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.

Crystallization of the polyethylene block in polystyrene-b-polyethylene-b-polycaprolactone triblock copolymers, 1. Self-nucleation behavior

Abstract: The self-nucleation of branched polyethylene chains of different degrees of chain mobility was studied. The polyethylene block (PE block) within poly(styrene-b-ethylene-b-caprolactone) triblock copolymers (SEC) of varying compositions was studied. Differential scanning caloriometry was used to determine the self-nucleation domains as a function of the self-nucleation temperature (T s ). The self-nucleation behavior of PE chains within SEC block copolymers was found to be anomalous in comparison to the classical self-nucleation behavior exhibited by homopolymers. When the degree of chain constraint is high, as in the the case where the SEC copolymer only contains 15% of PE, domain II (only self-nucleation domain) completely disappears and annealing can take place before self-nucleation occurs. This means that chain constraint complicates the self-nucleation process and this situation persists until, upon decreasing the self-nucleation temperature (T s ), annealing has generated crystals tha are big enough to act a self-nuclei for the less restricted portions of the chain. If the PE content in the copolymer is very low (15%), two crystal populations can be distinguished. This may reflect the differences in diffusion of the PE chain segments close to the interfaces with the other two blocks and those segments that are close to the middle of the PE block. the influence of chain constraint on determining the difficulty of the chains to self-nucleate was further explored using a crosslinked low-density polyethylene (XLDPE). In this case, crosslinking junctions instead of covalent links with other blocks restrict chain mobility. Nevertheless, a similar difficulty in self-nucleation was found as in the case of the PE block within SEC triblock copolymers in contrast to neat LDPE, a polymer that exhibited the classical self-nucleation behavior with the usual three domains.

Linamarase Expression in Cassava Cultivars with Roots of Low- and High-Cyanide Content

Abstract: This paper reports the expression and localization of linamarase in roots of two cassava (Manihot esculenta Crantz) cultivars of low and high cyanide. Two different patterns of linamarase activity were observed. In the low-cyanide type, young leaves displayed very high enzyme activity during the early plant growing stage (3 months), whereas in root peel, the activity increased progressively to reach a peak in 11-month-old plants. Conversely, in the high-cyanide cultivar (HCV), root peel linamarase activity decreased during the growth cycle, whereas in expanded leaves linamarase activity peaked in 11-month-old plants. The accumulation of linamarin showed a similar pattern in both cultivars, although a higher concentration was always found in the HCV. Linamarase was found mainly in laticifer cells of petioles and roots of both cultivars with no significant differences between them. At the subcellular level, there were sharp differences because linamarase was found mainly in the cell walls of the HCV, whereas in the low-cyanide cultivar, the enzyme was present in vacuoles and cell wall of laticifer cells. Reverse transcriptase-PCR on cassava tissues showed no expression of linamarase in cassava roots, thus, the transport of linamarase from shoots to roots through laticifers is proposed.

A comparison of the electrooxidation kinetics of p-methoxyphenol and p-nitrophenol on Sb-doped SnO2 surfaces: Concentration and temperature effects

Abstract: We discuss the role of the phenol substituent on the kinetics of electrochemical oxidation on metal oxide surfaces. Using UV–vis spectroscopic and high performance liquid chromatographic analyses of solutions during electrolysis on antimony-doped tin oxide (Sb-SnO2) electrodes, we established that the oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) follow different reaction mechanisms. The initial PNP and PMP oxidation rates are well represented as functions of the initial phenol concentrations by the Langmuir–Hinshelwood mechanism. At high concentrations of phenol in solution, the oxidation rates are controlled by surface processes. Under surface saturation conditions the formation of OH surface species required for complete oxidation is hampered, lowering the extent of PNP oxidation. The effect of temperature on the oxidation of PNP and PMP on Sb-SnO2 electrodes was also studied. UV–vis spectroscopy of solutions during electrolysis show that the oxidation rates of both compounds follow Arrhenius behaviour; however, the fraction of the PMP initially present oxidized after the passage of a given electric charge was not affected by temperature, whilst the fraction of PNP oxidized increased with temperature. The dissimilar behaviour is ascribed to differing adsorption of PMP and PNP on the surface of the SnO2 electrode, as established from the correspondence of kinetic data to the Langmuir–Hinshelwood mechanism.

Extraction of non-ideal junction model parameters from the explicit analytic solutions of its I–V characteristics

Abstract: In modeling semiconductor junctions the extraction of the models parameters is often hindered by the presence of parasitic series resistance and shunt conductance. We propose a method to extract the intrinsic and extrinsic model parameters using the exact explicit analytic solutions for current and voltage of the junctions I–V characteristics, which are expressed in terms of Lambert W functions. However, direct numerical fitting of these solutions to extract the models parameters would be an unwieldy and computationally inefficient task. To circumvent this difficulty, the proposed method is based on first calculating the Integral Difference Function, D, from the explicit analytic solutions for I and V. This produces a purely algebraic equation in I and V whose coefficients contain the models parameters. The coefficients of this auxiliary equation can be quickly determined by direct numerical fitting. From them, all the intrinsic and extrinsic model parameters are then readily obtained at once. The method is tested on representative synthetic I–V characteristics to illustrate the computation process. 2004 Elsevier Ltd. All rights reserved.

Cytotoxicity and genotoxicity induced by coal and coal fly ash particles samples in V79 cells

Abstract: Exposure to coal and coal ashes can cause harmful effects in in vitro and in vivo systems, mainly by the induction of oxidative damage. The aim of this work was to assess cytotoxic and genotoxic effects using the V79 cell line treated with coal and coal fly ash particles derived from a coal power plant located in Santa Catarina, Brazil. Two coal samples (COAL11 and COAL16) and two coal fly ash samples (CFA11 and CFA16) were included in this study. COAL16 was co-firing with a mixture of fuel oil and diesel oil. The comet assay data showed that exposure of V79 cells to coal and coal fly ash particles induced primary DNA lesions. Application of lesion-specific endonucleases (FPG and ENDO III) demonstrated increased DNA effects indicating the presence of high amounts of oxidative DNA lesions. The cytokinesis-block micronucleus cytome assay analysis showed that exposure of V79 cells to high concentrations of coal and coal fly ash particles induced cytotoxic effects (apoptosis and necrosis) and chromosomal instability (nucleoplasmic bridges, nuclear buds, and micronucleus (MN) formation). These results may be associated with compounds contained in the surface of the particles as hazardous elements, ultrafine/nanoparticles, and polycyclic aromatic hydrocarbons (PAHs) which were detected in the samples. Graphical abstract ᅟ.