Showing papers by "Rosario Benavente published in 2001"

Influence of the polymer microstructure on the thermal properties of cycloolefin copolymers with high norbornene contents

Abstract: This study involves research on possible relationships between the composition of ethylene/norbornene copolymers and their thermal properties. The copolymers were fully characterised by differential scanning calorimetry (DSC), 13 C NMR spectroscopy, Fourier transformation infra-red (FTIR) and Raman spectroscopy. The results of this study show that there is no linear correlation between the amount of norbornene incorporated in copolymers and thermal behaviour when copolymers containing high norbornene content and different microstructures are considered. The latter of which was found to be extremely sensitive to the catalyst used in its preparation.

Stress‐Strain Behaviour, Microhardness, and Dynamic Mechanical Properties of a Series of Ethylene‐Norbornene Copolymers

Abstract: Ten copolymers of ethylene and norbornene, synthesised by a metallocene catalyst, covering the composition range from 31 to 62 mol-% of norbornene, have been studied by stress-strain, microhardness and dynamic mechanical measurements. These copolymers show a single glass transition and the correlation between T g and the norbornene content is quite good. The inclusion in the polymer chain of rigid norbornene units, with the corresponding increase in T g , leads to considerably higher Young moduli and microhardness values in these cyclo-olefin copolymers. The same result is found for the yield stress: this parameter shows an increase by a factor of two in studied copolymers. Two relaxations, α and γ have been found by means of dynamic mechanical analysis of these copolymers. The results show that the γ relaxation has a similar origin to that of polyethylene and therefore its intensity diminishes as the proportion of ethylene segments in the copolymers decreases. On the contrary, the α relaxation of the copolymers has a completely different origin than that of polyethylene. Thus, this relaxation represents the glass transition in the copolymers, associated with the amorphous character, while it is attributed to motions in the crystalline regions of the parent homopolymer, polyethylene.

Thermal and dynamic mechanical behavior of ethylene/norbornene copolymers with medium norbornene contents

Abstract: A range of ethylene/norbornene copolymers were synthesized using the commercially available rac-Et(Ind)2ZrCl2 metallocene catalyst. A large window of norbornene contents, between 30 and 55 mol % was used to facilitate the interpretation of the results. The polymers were characterized by means of wide-angle X-ray scattering, differential scanning calorimetry, and dynamic mechanical thermal analysis. The X-ray diffractograms showed two amorphous halos, the low-angle one increasing in the intensity with norbornene content. Calorimetric and dynamic mechanical results led to a linear relation between the glass transition temperature and the norbornene content. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2159–2165, 2001

Effect of the comonomer content on the mechanical parameters and microhardness values in poly(ethylene-co-1-octadecene) synthesized by a metallocene catalyst

Abstract: Stress–strain and microhardness measurements were carried out on a series of copolymers of ethylene and 1-octadecene with different comonomer contents in the corresponding homopolymer of ethylene, synthesized with a metallocene catalyst. The different mechanical properties, deduced from the stress–strain curves (Young's modulus, yield stress, deformation at break, and energy to break) are interpreted in terms of the crystallinity and molecular weight of the samples because these two characteristics show considerable variations with the comonomer content. The microhardness values are explained in terms of these properties, and they are also correlated with Young's moduli and yield stresses deduced from the stress–strain curves. Linear relations are found between microhardness and yield stress and between the logarithm of the microhardness and the logarithm of the elastic modulus. The properties deduced from these lines are compared with literature values. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 277–285, 2001

Effect of Short Glass Fiber on Structure and Mechanical Behavior of an Ethylene–1-Octene Copolymer

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 60 wt.-%. The effect of the fiber on the structure and mechanical behavior has been analyzed. Though the peak melting and crystallization temperatures are not altered, the incorporation of the fibers leads to a slightly higher crystallinity in the composites, and the mechanical behavior is modified. Since fibers introduce rigidity, both Young's modulus and the microhardness increase and either elongation or stress-at-break decrease as the glass fiber content rises in the composite. However, the dependence of Young's modulus on the fiber content is very different from that of the microhardness. Moreover, an essentially homogeneous deformation takes place along the different materials analyzed, but, depending upon reinforcement composition, one or two yield stresses are observed. Finally, all specimens show three viscoelastic mechanisms. Due to the rigidity introduced by fibers and their influence on the crystallinity, the distinct relaxations in the composites are shifted to higher temperatures and the apparent activation energies associated with them are higher. The effect of fiber is more significant in either the β or α processes than in that of the γ. The increase of the temperature of the α relaxation is particularly important and thus the incorporation of the fibers considerably enlarges the useful temperature range, which can expand the application fields of the plain copolymer.

Influence of thermal history on morphology and viscoelastic behavior of ethylene–1-octene copolymers synthesized with metallocene catalysts

Abstract: The relationships among structure and properties have been established in copolymers of ethylene and 1-octene, synthesized by a particular metallocene catalyst system. The most important factor affecting the structure and properties of these copolymers is, evidently, the comonomer content. However, the cooling treatment from the melt has been found to be also a very significant factor and, although its influence on the thermal properties is rather small, some structural parameters and the viscoelastic behavior are clearly dependent upon thermal history. These parameters include the degree of crystallinity, lattice constants, relaxation processes, stiffness, and microhardness of the samples. Regarding the viscoelastic behavior, the β relaxation is shifted to lower temperatures and its intensity is increased as 1-octene content raises. On the other hand, the α mechanism, associated with motions within the crystalline regions, is also moved to lower temperatures. Such a relaxation is only observed up to a certain 1-octene content in the copolymer.

The effect of annealing on the structure and relaxation processes of vinyl alcohol-ethylene copolymers

Abstract: An annealing process has been applied to three samples of vinyl alcohol–ethylene (VAE) copolymers, richer in the former comonomer. The effect of such a process on the structure and on the relaxation mechanisms is studied. The structure of the three VAE copolymers has changed slightly. Nevertheless, the viscoelastic relaxation processes have been significantly affected for the thermal treatment. Two additional relaxations have appeared: one of them at temperatures above the relaxation associated to the glass transition, and the other at temperatures below the β mechanism of these copolymers. The temperature location, intensity, and apparent activation energy of the distinct relaxations found are discussed and compared with those in the original copolymers and the homopolymers, poly(vinyl alcohol) and polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1–12, 2001

A Significant Enhance of Impact Strength with Thermal Annealing in High Density Polyethylene

Abstract: Different structural, thermal, mechanical and viscoelastic characteristics established in an annealed commercial high density polyethylene (HDPE) are described and compared to those in the as molded sample. The main features found in the annealed specimen are a significant improvement in the impact strength and a greater intensity of the β relaxation process. In addition, a loss of some low molecular weight products is also found joined to a slight perfection in the crystalline phase. The causes for those features are discussed.