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.

Synthesis and morphological characterization of miktoarm star copolymers (PCL)2(PS)2 of poly(ε-caprolactone) and polystyrene

Abstract: An heterofunctional initiator combining two reactive sites for ring opening polymerization and two for atom transfer radical polymerization was used to prepare three A2B2 miktoarm star copolymers of poly(e-caprolactone) (PCL) and polystyrene (PS). The morphology and thermal properties were studied by transmission electron microscopy, polarized light optical microscopy, and differential scanning calorimetry. The (PCL)2(PS)2 72/28 (72 wt % PCL) sample was crystallized from a disordered melt. In this case, crystallization drove the structure formation and a lamellar morphology was obtained at the microdomain level, while spherulites were observed at a superstructural level. The other two samples, 39/61 and 27/73, with lower PCL content and higher total molecular weight, were not able to form spherulites. Surprisingly, these miktoarm star copolymers exhibited hexagonally packed cylinders and spheres morphologies, respectively, instead of lamellar and cylindrical morphology. Such unexpected and novel behavior was explained in terms of the higher resistance of the arms to be stretched in a miktoarm star copolymer when compared with the corresponding linear diblocks. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5387–5397, 2007

Chromosomal speciation in a rassenkreis of venezuelan spiny rats (genus proechimys, rodentia, echimyidae)

Abstract: A series of populations of spiny rats (Proechimys) of NW and N Central Venezuela were found to belong to a circle of species and subspecies which surrounds the Andes of M6rida-CordiUera de la Costa mountain axis. This 'Rassenkreis' consists of 6 successive karyomorphs which exhibit a stepped-dine distribution (2n = 42, 44, 46, 48, 50 and 62). Each of the karyomorphs occupies a discrete subarea in an almost continuous territory. Polymorphism is very rare and no intermediate karyotypes have been found in the borders of each subarea. Although contact zones have not been studied in detail, the distribution seems to be mostly parapatric. Areas are 12-100 km apart, in most cases without effective barrier between them. Only 1 Robertsonlan rearrangement accounts for each of the steps from 2n = 44 to 2n = 50. The transition from 2n = 42 to 2n = 44 involves 1 Robertsonian change and 3 pericentric inversions. The transition from 2n = 50 to 2n = 62 requires 6 Robertsonian rearrangements and 2 pericentric inversions. It is inferred that successive populations of the 2n = 44 2n = 50 series may be at an incomplete stage of reproductive isolation by partial reproductive failure of the FI hybrids. Reproductive isolation may be almost complete between the 2n = 42 and 2n = 44 and the 2n = 50 and 2n = 62 karyomorphs. This 'Rassenkreis' is classified as a superspecies, Proechimys [guairae], comprising three allospecies: P. poliopus, P. guairae, and a new unnamed species (Barina's~ sp. n.). Within P. guairae 4 subspecies (or semispecies) are recognized, 2 of which are named (P. guairae guairae and P. guairae ochraceus). It is postulated that differentiation within this superspecies occurred during the Late Pleistocene. The tentative interpretation is advanced that rapid colonization occurred over most of the present territory by a chromosomically uniform invading stock, which subsequently became differentiated by parapatric chromosome raciation. This was followed by further chromosome differentiation in peripheral isolates which formed in unfavorable, dry climatic phases. During humid climatic phases the ranges expanded and zones of secondary contact with the original populations were established.

Marine Biodiversity and Biogeography – Regional Comparisons of Global Issues, an Introduction

Abstract: The Census of Marine Life grew out of a series of global concept meetings, following a 1995 U.S. National Research Council report indicating that no nation in the world could list the species that live in its offshore exclusive economic zone, as required under the Convention on Biological Diversity. A series of “Known, Unknown, Unknowable” meetings convened in every ocean region enabled the Census Scientific Steering Committee to compile information and enlist people from around the world to form a network of National and Regional Implementation Committees in support of a global effort to resolve this knowledge void. This collection represents the assembled results of their activities. As these data are incorporated into the Census' Ocean Biogeographic Information System and made available through its new home at the UN's Intergovernmental Oceanographic Commission, they will be the best records of most countries' marine biodiversity. The people, technologies, and associations of the Census will play a key role the UN General Assembly's future Regular Marine Assessments. This collection is a major legacy of the Census' US$650 million “Decade of Discovery” to be released in London in October 2010.