Kinetics and Mechanistic Investigation of Hydrotalcite‐Catalyzed Melt Synthesis of Poly(ethylene terephthalate)

Summary: Hydrotalcite‐catalyzed polycondensation of BHET was studied by thermogravimetry to elucidate the kinetics. The reaction was found to follow second‐order kinetics with respect to hydroxyl end groups. The overall activation energy of the polycondensation was found to decrease with increasing catalyst concentration before it leveled out to the value of 93 kJ · mol⁻¹ at high catalyst concentration. This is a result of the uncatalyzed reaction that takes place parallel to the catalyzed one. The activation energy of the uncatalyzed path was found to be 156 kJ · mol⁻¹. IR spectroscopy and XRD showed that the monomer intercalates between the layers of hydrotalcite at the beginning of the reaction enabling the complexation of oxygen‐containing functional groups with the hydroxide groups of the catalyst. Based on these findings a polycondensation mechanism is proposed. One end group of the monomer is activated in the form of alkoxide that counterbalances the positive charge of the hydrotalcite layer. This alkoxide group attacks an ester carbonyl group fixed close to it, generating a new ester bond and a glycoxide species. The role of hydrotalcite is to activate the reactants, rendering the attacking hydroxyl group more nucleophilic and the ester carbonyl group more electrophilic, and at the same time fixing the reactant together in a favorable geometry.

Intercalation of BHET in the gallery of HT layers in a twisted alkoxide form.     

Development of a thermogelling ophthalmic formulation of cysteine

Preliminary studies carried out with cysteine 2% solution showed that pH adjusted to isoelectrical pH (i.e., 4.9) led to enhance stability during autoclaving and ensured no significant degradation during at least 14 days if stored at 2-8°C protected from light. Optimized formulations combined either cysteine(2%)/Poloxamer407(16.5%) or cysteine(2%)/Poloxamer407(20%)/Poloxamer188(5%) and were characterized by an adequate temperature of gelification (TG) (25.9°C and 26.9°C, respectively), an important gel strength (5.1daN and 5.3daN, respectively) and a drastic increase in the apparent viscosity between 24°C and 32°C (multiplication factor of 78 and 77-fold, respectively). Cysteine addition produced only slight but significant decrease in temperature of gelification and increase in gel strength.     

Occurrence of mycotoxins (ochratoxin A, deoxynivalenol) and toxigenic fungi in Moroccan wheat grains: impact of ecological factors on the growth and ochratoxin A production

The aim of the present work was to evaluate the contamination of some samples, taken from Moroccan wheat grains, by ochratoxin A (OTA), deoxynivalenol (DON) and the associated toxigenic fungi. Moreover, we focused on the influence of environmental factors on both the growth and OTA production by three strains of Aspergillus. The results showed that only few samples were contaminated by the two mycotoxins (2 samples for OTA and 7 for DON). The main isolated fungi belong to the Aspergillus, Penicillium and Fusarium genus; 74 Aspergillus and 28 Penicillium isolates were tested for their ability to produce OTA. Only 2 A. alliaceus and 14 A. niger were able to synthesize OTA. However, none of Penicillium isolates can produce this toxin under the conditions mentioned. In respect of the effects of the temperature and water activity (aw), the optimal conditions for the growth and OTA production were different. While the optimal conditions of growth for A. alliaceus and A. terreus are 30 degrees C and 0.98 aw, A. niger preferred 0.93-0.95 aw at 25 degrees C, whereas the optimal production of OTA was observed at 30 degrees C for both A. alliaceus and A. niger at 0.93 and 0.99 aw, respectively.     

Pore network model of the cathode catalyst layer of proton exchange membrane fuel cells: Analysis of water management and electrical performance

A pore network modeling approach is developed to study multiphase transport phenomena inside a porous structure representative of the Cathode Catalyst Layer (CCL) of Proton Exchange Membrane Fuel Cell. A full coupling between two-phase transport, charge transport and heat transport is considered. The liquid water evaporation is also taken into account. The current density profile and the liquid water distribution and production are investigated to understand the liquid production mechanism inside the CCL. The results suggest that the wettability and the pore size distribution have an important impact on the water management inside the cathode catalyst layer and thus on the performances of the proton exchange membrane fuel cell. Simulations show also that Bruggemann correlation used in classical models does not predict correctly gas diffusion.     

Microstructure of porous Cu fabricated by freeze-drying process of CuO/camphene slurry

Porous Cu with macroscopically aligned channels was synthesized using a freeze-drying process.Camphene-based CuO slurry was prepared by milling at 60 °C with a small amount of dispersant.Freezing of a slurry was done at 25 °C while unidirectionally controlling the growth direction of the camphene.Pores were generated subsequently by sublimation of the camphene during drying.The green body was hydrogen-reduced at 300 °C for 30 min,and sintered in the furnace at 700 °C for 1 h under a hydrogen atmosphere.Microstructural observation reveals that all of the sintered samples are composed of only Cu phase and show macroscopic open pores with an average size of 100 μm which are aligned along its macroscopic growth direction.The internal wall of the macroscopic aligned pore shows relatively small pores due to the traces of the camphene left between the concentrated Cu particles on the internal wall.Increase in the porosity and pore size with increasing camphene content was explained by the change of the growth behavior of the camphene crystals.     

Biohydrogen production by gas phase reforming of glycerine and ethanol mixtures

In this paper the steam reforming of bioalcohols over Ni and Pt catalysts supported on bare Al₂O₃ and La₂O₃ and CeO₂-modified Al₂O₃ to produce H₂ was studied. Catalytic activity results showed that the glycerine and the intermediate liquid products may hinder the ethanol adsorption on metal active sites of the catalysts, especially at temperatures below 773 K. In fact, ethanol conversion was lower than glycerine conversion in the steam reforming reaction at low temperatures. H₂ chemisorption revealed that La₂O₃ doping of the Ni/Al₂O₃ catalyst improved the metal dispersion providing a better behaviour to the Ni/Al₂O₃-O2 catalyst towards H₂ production. In the case of Pt catalysts, the good reducibility and the H₂ spillover effect provided to the Pt/Al₂O₃-O1 catalyst the capacity to produce higher H₂ yields.     

Interface structure of deposited GaSb on GaAs (001): Monte Carlo simulation and experimental study

The growth of GaSb thin films by MBE on GaAs (001) is investigated experimentally, using TEM, and theoretically, using KMC simulations. The atomic scale mechanisms inherent to the growth are discussed and described in the KMC model in which the strain is introduced through an elastic energy term based on a valence force field approximation. We observe that the first two monolayers of the deposited films form strained three-dimensional clusters, but further deposition induces film relaxation and rough 3D growth with valley formation presenting (111) facets with unstable bottoms. We show that the roughening morphology and creation of grooves during growth are in agreement with experimental TEM observations.     

Re-realization of the NIS National Viscosity Scale and Its Range Extension up to 250,142?mm2/s

A wide-range viscosity standard system has generally been realized with capillary viscometers, which are calibrated by the stepping-up method on the basis of viscosity value of distilled water at 20?°C. As per recommendation of Bureau International des Poids et Mesures (BIPM), the national primary viscosity scale must be realized every 10?years. The present work is an attempt to re-realize the existing scale and extend it to from 70,000 to 250,142?mm²/s. This would enable NIS to calibrate different types of viscometers with the highest constant. The NIS viscosity scale was first realized in 2001 [1] using glass capillary viscometers in the range from 1 to 70,000?mm²/s. From the present calibration results of the NIS viscometers, the present scale is realized with an uncertainty of about 0.07?% at low viscosities (0.9?mm²/s) rising to about 0.25?% at higher viscosities (250,142?mm²/s).     

A thermomechanical crystal plasticity constitutive model for ultrasonic consolidation

We present a micromechanics-based thermomechanical constitutive model to simulate the ultrasonic consolidation process. Model parameters are calibrated using an inverse modeling approach. A comparison of the simulated response and experimental results for uniaxial tests validate and verify the appropriateness of the proposed model. Moreover, simulation results of polycrystalline aluminum using the identified crystal plasticity based material parameters are compared qualitatively with the electron back scattering diffraction (EBSD) results reported in the literature. The validated constitutive model is then used to simulate the ultrasonic consolidation process at sub-micron scale where an effort is exerted to quantify the underlying micromechanisms involved during the ultrasonic consolidation process.