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

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.