Ernesto Pérez

Influence of the Polymer Microstructure on the Thermal Properties of Cycloolefin Copolymers with High Norbornene Contents

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.

Obtaining the γ phase in isotactic polypropylene: effect of catalyst system and crystallization conditions

Abstract Five samples of isotactic polypropylene, iPP, and two copolymers of iPP with 1-hexene, synthesized with different catalyst systems, both heterogeneous and homogeneous, have been studied in order to analyse the effect of the catalyst system, the presence of comonomer units and the crystallization conditions on the phase structure of iPP and, in particular, on the amount of y modification obtained. Minor amounts, if any, of γ modification are present in iPP samples synthesized with highly isospecific Ziegler-Natta catalysts and crystallized from the melt at different cooling rates, ranging from 100 to 3°C/min. On the contrary, considerable amounts of the γ form have been obtained both in samples prepared with catalysts of very low isospecificity or in those prepared with homogeneous metallocene catalysts. It has been shown that the relative proportion of the α and γ modifications can be controlled just by changing the crystallization conditions. A clear influence of the presence of comonomer units on favouring the formation of the γ phase has not been ascertained. Moreover, attempts to deduce the γ content from the d.s.c. melting patterns have been unsuccessful, since the two modifications exhibit very similar melting temperatures.

Synthesis and properties of elastomeric poly(propylene)

Elastomeric poly(propylene) with low percentages of isotactic pentads and high molecular weight was synthesized using unbridged “oscillating” metallocenes. The polymer sample with the highest [mmmm] content (22%) shows a small amount of crystallinity, which is highly influenced by the thermal history, and differs significantly in stress-strain and dynamic mechanical properties from all other samples. Polymers with lower isotacticity index, although lacking crystallinity in thermal analysis, differ in their viscoelastic behaviour from those of atactic poly(propylene). In dynamic mechanical analysis the γ relaxation at low temperature shows high sensitivity to low isotactic contents.

Synthesis and characterization of copolymers of ethylene and 1-octadecene using the rac-Et(Ind)2ZrCl2/MAO catalyst system

The copolymerization of ethylene and 1-octadecene using a bridged metallocene was studied in order to observe the effect of the comonomer on the catalytic activity. A noticeable increase in activity is seen as the concentration of 1-octadecene in the reaction medium increases. 13 C NMR analysis shows 6.4 mol-% incorporation of comonomer at the highest 1-octadecene concentration in the feed used here. The molecular weight of the copolymers shows a drastic decrease that may be attributed to chain termination by transfer or β-elimination of the comonomer. As to the molecular weight distribution, it remains within a narrow range, as expected with metallocene catalysts. The melting temperature and the enthalpy of melting of the copolymers show a decrease with increasing comonomer content. As usual for ethylene copolymers, the X-ray crystallinities are higher than those determined from the enthalpy of melting.

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

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.

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

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.

The effect of thermal treatment on the structure and relaxation processes of olefinic polymers synthesized with metallocene catalysts

The relationships between the structure and properties have been established in several copolymers of ethylene and 1-hexene, synthesized by a metallocene catalyst. A dependency with composition and thermal history has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as crystallinity and crystal lattice constants are considerably affected as 1-hexene content increases in the copolymer. The β-relaxation does not appear in the homopolymer. In the copolymers, it is shifted to lower temperature and its intensity is increased as 1-hexene content raises. On the other hand, the α-mechanism associated to motion within the crystalline regions is also moved to lower temperatures but its intensity is diminished as 1-hexene content increases in the copolymer.

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

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 (Youngs 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 Youngs 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.

The γ relaxation in polymers containing ether linkages: conformational dynamics in the amorphous phase for a series of polybibenzoates containing oxyethylene spacers

Restricted conformational transitions in oxyethylene chains have been investigated using a molecular mechanics approach. Geometries and energies of transition pathways have been evaluated for kink inversion and formation, and for crankshaft type motions which are feasible in anisotropic glassy matrices. Barrier heights for these processes are found to be similar to those determined for the corresponding processes in polyethylene chains although geometries of the conformations at the energy minima are significantly different. Results of the calculations are broadly consistent with dynamic mechanical measurements which indicate a γ relaxation in the amorphous phase of thermotropic liquid crystal polymers containing oxyethylene spacers. They support the hypothesis that kink formation, inversion and migration are primarily responsible for the low temperature relaxation in these polymers.

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

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 Youngs 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 Youngs 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.