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The thermal degradation of poly(vinyl acetate) (PVA), poly(vinyl alcohol) (PVAL), vinyl acetate-vinyl alcohol (VAVAL), vinyl acetate-vinyl-3,5-dinitrobenzoate (VAVDNB) and vinyl alcohol-3,5-dinitrobenzoate (VALVDNB) copolymers have been studied using differential thermal analysis (DTA) and thermogravimetry (TG) under isothermal and dynamic conditions in nitrogen. Thermal analysis indicates that PVA and PVAL are thermally more stable than VAVAL copolymers, being PVAL the most stable polymer. The presence of small amounts of vinyl-3,5-dinitrobenzoate (VDNB) in PVA or PVAL produces a marked decrease in the thermal stability of both homopolymers, being VALVDNB copolymers the less stable materials. The apparent activation energy of the degradative process was determined by the Kissinger and Flynn-Wall methods which agree well.
The thermal degradation of ethylene-vinyl acetate (EVA), ethylene-vinyl-3,5-dinitrobenzoate (EVDNB) and ethylene-vinyl alcohol (EVAL) copolymers have been studied using differential thermal analysis (DTA) and thermogravimetry (TG) under isothermal and dynamic conditions in nitrogen. Thermal analysis indicates that EVA copolymers are thermally more stable than EVDNB samples. The degradation of the copolymers considered occurs as an additive degradation of each component polyethylene (PE) and poly(vinyl acetate) (PVA), poly(vinyl-3,5-dinitrobenzoate) (PVDNB) or poly(vinyl alcohol) (PVAL). The apparent activation energy of the decomposition was determined by the Kissinger and Flynn-Wall methods which agree well.
This paper describes an unusual halo observed on the morning of 19 January 1787 from Caspe, Spain. The most striking characteristic of this phenomenon was a vertical tail of light that appears under one of the false suns.
A new Rondeletia species (Rondeletieae, Rubiaceae) was collected by the combined German- Cuban research group of the National Botanical Garden of Cuba in the dry southern region of the historical province Oriente, actually Prov. Guantánamo, in the valley of Rio Seco between Imias and Cajobabo. The region is rich in small leaved sclerophyllous species belonging to the section Hypoleucae Fernandez-Zequeira, like R. acuminata, R. baracoensis, R. ingrata. It is also closely related to R. rugelii of the province Matanzas, and to the R. fuertesii of Hispaniola. From these mentioned species the new taxon differs in having naked corolla tube, a character completely unique in the section.
Impact factor is a quasi-qualitative indicator, which provides a measurement of the prestige and international visibility of journals. Although the use of impact factor-based indicators for science policy purposes has increased over the last two decades, several limitations have been pointed out and should be borne in mind. The use of impact factor should be treated carefully when applied to the analysis of peripheral countries, whose national journals are hardly covered by ISI databases. Our experience in the use of impact factor based indicators for the analysis of the Spanish scientific production is shown. The usefulness of the impact factor measures in macro, meso and micro analyses is displayed. In addition, the main advantages, such as the great accessibility of impact factor and its ready-to-use nature are pointed out. Several limitations such as the need to avoid inter-field comparisons or the convenience of using a fixed journal set for international comparisons are also stressed. It is worth noting that the use of impact factor in the research evaluation process has influenced strongly the publication strategy of scientists.
We give a simple proof of the existence of an almost contact metric structure on any orientable 3-dimensional Riemannian manifold (M 3, g) with the prescribed metric g as the adapted metric of the almost contact metric structure. By using the key formula for the structure tensor obtained in the proof this theorem, we give an application which allows us to completely determine the magnetic flow of the contact magnetic field in any 3-dimensional Sasakian manifold.