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Gracia-Fernandez C.,TA Instruments Waters Cromatografia | Gomez-Barreiro S.,University of La Coruna | Lopez-Beceiro J.,University of La Coruna | Naya S.,University of La Coruna | Artiaga R.,University of La Coruna
Journal of Thermal Analysis and Calorimetry

Interest in polymeric materials with dispersed nanotubes has increased in recent years. There are several methods to characterize this kind of dispersions that may be based on evaluating the percolation concentration, the "goodness" of the dispersion, or the matrix-nanotube interphase. Among other techniques, rheology and conductivity are used to this aim. Commonly, the oscillatory rheology measurements are performed within the linear viscoelastic range, which is achieved by operating at small amplitude oscillation shear. Nevertheless, these measurements do not fully describe the behavior of the dispersion structure. In this work, we propose the use of medium amplitude oscillation shear and large amplitude oscillation shear to characterize the dispersion/structure of a thermoplastic polyurethane matrix filled with multiwalled carbon nanotubes. The Ewoldt framework mathematical approach is used to analyze the non-linear stress response. That approach allows obtaining physically grounded magnitudes from the experimental data. These magnitudes allow for a better understanding of the effects of the filler content. © 2013 Akadémiai Kiadó, Budapest, Hungary. Source

Rios-Fachal M.,CPI Cruz Do Sar Bergondo | Gracia-Fernandez C.,TA Instruments Waters Cromatografia | Lopez-Beceiro J.,University of La Coruna | Gomez-Barreiro S.,University College Dublin | And 3 more authors.
Journal of Thermal Analysis and Calorimetry

It has been shown that introducing carbon nanotubes (CNT) into a polymer matrix has a beneficial effect on thermal stability of composites. While the specific effects noted differ depending on many parameters, the general trend is an increase in the degradation temperature and reduction in the degradation rate, quantified as the mass decline over time. The purpose of this study is to evaluate how CNTs influence the main degradation process of composites made with polystyrene containing 2, 3 and 5 % of CNTs. Thermogravimetric experiments are performed, with nitrogen purge, at multiple linear heating ramps. The effects of the nanotubes on the degradation of polystyrene are evaluated. Insightful kinetic parameters were obtained for the main process making use of a recently developed model, which is adapted to the thermogravimetric context. The model allows the means to separate the main process from other processes, which could interfere with the kinetic analysis, and also subtract the residual mass, which could produce an apparent stabilizing effect. The main degradation process is clearly stabilized by the presence of nanotubes, although the stabilization is more pronounced at the lowest of the filler contents considered. Clear effects of nanotubes on kinetic parameters were observed. © 2013 Akadémiai Kiadó, Budapest, Hungary. Source

Gracia-Fernandez C.,TA Instruments Waters Cromatografia | Gomez-Barreiro S.,CESUGA | Elmoumni A.,W L Gore and Associates | Alvarez A.,University of La Coruna | And 2 more authors.
Journal of Rheology

Orthogonal superposition (OSP) rheology is successfully used in this work to study an electrorheological (ER) fluid composed of a starch/silicone oil suspension. Interactions between hydrodynamic and polarization forces and other ER effects, such as the typical two-step yield stress observed when ER fluids are subjected to variable shear rates with simultaneous application of an electrical field, were evaluated. In order to conveniently investigate these effects, a specially designed geometry was mounted on a strain controlled rheometer. The OSP tests basically consisted of a continuous rotation and a simultaneous oscillation along the rotation axis (orthogonal to the rotation plane). On the other hand, by combining electro and OSP it is possible to evaluate how the electric potential influences the typical two-step yield stress of ER fluids in continuous rotation while the viscoelastic properties are simultaneously studied by OSP. Obtaining reliable storage and loss moduli in the axial direction while applying a rotational shear rate and an electric field allows to find out which part of the two-step yield corresponds to the elastic or viscous components. It is also observed that the moduli crossover varies linearly with the applied shear rate and there is a linear relation of the moduli crossover with the electric potential at which the crossover is observed. © 2016 The Society of Rheology. Source

Gracia-Fernandez C.A.,TA Instruments Waters Cromatografia | Gomez-Barreiro S.,University College Dublin | Alvarez-Garcia A.,University of La Coruna | Lopez-Beceiro J.,University of La Coruna | And 3 more authors.
Journal of Thermal Analysis and Calorimetry

Dielectric analysis (DEA) is a very sensitive technique, which allows for detection of small structural changes at the low scale. An advantage of DEA, with respect to other modulated techniques, is the possibility of using a wider frequency range. Molecular relaxations of the order of only a few nanometers are not observed by any other thermoanalytic method. Nevertheless, these small relaxations involve dipole changes that can be observed by DEA. Thus, this technique is used here, in combination with temperature-modulated differential scanning calorimetry (TMDSC) to obtain insightful information about the thermal transitions of poly-L-lactic acid (PLLA), one of the stereo-isomers of polylactide. Its complex thermal behavior is the subject of ongoing debate, with several overlapping crystallization and melting processes. The combined use of TMDSC and DEA provides a better insight of three important transitions of this polymer: the alpha relaxation, the enthalpic relaxation, and the cold crystallization. The dependences of the enthalpy relaxation on the dynamic glass transition relaxation and on the glass transition as a thermal event are evaluated. On the other hand, it will be shown how the cold crystallization can be identified by TMDSC, and DEA helps us understand the effect of crystallization on the dipole movements. The shape of the dielectric permittivity curve at low frequencies is compared to that of the reversing heat capacity to check whether both signals are sensitive or not to the same events. It is also verified how the experimental results of alpha relaxation of PLLA follow an Arrhenius or a Vogel trend. © Akadémiai Kiadó, Budapest, Hungary 2014. Source

Lopez-Beceiro J.,University of La Coruna | Fontenot S.A.,University of Oregon | Gracia-Fernandez C.,TA Instruments Waters Cromatografia | Artiaga R.,University of La Coruna | Chartoff R.,University of Oregon
Journal of Applied Polymer Science

A model describing the low-temperature crystallization kinetics observed for thermoplastic polymers from the melt by differential scanning calorimetry (DSC) was shown to accurately predict the cooling curves as a function of time and temperature. The model was successful for treating data for several cooling rates as well as for isothermal DSC data. In this article, we extended the model to cure reactions of thermosetting polymers. The parameters representing lower and upper exotherm reference temperatures in crystallization events have a different meaning for curing events. Thus, the model was modified to account for this change of context. The new model was tested for exothermic reactions of a Hysol® FP4527 epoxy adhesive system using data from DSC ramp heating experiments at several heating rates and also from isothermal experiments. Good fits were obtained for all the varied experimental conditions. The model made use of three fitting parameters with physical significance: a lower critical temperature (Tc) an activation energy (Eb), and a reaction order (τ + 1). Additionally, to complete the kinetic fitting, the dependence of the time to reach the reaction peak maximum for isothermal cure was considered. That dependence was found to follow a more simple model which is formally equivalent to that observed in isothermal crystallization, and which makes use of two parameters related to the limits of the temperature range in which the polymerization may occur. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40670. Copyright © 2014 Wiley Periodicals, Inc. Source

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