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.

The Influence of Blend Morphology (Co-Continuous or Sub-Micrometer Droplets Dispersions) on the Nucleation and Crystallization Kinetics of Double Crystalline Polyethylene/Polyamide Blends Prepared by Reactive Extrusion

Abstract: Reactive blending of functionalized polyethylenes (PE) and polyamides (PA) is performed by carefully choosing extrusion conditions and polymeric materials in order to obtain two types of stable morphologies. The first blend type yields a co-continuous morphology and the second type a dispersion of sub-micron droplets of the PA phase in a PE matrix. The crystallization kinetics of the PA change from a sigmoidal classical type (for the co-continuous blend) to first-order kinetics in the case of the sub-micron PA droplets. The results demonstrate an intimate relationship between blend morphology and nucleation and crystallization kinetics of the blend components.

Foraging patterns of capybaras in a seasonally flooded savanna of Venezuela

Abstract: The foraging behaviour of capybaras (Hydrochaeris hydrochaeris) was assessed in relation to habitat and season in a flooded savanna of Venezuela from February (dry season) 1993 to June (rainy season) 1994. Direct observations were carried out to record group movements and foraging activities of individual capybaras when feeding on specific plant species. The time devoted to feeding upon some species was recorded in relation to total foraging time (feeding + searching). Capybaras spent similar times feeding on reed (Eleocharis interstincta; Cyperaceae) and Paratheria prostrata (Poaceae) patches during the dry season though plant quality was different. During the rainy season, reeds were almost ignored by the animals which fed mainly on Hymenachne amplexicaulis, a grass with a significantly higher content of energy and protein. Capybaras fed for longer time during the dry season but more selectively during the rainy season. Capybaras were the dominant grazers in this flood savanna as inferred from the comparative effects they produced in the height, biomass and quality of the pasture in relation to those effects produced by other vertebrate herbivores, namely, cattle, feral horses and asses.

Productivity in Physical and Chemical Science Predicts the Future Economic Growth of Developing Countries Better than Other Popular Indices

Abstract: Scientific productivity of middle income countries correlates stronger with present and future wealth than indices reflecting its financial, social, economic or technological sophistication. We identify the contribution of the relative productivity of different scientific disciplines in predicting the future economic growth of a nation. Results show that rich and poor countries differ in the relative proportion of their scientific output in the different disciplines: countries with higher relative productivity in basic sciences such as physics and chemistry had the highest economic growth in the following five years compared to countries with a higher relative productivity in applied sciences such as medicine and pharmacy. Results suggest that the economies of middle income countries that focus their academic efforts in selected areas of applied knowledge grow slower than countries which invest in general basic sciences.

Automatic heart sound recording classification using a nested set of ensemble algorithms

Abstract: Automated phonocardiogram (PCG) analysis may provide better clinical information to physicians for analyzing and diagnosing different heart abnormalities. However, despite recent advances in PCG analysis methods, it is still a challenging task to extract accurate and useful information from contaminated heart sound recordings. The main objective of this paper is to introduce a new approach for classification of normal and abnormal heart sound recordings using a nested ensemble of algorithms that includes Random Forest, LogitBoost and a Cost-Sensitive Classifier. The approach consisted of three stages: preprocessing, classification and evaluation. In the preprocessing stage, PCG signals were first downsampled to 1 kHz using a polyphase antialiasing filter. Next, each heart sound was segmented using Springer's improved version of Schmidt's method to identify four states; S1, S2, systole and diastole. Thereafter, 131 features in time, frequency, wavelet and statistical domains were extracted from the entire signal and from the timings of the states. In the classification stage, the meta-classifier was cross validated on the entire training dataset provided by Physionet Challenge 2016. In the evaluation stage, the sensitivity and specificity of the trained algorithm was tested with unseen signals selected randomly by the Challenge testing environment. Experimental results showed that the proposed approach achieved an overall score of 84.48%, ranking 5th. The use of a nested set of ensemble classifier with a combined set of features extracted from different domains helped reduce overfitting and improved classification performance.

Rheological and calorimetric evidences of the fractionated crystallization of iPP dispersed in ethylene/α‐olefin copolymers

Abstract: In this work, the melt crystallization of immiscible blends of isotactic poly- propylene (iPP) and branched polyethylenes (PE) was followed by oscillatory shear measurements during controlled cooling. All the blends contained 20% iPP finely dis- persed in several ethylene/a-olefin copolymer matrices (with and without a nucleating agent) with densities ranging from 0.88 to 0.92 g/cm 3 (linear low, very low, and ultra low density polyethylenes: LLDPE, VLDPE, and ULDPE). The rheological results were compared with parallel differential scanning calorimetry (DSC) measurements at the same cooling rate. During preliminary evaluations of the neat resins, no effect was found of the variation of the frequency of oscillation or the applied shear strain on their crystallization (at least in the range explored in this work). In the case of the blends, when the iPP crystallized in a fractionated fashion, only one sudden increase in the storage modulus (G*) was observed during cooling due to the partial coincident crystal- lization of both iPP and the PE matrix. In the presence of a nucleating agent, an almost complete separation between the crystallization of both components in the blend was achieved and two increases in G* were clearly observed upon cooling. A close match between the dynamic crystallization kinetics obtained by DSC and torsion rheometry was demonstrated by a direct comparison between calorimetrically measured solid conversion and G* during cooling from the melt. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2481-2493, 1997