Showing papers by "Rosario Benavente published in 2002"

Effect of composition and molecular weight on the crystallization behavior of blends of iPP and a metallocenic ethylene/1-octene copolymer

Abstract: Blends covering the entire range of compositions of a metallocenic ethylen/1-octene copolymer and two conventional isotactic poly(propylene)s (iPPs) of different molecular weight have been prepared, analyzing (by differential scanning calorimetry and X-ray diffraction) the effect of composition and molecular weight on the crystallization behavior. It was found that those blends rich in the iPP component show a behavior practically coincident with the weighted addition of the two components. On the contrary, significant deviations were found for the blends where the ethylene copolymer is the major component. These deviations are considerably more important in the case of the blends with iPP of higher molecular weight. In such a case, the blend with a 25% by weight of iPP presents a normalized crystallinity degree of only 0.20, in comparison with the value of 0.57 exhibited by the pure iPP. After annealing at 120°C for 10 minutes, and cooling back to room temperature, that blend increases its crystallinity degree up to only 0.40.

Crystalline Structure and Viscoelastic Behavior in Composites of a Metallocenic Ethylene‐1‐octene Copolymer and Glass Fiber

Abstract: Several composites with short glass fiber of a copolymer of ethylene and 1-octene, synthesized with a metallocene catalyst, have been prepared, up to a fiber content of 40 wt.-%. The effect of the fiber on the crystalline structure and on the viscoelastic response are analyzed. The role of glass fiber incorporation on crystallite development is discussed based on real-time variable-temperature WAXS and SAXS experiments. The viscoelastic behavior is considerably modified by incorporation of the glass fiber when compared with the plain copolymer. Thus, both the intensity and location of the γ and β relaxations are dependent on the fiber content. Moreover, the stiffness is considerably increased in the composites, allowing the analysis of the viscoelastic behavior up to temperatures well inside the melting region of the copolymer crystals, and thus enlarging substantially the service temperature of these composites, The β relaxation is assigned to the glass transition temperature, in agreement with the calorimetric results (DSC and MDSC) in these composites.

Mechanical Properties of Ultra High Molecular Weight Polyethylene Obtained with Different Cocatalyst Systems

Abstract: Three types of semicrystalline ultra high molecular weight polyethylene, obtained with amorphous SiO 2 -supported Ziegler-Natta catalyst systems modified by vanadium and titanium, have been studied by wide angle X-Ray scattering, dynamic mechanical thermal analysis and microhardness methods. In all the cases a significant amount of monoclinic modification is observed, which is estimated to be of the order of 10%. With increasing the degree of crystallinity and quantity of entanglements, storage modulus and α relaxation increase and y relaxation decreases. A weak β relaxation occurs only in the sample obtained with vanadium catalytic systems. Microhardness measurements give the possibility of distinguishing the effect of crystallinity from the effect of entanglements: Vickers microhardness is sensitive predominately to crystalline phase, while total microhardness is sensitive also to the structural peculiarities in the amorphous phase.

Toughening of a propylene-b-(ethylene-co-propylene) copolymer by a plastomer

Abstract: Several blends, covering the entire range of compositions, of a metallocenic ethylene-1-octene copolymer (CEO) with a multiphasic block copolymer, propylene-b-(ethylene-co-propylene) (CPE) [composed of semicrystalline isotactic polypropylene (iPP) and amorphous ethylene-co-propylene segments], have been prepared and analyzed by differential scanning calorimetry, X-ray diffraction, optical microscopy, stress-strain and microhardness measurements, and dynamic mechanical thermal analysis. The results show that for high CEO contents, the crystallization of the iPP component is inhibited and slowed down in such a way that it crystallizes at much lower temperatures, simultaneously with the crystallization of the CEO crystals. The mechanical results suggest very clearly the toughening effect of CEO as its content increases in the blends, although it is accompanied by a decrease in stiffness. The analysis of the viscoelastic relaxations displays, first, the glass transition of the amorphous blocks of CPE appearing at around 223 K, which is responsible for the initial toughening of the plain CPE copolymer in relation to iPP homopolymer. Moreover, the additional toughening due to the addition of CEO in the blends is explained by the presence of the β relaxation of CEO that appears at about 223 K. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1869–1880, 2002

Norbornene-Ethylene Copolymers Studied by Non-Destructive Methods

Abstract: Norbornene-ethylene copolymers with relatively high norbornene content, synthesised using different metallocene catalysts, were studied by Microhardness methods and Positron Annihilation Lifetime Spectroscopy. It has been established that plastic and elastic properties as well as free volume of the investigated materials do not depend linearly on the norbornene content since the different molecular structures, as a result of the various catalytic systems used, have also an important influence on those parameters. Vickers microhardness and plastic properties of the samples investigated are not sensitive to the quantity of ethylene blocks and are influenced predominantly by micromechanical properties of polymer chains. Vickers microhardness is not sensitive either to dimensions and quantity of the pores or to the free volume. On the other hand, long ethylene sequences are related to total microhardness, i.e., during penetration they contribute to the elastic deformation. The sizes, distribution and quantity of the nanopores have been determined and explained in relation to the structure of the norbornene-ethylene copolymers.

Influence of the Type of Fiber on the Structure and Viscoelastic Relaxations in Composites Based on a Metallocenic Ethylene-1-octene Copolymer.

Abstract: Several composites of a copolymer of ethylene and 1-octene, synthesized with a metallocene catalyst, have been prepared with a 5% in weight of different types of fiber. The effect of the fiber on the crystalline structure and on the viscoelastic response is analyzed. The role of the fiber on crystallite development is discussed based on WAXS, SAXS, optical microscopy and DSC experiments. The viscoelastic behavior is modified by incorporation of the fiber when compared with the plain copolymer. Thus, both the intensity and location of the γ, β and α relaxations are depen- dent on the introduction of the fiber. The β relaxation is assigned to the glass transition temperature, in agreement with the calorimetric results (DSC and MDSC) in the copolymer. Moreover, the stiffness is increased in the composites above the glass transition temperature of the copolymer, allowing the analysis of the viscoelastic behavior up to temperatures well inside the melting region of the copolymer crystals, and thus enlarging substantially the service temperature of these composites.

Physical aging of poly(diethylene glycol-p,p′bibenzoate)

Abstract: The structure of a quenched sample of a liquid crystalline polyester, poly(diethylene glycol-p,p′-bibenzoate) (PDEB), and its physical aging was studied by microhardness methods (MH), wide-angle X-ray scattering (WAXS), and positron annihilation lifetime spectroscopy (PALS). It was established that the smectic layers in the liquid crystalline domain are oriented parallel to the sample plane. Physical aging leads to an increase of either the Vickers microhardness, directly related to the elastic modulus of the material, or the total microhardness, which includes all the components of the overall deformation. These mechanical changes are due to a decrease in the sizes of nanopores and the consequential increase of the density of the disordered phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2363–2368, 2002