Crystallization of isotactic poly(propylenes) with enhanced melt strength.

摘要

Copolymerization of propylene with 1,9 decadiene using an isospecific metallocene catalyst leads to isotacic poly(propylenes) with H-type intermolecular linkages, resembling chemical crosslinks and leading to enhanced melt strength. In this work we analyze the effect of very small concentrations of diene (0--400 ppm) added to the iPP chain on the crystallization, polymorphism and morphological behavior of two series of copolymers of this nature crystallized from their quiescent melts. These series differ in the intermolecular distribution of the incorporated diene. In reference to the homopolymer the H-linkages enhance the primary nucleation density and the overall crystallization rate increases substantially. This effect is particularly strong when the chains were exposed to diene from the start of the polymerization. A minimal variation in the linear growth rate between the linear and H-linked samples in both series indicated that the addition of ppm levels of diene does not change the chain microstructure significantly as to impact growth. Some structural changes caused by the H linkages led to higher contents of gamma polymorphs. The difference in distribution of the diene between the two series was confirmed by polymorphic analysis. Crystallinity was not affected by the addition of H-links.;The application of Avrami models to the overall crystallization data ruled out significant differences between nucleation mode and the crystal geometry of the linear and H-linked iPPs. Solid state 13 C NMR indicated that the H-links in the amorphous regions slows short segmental relaxations of the diene containing samples in this region compared to the linear ones. The wide difference in nucleation rates is correlated with differences in melt dynamics, as the longest more branched molecules may retain some order prior to crystallization from their melt state. The addition of extra entanglements in the linear chain by diene incorporation affects the spherulitic morphology and lamellar stacking. Dynamic mechanical analysis suggested minimal variation in the mechanical properties between the linear and H-linked polypropylenes.