On nucleation and the evolution of morphology in a propylene/ethylene copolymer

The morphology and crystallisation behaviour of a propylene/ethylene copolymer have been studied as a function of both crystallisation temperature and melt history, in the absence of any nucleating additives. In contrast to the previous paper, here melt temperatures were chosen to vary the number of residual lamellar fragments remaining prior to the onset of crystallisation, to permit so-called self-seeding effects to be explored. DSC results indicate that, under all circumstances, two lamellar populations develop isothermally. At sufficiently high temperatures, these are accompanied by a third quenched component.,Although this general pattern of crystallisation is not qualitatively affected by the choice of self-seeding conditions, melting at 168 degreesC (to remove self-seeding effects) does markedly reduce the proportion of primary lamellae. In all samples, TEM examination reveals the presence of two distinct lamellar types, extensive lath-like crystals and smaller, cross-hatching lamellae. The extensive laths result from primary crystallisation, whereas the cross-hatching is associated with secondary crystallisation. Quenched material is assumed to be located between individual lamellae since, where isothermal crystallisation has proceeded to completion, there is no evidence of macroscopic segregation consistent with the size of the BSC quench peak. The architecture of the lamellar aggregates, when small, is identical to the quadrites we have described previously. As the self-seeding temperature is increased, the nucleation density decreases and larger structures develop. These exhibit a number of characteristic forms, depending upon the direction in which they are viewed. Finally, when crystallisation occurs from non-seeded melts (melt temperature of 168 degreesC) irregular spherulitic structures are seen to develop, The form of these structures is markedly influenced by the non-crystallisable fraction of the melt, which serves to suppress primary crystallisation. (C) 2001 Elsevier Science Ltd. All rights reserved.