José M. Pereña

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.

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 orientation on the morphology and viscoelastic response of vinyl alcohol-ethylene copolymers

The influence of the main variables (temperature and strain rate) of the drawing process on the structure and dynamic mechanical relaxations of vinyl alcohol-ethylene (VAE) copolymers with higher content in the former counit is studied. The structure of VAE copolymers is considerably modified by drawing, which produces a disordered crystalline form, as revealed by WAXD and DSC measurements. The structural changes have significant effects on the viscoelastic relaxations of stretched VAE copolymers. A remarkable increase in the storage moduli is observed in the direction parallel to the uniaxial orientation, depending on composition and drawing parameters. The drawn copolymers show an additional α relaxation taking place at temperatures higher than that of the α relaxation (glass transition), attributed to motions within the crystalline regions. The β mechanism is moved to higher temperatures for the stretched specimens and its intensity is considerably increased. Finally, the γ relaxation is very weak and only displayed in the sample with the higher ethylene content.

Microhardness of polyethylene surface modified by chlorosulphonic acid

Chlorosulphonic acid etching of polyethylene produces changes in its surface structure that can be tested by means of microhardness measurements. The results of different microhardness parameters (Mayers lines, penetration curves, Vickers microhardness and total microhardness) are compared, discussed and related to the structure of polyethylene.

Thermal and viscoelastic behaviour of copolymers of propene and 1-hexene

SummaryThe thermal and viscoelastic behaviour of several copolymers of propene and 1-hexene have been studied in a wide range of compositions. The samples were prepared with a Ziegler-Natta catalyst with both internal and external donors. A significant amount of crystallinity has been found for the sample with a comonomer content as high as 19 mol %. The dynamic mechanical results show a considerable decrease of the storage modulus in the copolymers in relation to that of iPP. A single relaxation, corresponding to the glass transition, has been found in all the samples. Only the copolymer with the highest hexene content seems to display the beginning of another relaxation at low temperature.

Synthesis and phase behaviour of liquid crystalline polyesters derived from p,p′-bibenzoic acid and meso and R-3-methylhexanediol

The synthesis and thermal properties of two new liquid crystalline polyesters derived from bibenzoic acid and (R,S)- or (R)-3-methyl-1,6-hexanediol are reported. The phase behaviour was investigated by means of differential scanning calorimetry, X-ray diffraction and polarizing microscopy. The effect of the asymmetry introduced by the branched methyl group and that of the stereoisomerism on the type and stability of the mesophases formed are analysed. Both polymers exhibited smectic mesophases of different types depending on thermal history. The presence of an optically active pure diol along the chain gives rise to a more regular structure in comparison with the polymer synthesized from the racemic mixture.

Evaluation of orientation in vinyl alcohol‐ethylene copolymer films by means of infrared dichroism and birefringence

Coupled measurements of birefringence and IR dichroism of drawn films of vinyl alcohol-ethylene copolymers have led to a value of around 46° for the angle α between the dipole moment vector of the IR band at 610 cm-1 and the chain axis. The increase of the second moment of the orientation function is well fitted to the pseudoaffine scheme of deformation.