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

The crystallization behaviour, morphology and electrical and mechanical pro perties of a propylene/ethylene copolymer containing a clarifying additive have been studied. The addition of the clarifier generally results in enhan ced nucleation and consequent formation of a more uniform structure. The me lting behaviour is analysed and it is shown that, only following quenching, are the observed multiple melting peaks associated with dynamic reorganiza tion effects. Otherwise, the various endotherms are associated with the ini tial crystallization process itself. The morphology of the copolymer is sho wn to change abruptly from a fine to a coarse structure when the crystalliz ation temperature exceeds 128 degreesC. This is associated with a marked re duction in the nucleating efficiency of the additive, which permits the gro wth of distinct, spatially separated morphological features that are best t ermed quadrites. Through a combination of the increased crystallization tem perature and the growth of these relatively large structural units, appreci able molecular fractionation is also observed. These changes in crystalliza tion behaviour should have important consequences for the macroscopic physi cal properties; dielectric breakdown and mechanical failure have been inves tigated. Although the electric strength of the material exhibits a clear de pendence on sample morphology, most of the mechanical parameters do not var y in the same discontinuous manner. The only significant correlations seen between electrical and mechanical parameters involve the ultimate failure s tress and strain, as recorded at a relatively high strain rate of 100 mm/mi n. Consequently, although this suggests that dielectric breakdown and mecha nical failure may be weakly related, no agreement with existing theoretical predictions has been found. (C) 2001 Elsevier Science Ltd. All rights rese rved.