S. Gómez-Barreiro

Study of gypsum by PDSC

Calcium sulfate dihydrate has been widely characterized by both differential scanning calorimetry and thermogravimetry (TG). Two dehydration processes were reported to be partially overlapping. High resolution TG and water vapor self generated atmosphere pin-hole lid strategies were used to increase the resolution of both dehydration processes. In this study, isobaric experiments were carried out in a pressure differential scanning calorimetry cell. The approach consisted in combining the pin-hole lid with different pressures with nitrogen atmosphere. Resolution was improved at moderately low pressures. At higher pressures other processes were observed.

Simultaneous application of electro and orthogonal superposition rheology on a starch/silicone oil suspension

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 ap...

Electrorheological behaviour of a starch-oil system

A simple system consisting of a suspension of 10 wt % of starch in silicone oil was subjected to electrorheological testing. The system exhibits a complex behaviour depending on the electrical field. On one hand, the application of an external electric field induced the formation of a linear structure, aligned with the electric field. The formation of that structure was studied by several rheological methods. The rate of modifying the electric field intensity resulted to be related to the electric field value at which percolation is observed. On the other hand, master curves for the structure formation and breakdown in squeeze experiments were obtained.

Characterization of MWCNT/TPU systems by large amplitude oscillation shear

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.

Thermal characterization of poly- l -lactide by dielectric analysis and modulated DSC

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.