Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests

Abstract: This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera All these species coexist in a tropical dry forest in Venezuela Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm2 g-1 in evergreen and deciduous species, respectively Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species This was related to a lower osmotic potential at full turgor in evergreen (≃-30 MPa)_than in deciduous (≃-20 MPa) species, rather than to the elastic properties of leaf tissue The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7-10 MPa in deciduous species Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest

Rheology, Processing, Tensile Properties, and Crystallization of Polyethylene/Carbon Nanotube Nanocomposites

Abstract: A nanocomposite sample was prepared by melt mixing a high density polyethylene (HDPE) with an in situ polymerized HDPE/multi wall carbon nanotube (MWNT) masterbatch. The nanocomposite had an approximate content of 0.52 wt % MWNT. Rheological, thermal, and mechanical properties were investigated for both neat HDPE and nanocomposite. The nanocomposite, when compared to the neat polymer, exhibits lower values of viscosity, shear modulus and shear stress in extrusion and a concurrent delay of the distortion regimes to higher shear stresses and rates. The nanocomposite presents also improved dimensional stability after processing, and lower values of the melt strength, draw ratio and viscosity in elongational flow. This behavior has been observed in composites in which an adsorption of a fraction (that with the highest molecular weight or relaxation time) of the polymer chains is considered. Furthermore, the enhancement in the crystallization kinetics, probed by rheometry and DSC, suggests that the carbon nano...

New procedure for the extraction of basic a-Si:H TFT model parameters in the linear and saturation regions

Abstract: A new procedure is proposed to extract basic parameters for the AIM-Spice amorphous thin film transistor model in the above-threshold region. Our method avoids non-linear optimization, which is mainly the method utilized up to now, when using a program extractor included in AIM-Spice. The present extraction procedure is based on the integration of the experimental data current. The integration method as in known is convenient to decrease the effects of experimental noise. The method is applied to the linear and saturation regions for the above-threshold regime and allows the extraction of all the above-threshold parameters. The accuracy of the simulated curves using the parameters extracted with the new procedure is verified with measured and calculated data using the expressions contained in the model.

Atypical Human Infections by Animal Trypanosomes

Abstract: The two classical forms of human trypanosomoses are sleeping sickness due to Trypanosoma brucei gambiense or T. brucei rhodesiense, and Chagas disease due to T. cruzi. However, a number of atypical human infections caused by other T. species (or sub-species) have been reported, namely due to T. brucei brucei, T. vivax, T. congolense, T. evansi, T. lewisi, and T. lewisi-like. These cases are reviewed here. Some infections were transient in nature, while others required treatments that were successful in most cases, although two cases were fatal. A recent case of infection due to T. evansi was related to a lack of apolipoprotein L-I, but T. lewisi infections were not related to immunosuppression or specific human genetic profiles. Out of 19 patients, eight were confirmed between 1974 and 2010, thanks to improved molecular techniques. However, the number of cases of atypical human trypanosomoses might be underestimated. Thus, improvement, evaluation of new diagnostic tests, and field investigations are required for detection and confirmation of these atypical cases.