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.

Effect of annealing time on the self‐nucleation behavior of semicrystalline polymers

Abstract: It is widely known that when a polymer is heated just above its melting point and is kept at a given temperature (denoted Ts) for a short time, when it is cooled down its nucleation density increases and its peak crystallization temperature shifts to higher temperatures, as detected for instance by differential scanning calorimetry (DSC). The Ts temperature range where the described process occurs has been named Domain II self-nucleation (SN) because the selected Ts temperatures are high enough to melt the polymer without causing detectable annealing of any remnant crystals by DSC. Experimental results obtained by DSC, polarized light optical microscopy (PLOM), and rheology indicate that these techniques are unable to detect any remaining crystal fragments in Domain II. Our kinetic results demonstrate that Domain II SN is a transient phenomenon that can even disappear if enough time at Ts is allowed. Results of the study of the time dependence of the SN effect indicates two possibilities: (a) if crystal fragments are present (even if undetected by the employed techniques) their final melting is a very slow process (in the order of hours); (b) if all crystallites have melted in Domain II, then it may be more plausible to reinterpret self-nuclei as arising from “precursors” whose detail nature has not been the subject of this investigation but that can be regarded as either a residual segmental orientation in the melt (i.e., a melt memory effect) or a mesophase in a preordered state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1738–1750, 2006

Homogeneous nucleation and fractionated crystallization in block copolymers

Abstract: The confinement of crystallizable blocks within AB or ABC microphase-separated block copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight, and chemical structure. In this work we have examined the crystallization behavior of a series of AB and ABC block copolymers incorporating one or two of the following crystallizable blocks: polyethylene, poly(e-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or any other isolated morphology, is much higher than the number of heterogeneities available in each crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from homogeneous nuclei was obtained. The results show that, regardless of the specific morphologica...

Globally consistent quantitative observations of planktonic ecosystems

Abstract: In this paper we review on the technologies available to make globally quantitative observations of particles, in general, and plankton, in particular, in the world oceans, and for sizes varying from sub-micron to centimeters. Some of these technologies have been available for years while others have only recently emerged. Use of these technologies is critical to improve understanding of the processes that control abundances, distributions and composition of plankton, provide data necessary to constrain and improve ecosystem and biogeochemical models, and forecast changes in marine ecosystems in light of climate change. In this paper we begin by providing the motivation for plankton observations, quantification and diversity qualification on a global scale. We then expand on the state-of-the-art, detailing a variety of relevant and (mostly) mature technologies and measurements, including bulk measurements of plankton, pigment composition, uses of genomic, optical, acoustical methods and analysis using particles counters, flow cytometers and quantitative imaging devices. We follow by highlighting the requirements necessary for a plankton observing system, the approach to achieve it and associated challenges. We conclude with ranked action-item recommendations for the next ten years to move towards our vision of a holistic ocean-wide plankton observing system. Particularly, we suggest to begin with a demonstration project on a GO-SHIP line and/or a long-term observation site and expand from there ensuring that issues associated with methods, observation tools, data analysis, quality assessment and curation are addressed early in the implementation. Global coordination is key for the success of this vision and will bring new insights on processes associated with nutrient regeneration, ocean production, fisheries, and carbon sequestration.

Communicating and Displaying Real-Time Data with WebSocket

Abstract: Internet communication provides a convenient, hyperlinked, stateless exchange of information, but can be problematic when real-time data exchange is needed. The WebSocket protocol reduces Internet communication overhead and provides efficient, stateful communication between Web servers and clients. To determine whether WebSocket communication is faster than HTTP polling, the authors built a Web application to measure the one-way transmission latency of sending real-time wind sensor data at a rate of 4 Hz. They implemented a Jetty servlet to upgrade an HTTP connection to a WebSocket connection. Here, they compare the WebSocket protocol latency to HTTP polling and long polling.