Search Results

You are looking at 1 - 10 of 58 items for

  • Author or Editor: M. Fernández x
Clear All Modify Search

Abstract  

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.

Restricted access
Authors: M. Obregón, M. Fernández-Fernández and G. Sáenz

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.

Restricted access

Abstract  

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.

Restricted access
Authors: J. L. Cabrerizo, L. M. Fernández and M. Fernández
Restricted access
Authors: J. L. Cabrerizo, L. M. Fernández and M. Fernández
Restricted access

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.

Restricted access
Authors: J. Cabrerizo, L. Fernández, M. Fernández and Guo Zhen
Restricted access
Authors: C. Benítez, M. Fernández and M. Soriano
Restricted access

Abstract  

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.

Restricted access

Abstract  

The thermal behaviour of a new kind of hybrid system based on silanized poly(ether-urethanes) (SPURs) has been analyzed by thermogravimetric analysis (TG). The influence of the chemical nature of employed alkoxysilanes, polyether diol molecular weight and the physical state of the obtained hybrids (cured and non-cured) has been studied. The results show that in the non-cured state, aminosilane-based systems present a higher stability compared with those based on isocyanatesilane. However, in the cured state, both types of hybrids present a similar thermal stability, but much higher than their corresponding partners before the curing process. The presence of the inorganic silica network improves the thermal stability of all the systems studied.

Restricted access