Showing papers by "Rosario Benavente published in 2007"
TL;DR: In this article, the competition between α, β, and γ polymorphs was studied in a β-nucleated metallocenic isotactic polypropylene, iPP, as a function of the cooling rate and of the isothermal crystallization temperature, by performing X-ray diffraction and DSC experiments.
Abstract: The competition between α, β, and γ polymorphs has been studied in a β-nucleated metallocenic isotactic polypropylene, iPP, as a function of the cooling rate and of the isothermal crystallization temperature, by performing X-ray diffraction and DSC experiments. It was found that the addition of a 1% by weight of a typical β-nucleating agent is not enough to develop any appreciable amount of β modification, at least under the crystallization conditions used, which cover a wide range of cooling rates. In comparison, the same amount of nucleating agent added to a Ziegler−Natta iPP leads to almost 100% of β form at low cooling rates. It seems that such amount of β nucleating agent is not enough to counterbalance the well-known γ nucleation ability of the relatively high content of defects (stereo- and regioerrors) which are present in the studied metallocenic iPP, and only different proportions of γ and α modifications are obtained in this sample, the relative amount of them depending on the cooling rate. On ...
160 citations
TL;DR: In this paper, the relationship between structure and properties of isotactic polypropylene, and homopolymers synthesized by metallocene catalyst systems was established. But, the relationship was not extended to other types of polymers.
60 citations
TL;DR: In this paper, a comprehensive study of the structure and properties of copolymers of propylene-1-hexene, CiPH, and propyleneethylene (CiPE), synthesized by an isotactic metallocene catalyst system was performed.
Abstract: A comprehensive study of the structure and properties has been performed for copolymers of propylene-1-hexene, CiPH, and propylene-ethylene, CiPE, synthesized by an isotactic metallocene catalyst system. The comonomer content constitutes the most important factor affecting the structure and properties of these CiPH and CiPE copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity is observed in the two kinds of copolymers as the comonomer content increases. The structure in the CiPH copolymers evolves, however, from the typical, monoclinic crystal lattice to mesomorphic-like, ordered entities for the highest 1-hexene molar fraction, whereas in the CiPE copolymers the structural evolution with molar fraction goes from a monoclinic lattice to an almost amorphous material. All of these variations in crystal structure significantly influence the viscoelastic and mechanical behavior of these CiPH and CiPE copolymers. Consequently, the location and intensity of the different relaxation mechanisms, as well as the rigidity parameters (storage and Young's moduli and microhardness) and deformation mechanism are strongly dependent upon composition.
15 citations
TL;DR: In this article, the synthesis and characterisation of polyethylenes obtained by direct copolymerisation of ethylene with 10-undecenoic acid (UA) have been carried out.
Abstract: The synthesis and characterisation of several functionalised polyethylenes, obtained by direct copolymerisation of ethylene with 10-undecenoic acid (UA), have been carried out. Two metallocene complexes of C s symmetry with different bridges were used: a dimethylsilyl bridge complex Me 2 Si(Cp)(Flu)ZrCl 2 and a phenylidene bridge complex Ph 2 C(Cp)(Flu)ZrCl 2 . For comparison, results obtained with the Cp 2 ZrCl 2 / MAO system are also reported. The copolymerisation activity depends strongly on the metallocene complex and on the added amount of the UA comonomer. High incorporation levels of up to 7.3 mol-% are achieved when using the phenylidene bridged catalyst presenting a rigid structure and a wide bite angle. The effect of comonomer content and catalyst geometry on the crystalline structure and mechanical behaviour of the different copolymers are comprehensively discussed.
10 citations
TL;DR: In this paper, the influence of the milling process on the mechanical and electrical properties of composite materials was investigated using X-ray diffraction and scanning and transmission electron microscopy.
Abstract: Composites materials consisting of pure copper reinforced with 1 vol.% of NbC were prepared by the powder metallurgy route to determine the influence of the milling process on the mechanical and electrical properties. For comparative purpose different milling times at four different rotational speeds were used. The resulting powders were consolidated by hot uniaxial pressing under 90MPa for 2h at 923K to obtain materials with a fine microstructure without residual porosity. It was found that the microstructure and properties of composite materials could be principally related to the amount of Fe, Cr, C and O incorporated as impurities during the milling process. Therefore, the rotational speeds used for milling has an important influence on the properties of the final product. A lower energy-ball milling is accompanied by a smaller amount of impurities (Fe, C and O) incorporated during milling. Composites materials combine electrical conductivity above 40% IACS with high strength. A detailed microstructural analysis by scanning and transmission electron microscopy and X ray diffraction showed that these properties are related not only to NbC particles, but also to the presence of very fine particles of carbides and oxides.