On the crystallization, morphology and physical properties of a clarified propylene/ethylene copolymer

The crystallization behaviour, morphology and electrical and mechanical properties of a propylene/ethylene copolymer containing a clarifying additive have been studied. The addition of the clarifier generally results in enhanced nucleation and consequent formation of a more uniform structure. The melting behaviour is analysed and it is shown that, only following quenching, are the observed multiple melting peaks associated with dynamic reorganization effects. Otherwise, the various endotherms are associated with the initial crystallization process itself. The morphology of the copolymer is shown to change abruptly from a fine to a coarse structure when the crystallization temperature exceeds 128°C. This is associated with a marked reduction in the nucleating efficiency of the additive, which permits the growth of distinct, spatially separated morphological features that are best termed quadrites. Through a combination of the increased crystallization temperature and the growth of these relatively large structural units, appreciable molecular fractionation is also observed. These changes in crystallization behaviour should have important consequences for the macroscopic physical properties; dielectric breakdown and mechanical failure have been investigated. Although the electric strength of the material exhibits a clear dependence on sample morphology, most of the mechanical parameters do not vary in the same discontinuous manner. The only significant correlations seen between electrical and mechanical parameters involve the ultimate failure stress and strain, as recorded at a relatively high strain rate of 100 mm/min. Consequently, although this suggests that dielectric breakdown and mechanical failure may be weakly related, no agreement with existing theoretical predictions has been found.