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.

Confined crystallization of polymers within anodic aluminum oxide templates

Abstract: In this article, a review of recent literature on confined crystallization within nanoporous anodic aluminum oxide (AAO) templates is presented. For almost all infiltrated polymeric materials, crystal orientation within the nanopores is a function of pore diameter. Tc and Tm usually decrease and are a function of pore size. When no pore interconnection remains, the crystallization occur at large supercoolings in heterogeneity free environments. Hence, the nucleation mechanism changes from heterogeneous to surface or homogeneous nucleation. The crystallization kinetics of infiltrated polymers should be close to first order, since in confined environments nucleation is the determining step of the overall crystallization and Avrami indexes (n) of ∼1 (or lower in some cases) should be obtained. Examples are provided where these conditions have been met and first order kinetics (n = 1) were measured as opposed to higher orders (n = 3−4) for the same polymer in the bulk. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1179–1194

Interleukin-6 (IL-6) in seminal plasma of infertile men, and lipid peroxidation of their sperm.

Abstract: Concentrations of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in seminal fluid, as well as levels of sperm lipid membrane peroxidation, were investigated in fertile and infertile men. Semen samples, obtained by masturbation from 37 infertile and 14 fertile men, were examined for the presence of TNF-alpha and IL-6. The level of lipid peroxidation of the sperm membrane was measured by determining malondialdehyde (MDA) formation. The correlation between the IL-6 and the TNF-alpha concentrations in seminal plasma with the levels of lipid peroxidation of the sperm membranes was statistically evaluated. The IL-6 concentration in seminal plasma of infertile men was significantly higher than that of fertile men (p < .05). Similarly, the level of membrane lipid peroxidation was higher for the semen of infertile men than that of fertile men (p < .001). A significant positive correlation was found between IL-6 levels in seminal plasma and membrane sperm lipid peroxidation (p < .002), but not between this parameter and TNF-alpha levels in seminal plasma. These findings suggest a possible association between IL-6 seminal plasma levels and lipid peroxidation of sperm membrane. Stimulation of reactive species production by human sperm and leucocytes, induced by the high levels of IL-6, could explain these results.

A Nonstationary Multisite Model for Rainfall

Abstract: Estimation and prediction of the amount of rainfall in time and space is a problem of fundamental importance in many applications in agriculture, hydrology, and ecology. Stochastic simulation of rainfall data is also an important step in the development of stochastic downscaling methods where large-scale climate information is considered as an additional explanatory variable of rainfall behavior at the local scale. Simulated rainfall has also been used as input data for many agricultural, hydrological, and ecological models, especially when rainfall measurements are not available for locations of interest or when historical records are not of sufficient length to evaluate important rainfall characteristics as extreme values. Rainfall estimation and prediction were carried out for an agricultural region of Venezuela in the central plains state of Guarico, where rainfall for 10-day periods is available for 80 different locations. The measurement network is relatively sparse for some areas, and aggr...

Crystallization in Block Copolymers with More than One Crystallizable Block

Abstract: Recent results on the crystallization of block copolymers with more than one crystallizable block are reviewed. The effect that each block has on the nucle- ation, crystallization kinetics and location of thermal transitions of the other blocks has been considered in detail. Depending on the thermodynamic repulsion between the blocks, the initial melt morphology in weakly segregated double crystalline di- block copolymers can be sequentially transformed by the crystallization of the dif- ferent blocks. The crystallization kinetics of each block can be dramatically affected by the presence of the other, and by the crystallization temperature; the magni- tude of the effect is a function of thermodynamic repulsion. Also the morphology has been investigated and peculiar double crystalline spherulites with intercalated semi-crystalline lamellae of each component have been observed in weakly segre- gated diblock copolymers. In the case of ABC triblock copolymers with more than one crystallizable block, many interesting effects have been found; among them, self-nucleation, sequential or coincident crystallization, and fractionated crystalliza- tion can be mentioned. Additionally, the effect of the topological constrains due to the number of free ends has been studied. Factors like chemical structure, molec- ular weight, molecular architecture and number of crystallizable blocks provide a very large number of possibilities to tailor the morphology and properties of these interesting novel materials.