literature. © 2007 The American Physical Society.Peer Reviewe

Dispositivo portamuestras para medidas simultáneas utilizando radiación de sincrotrón

Referencia OEPM: P200001388.-- Fecha de solicitud: 31/05/2000.-- Titular: Consejo Superior de Investigaciones Científicas (CSIC).Dispositivo portamuestras para medidas simultáneas utilizando radiación de sincrotrón (ver figura en archivo de texto adjunto). La presente invención describe el desarrollo de un nuevo prototipo de portamuestras para utilización simultánea en una línea de difracción de rayos-X utilizando la para detección simultánea de dispersión en ángulos altos y bajos y espectroscopia dieléctrica. El prototipo consiste en: (i) dos bloques metálicos con cuatro resistencias calefactoras empotradas que permiten calentar los bloques metálicos y la muestra hasta 350ºC. Uno de los bloques esta atravesado por un tubo en forma de U por el cual circula aire de refrigeración y que al mismo tiempo conecta el bloque principal del portamuestras con la tapa; (ii) dos discos metálicos con orificios efectuados en el centro (electrodos); (iii) dos discos finos de aluminio; (iv) una placa superior (tapa), para cerrar el horno donde se introduce el portamuestras, con las correspondientes conexiones para el cableado.Peer reviewe

Micromechanical studies of styrene/butadiene block copolymer blends

The microhardness behavior of binary blends comprising a star block and a triblock copolymer, both consisting of polystyrene (PB) and polybutadiene (PB), was investigated over a wide composition range. In particular, a correlation between the microhardness H, the yield stress a y and the Young's modulus Ε was examined. The hardness was found to correlate with the mechanical parameters obtained by uniaxial tensile testing as follows: H/ay ~ 2 and Ε/Η ~ 30. In agreement with the studies performed in case of the pure microphase separated block copolymers, the microhardness behavior was found to be strongly dependent on the morphology of the blends. The glass transition temperature of the hard phase (Tg.PS) is shown to remain nearly constant in the blends with lower LN4 content. The glass transition temperature of the soft phase (Tg.PB), which varies with blend composition, is not related to the microhardness variation in the blends.Peer reviewe

Study of the morphology of semicrystalline poly(ethylene terephthalate) by hydrolysis etching

Semicrystalline poly(ethylene terephthalate) was hydrolysed in water at 180°C under elevated pressure and subsequently treated with ethanol, following the etching process first developed by Miyagi and Wunderlich. The weight loss, the wide-angle X-ray scattering and the molecular weight were measured as a function of etching time. It was found that even at the end of the etching process not all the amorphous material could be removed by the hydrolysis treatment. By comparing the obtained results with those derived from an elaborate small-angle X-ray scattering study and with wide-angle X-ray scattering measurements, it was concluded that only those amorphous regions lying outside of the lamellar stacks were removed. Subsequently, the lamellar stacks themselves were attacked. It was also found that at the very beginning of the hydrolysis process additional crystals were formed in the material.Peer reviewe

Simultaneous crystalline-amorphous phase evolution during crystallization of polymer systems

Despite the fact that polymer crystallization has been the object of intense research, this process is far from being fully understood. Traditional polymer crystallization studies using X-ray scattering techniques mainly provide information about the ordered regions. To obtain a more complete information about the time evolution of both the crystalline and the amorphous phase during polymer crystallization, we have developed a technique which allows one to obtain information about both the crystalline and the amorphous phase simultaneously. We report here simultaneous information about three key aspects of the isothermal polymer crystallization process: i) polymer chain ordering, through Wide-Angle X-ray Scattering; ii) lamellar crystals arrangement, through Small-Angle X-ray Scattering; iii) amorphous phase evolution through dielectric spectroscopy. Our results probe that during primary crystallization the average mobility of the amorphous phase is not notably affected. Upon passing through the crossover time, marking the transition from primary to secondary crystallization, the restriction to the mobility of the amorphous phase mainly occurs in the regions between the lamellar stacks regions and not in the amorphous regions within the lamellar stacks. We hypothesize that molecular mobility in the amorphous regions located between consecutive crystals become strongly arrested as soon as the lamellar stack is formed. © EDP SciencesMCYT (grants FPA2001-2139, BMF2000-1474), Spain. MEC “Científicos y Tecnólogos Extranjeros” program. IHP-Contract HPRI-CT-1999-00040 of the European Commission (EC(ERBFMGEDT 950059) and II-00-015 EC).Peer Reviewe

Molecular dynamics of ferroelectric polymeric systems as studied by incoherent quasielastic neutron scattering

11 págs.; 15 figs.We have studied by incoherent quasielastic neutron scattering the molecular dynamics of the ferroelectric copolymers of vinylidene fluoride and trifluoroethylene with two different molar ratios. Thermal hysteresis effects accompanying the structural changes observed in these materials are found. In the temperature range investigated three motions were identified: (i) a vibrational motion of Debye-Waller type which shows increasing amplitude until the vicinity of the Curie temperature and then it follows a Q4 law, (ii) a slow jump diffusive motion of each chain in the restricted volume defined by other chains, thermally activated with an activation barrier of 8 kcal/mol, and (iii) a fast local diffusive motion with only some fraction of the chains concerned. © 1994 The American Physical Society.Grateful acknowledgments is due to DGICYT (Grant No. PB91-0355), to CICYT (Grant No. MAT92-148), to CICYT (Grant No. MAT94-0740E), to the Consejeria de Educacion de la Comunidad de Madrid (Grant No. 284/92) and to Fundacion Domingo Martinez (Grant No. 1992/93) for the generous support of this investigation.Peer Reviewe

Role of a Compatibilizer in the Structure and Micromechanical Properties of Recycled Poly(ethylene terephthalate)/Polyolefin Blends with Clay

The comparison of the degree of crystallinity and the micromechanical properties in the blends of recycled amorphous poly(ethylene terephthalate) (PET)with isotactic polypropylene (iPP) and high-density polyethylene (HDPE) with a compatibilizer in different proportions is reported. The physical study of the composites of the compatibilized blends and clay is also discussed. The analysis, performed by means of wide-angle X-ray scattering and differential scanning calorimetry techniques, permits us to describe, at microscale level, the role of the compatibilizer on the structure and microhardness of the polymer blends that we studied. The results reveal that PET was incompatible with both iPP and HDPE. However, the presence of the compatibilizer, a styrene–ethylene/butylene–styrene block copolymer grafted with maleic anhydride, allowed the compatibilization of these polymers. In the PET/iPP blends, the clay seemed to have a nucleating effect on the iPP and also induced a hardness increase in the compatibilized blends. On the other hand, in case of PET/HDPE, the crystallinity of these samples (pure blends,blends with compatibilizer, and blends with compatibilizer plus clay) only depended on their composition. Similarly to the PET/iPP blends, the addition of clay induced an increase in the hardness of the ompatibilized blends.Peer reviewe

Molecular dynamics of the α relaxation during crystallization of a glassy polymer: A real-time dielectric spectroscopy study

9 págs.; 12 figs.; 1 tab.The change of the α relaxation process occurring during isothermal crystallization of a glassy polymer, poly(aryl ether ketone ketone), has been followed in real time through measurement of the dielectric complex permittivity. The dielectric strength has been correlated with the degree of crystallinity derived from real time wide angle x-ray patterns using synchrotron radiation. The phenomenological Havriliak-Negami description has been used to analyze the changes of the dielectric strength, central relaxation time and shape parameters describing the relaxation, as crystallization proceeds. The evolution of the dielectric magnitudes with crystallization time has been interpreted in the light of the Schönhals-Schlosser model. According to this model, a restriction of the long scale motions of the polymeric chains as the material is filled in with crystals is observed. The derived dipole moment time correlation functions have been calculated for different crystallization times and fitted by the Kohlrausch-Williams-Watts function. The variation of the stretching parameters with crystallization time can be interpreted in terms of the coupling model assuming an increase of the intermolecular cooperativity of the α relaxation as the polymeric chains are constrained to move between crystalline regions. These results offer a contribution to the understanding of molecular dynamics of a glassy polymers during crystallization. © 1994 The American Physical Society.The authors are indebted to the CICYT (Grant No. MAT 94-0740E), Spain, for generous support of this investigation. J.M. thanks the Tempus Program (JEP 0644) for the support of his investigations.Peer Reviewe

Relationship between Nanostructure and Deformation Behavior of Microphase-Separated Styrene/Butadiene Systems

The relationship between the morphology and the mechanical properties of nanostructured blends,comprising an asymmetric styrene/butadiene star block copolymer,exhibiting cocontinuous-like morphology and low molar mass homopolymers, was studied by electron microscopy, microindentation hardness, and tensile testing methods. Results show that the deformation behavior of these systems is significantly modified by the presence of unentangled homopolymer chains, leading partly to a decrease in mechanical properties. In contrast to common polymer blends, in which usually, the hardness values do not markedly deviate from the additivity law, the H-values in the system investigated show large deviations from the linear additivity behavior. The observed anomalous behavior is discussed in terms of the enhanced local flow processes induced by the phase-separated morphology at a nanometer scale.Peer reviewe

Aromatic polymers obtained by precipitation polycondensation: 4*. Synthesis of poly(ether ketone ketone)s

High molecular weight aromatic poly(ether ketone ketone)s were synthesized by the Friedel-Crafts polyacylation condensation of iso- and terephthaloyl chlorides with diphenyl ether, 1,4- and 1,3-bis(4- phenoxybenzoyl)benzenes. Depending on the monomers used for polycondensation, polyketones of regular structure with different iso-/tereisomer repeating unit ratio (100/0, 50/50, 0/100) in the main chain were obtained. Polymers of each repeating isomer unit were prepared in two different ways. All the polymer syntheses were performed as precipitation polycondensations and the resulting polymers were obtained in particle form. The influence of reaction conditions and preparation route on the polymer properties were examined. The monomer concentration and monomer stoichiometric ratio were found to affect the polymer viscosity. The size and shape of the polyketone particles obtained were also found to be governed by reaction conditions and preparation route.Peer reviewe