Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

Abstract

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α 1 and α 2 relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α 1 relaxation to α 2 relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α 1 and α 2 relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α 1 relaxation cannot be observed, the α 2 and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.