Thermal fractionation and effect of comonomer distribution on the crystal structure of ethylene–propylene copolymers

Abstract

In this paper the comonomer distributions of two series of ethylene–propylene copolymers with different propylene contents, which were prepared by a fluorinated bis(phenoxyimine) Ti catalyst (FI–EP copolymers) and a conventional Ziegler–Natta catalyst (ZN–EP copolymers), respectively, were characterized. It is found that the comonomer distribution of ethylene–propylene copolymers can still be characterized by thermal fractionation at a long scale, though the propylene units can be incorporated into the PE crystal lattice. The FI–EP copolymers exhibit a narrow and random comonomer distribution, whereas a broad comonomer distribution is observed for the ZN–EP copolymers. The crystal structures of the FI–EP and ZN–EP copolymers were studied by WAXD. The a-axis of the PE crystals of the FI–EP copolymers increases rapidly with propylene content, indicating that more propylene units are incorporated into the PE crystal lattice, whereas only a slight expansion in a-axis is observed for the ZN–EP copolymers. WAXD result also shows the presence of hexagonal phase in the FI–EP copolymers and the relative content of the hexagonal phase increases with the propylene content, while in the ZN–EP copolymers the hexagonal phase is negligible.