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

The morphology and crystallisation behaviour of a propylene/ethylene copoly mer have been studied as a function of both crystallisation temperature and melt history, in the absence of any nucleating additives. In contrast to t he previous paper, here melt temperatures were chosen to vary the number of residual lamellar fragments remaining prior to the onset of crystallisatio n, to permit so-called self-seeding effects to be explored. DSC results ind icate that, under all circumstances, two lamellar populations develop isoth ermally. At sufficiently high temperatures, these are accompanied by a thir d quenched component.,Although this general pattern of crystallisation is n ot 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 c rystallisation, whereas the cross-hatching is associated with secondary cry stallisation. Quenched material is assumed to be located between individual lamellae since, where isothermal crystallisation has proceeded to completi on, there is no evidence of macroscopic segregation consistent with the siz e 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 cryst allisation occurs from non-seeded melts (melt temperature of 168 degreesC) irregular spherulitic structures are seen to develop, The form of these str uctures is markedly influenced by the non-crystallisable fraction of the me lt, which serves to suppress primary crystallisation. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.