Melt memory in propene–pentene isotactic copolymers: the case of defects hosted in the crystals

Abstract

The memory of the crystalline state in the melt of propene–pentene isotactic copolymers (iPPC5) has been analyzed. Samples with pentene concentrations varying from 0.5 to 12.4 mol% have been prepared with a highly stereoselective metallocene catalyst. Pentene comonomeric units are largely included in the crystals of the polymorphic forms of isotactic polypropylene (iPP) and the relative amount of included comonomer increases with increasing pentene concentration, inducing crystallization of the trigonal δ form of iPP when the pentene content exceeds about 9–10 mol%. Self-nucleation experiments have demonstrated that high concentrations of pentene units produce a remarkable melt-memory that persists up to temperatures much higher than the melting temperature. The width of Domain II, where self-nucleation occurs, and the difference between the temperature of the beginning of the homogeneous melt, where the melt-memory is erased, and the melting temperature increase with increasing pentene concentration. These data indicate that a remarkable melt-memory of iPP crystals exists not only for the known cases of copolymers of iPP with noncrystallizable comonomers but also when comonomers are largely included in the crystals. Crystallization from a heterogeneous melt containing self-nuclei favors crystallization of the γ form, whereas crystallization from a homogeneous melt favors crystallization of the α form. For a high pentene concentration of 12.4 mol%, the trigonal δ form crystallizes from the homogeneous melt, whereas a small amount of the α form crystallizes from the heterogeneous melt when self-nucleation occurs and no trace of the γ form is observed.